MEDIATOR (PHILIPS) 66K365/16Z MULTISTANDARD CHASSIS KM2 CRT TUBE PHILIPS A66-410X.

 PHILIPS  A66-410X.110° COLOUR TELEVISION PICTURE TUBE

Three-gun temperature -compensated shadow-mask rectangular colour television tube
with electrostatic focus, magnetic deflection and convergence, metal-backed three colour phosphor dot screen and internal magnetic shield. A high white luminance is obtained at near unity current ratio. Bein¢ temperature compensated, the shadow-mask makes for optimum field purity and good uniformity during warm-up. The design is such that minimum occurence of the moire effect is ensured. The tube has a reinforced envelope and therefore no separate safety screen is necessary. Typically, a legible picture will appear within 5 s.

 

 

 

PHILIPS CRT TUBE A66-410X ELECTRON GUN TECHNOLOGY IMPROVEMENT BY
CATHODE-RAY TUBE HAVING AN ASTIGMATIC LENS ELEMENT IN ITS ELECTRON GUN:
PHILIPS CTV KM2

A cathode-ray tube, in particular a shadow mask colour tube, having at least one electron gun which comprises at least three grids. The second grid serves as an accelerating electrode. An astigmatic lens element is present in the region of the second and the third grid. As a result of this an elongate spot is formed on the picture display screen of an electron beam in the case of low beam current, while the spot remains substantially unchanged in the case of high beam current. This is of importance, inter alia, for removing moire patterns in a shadow mask colour tube without it being necessary to choose a particular distance of the mask holes.



1. A cathode ray tube for color television, comprising:

2. A cathode ray tube as claimed in claim 1 wherein said second grid comprises said astigmatic lens element and a circular aperture.

3. A cathode ray tube as claimed in claim 1 wherein said third grid comprises said astigmatic lens element and a circular aperture.

Description:

The invention relates to a cathode-ray tube comprising at least one electron gun for producing at least one electron beam and a picture display screen, each electron gun comprising a cathode and at least three grids, the second grid serving as an accelerating electrode.

In such a cathode-ray tube the electron beam is usually focused substantially on the picture display screen so that a spot is formed at that region which causes the relevant part of the picture display screen to luminesce. The electron beam scans the picture display screen according to a particular pattern so that the spot moves over the picture display screen. The size of the spot depends, inter alia, on the current strength in the beam. In order to obtain a greater brightness of the luminescing part of the picture display screen, a larger beam current is necessary, which goes hand in hand with a larger spot. The picture display screen is usually scanned along lines, namely horizontal lines. It is possible that these lines are visible during scanning of the picture display screen. This will occur in particular with a low beam current because in that case the spot is comparatively small. In the case of an increasing beam current because in that case the spot is comparatively small. In the case of an increasing beam current, the spots of two successive lines may partly overlap each other so that the separate lines become less visible and consequently the line structure of the reproduced picture as a whole is less conspicuous.

The line structure of the picture may give rise to difficulties in particular in a cathode-ray tube for displaying colour pictures which comprises a colour selection electrode having systematically arranged apertures. In such a cathode-ray tube a number of electron beams are produced and each electron beam causes a particular luminescent substance present on the display screen of the tube to luminesce, while the colour selection electrode (usually termed the mask) prevents the electrons of said beam from reaching one of the other luminescent substances. During operation of the tube, annoying moire patterns may occur as a result of interference between the line structure of the picture and the hole structure of the mask. It is known that the occurrence of moire patterns can be reduced by choosing the mutual distance of the mask holes in a particular manner in relation to the line distance. The line distance is a function of the dimension of the picture at right angles to the scanning lines (in the case of horizontal scanning lines this is the height of the picture) and of the number of scanning lines per picture. The mutual distance of the mask holes should hence be chosen in accordance with the height of the picture and the number of scanning lines per picture.

It has now been found, however, that the choice of the mutual distance of the mask holes is subject to certain restrictions for various reasons, so that the occurrence of moire patterns cannot always be reduced by means of the distance of the mask holes. If with a given height of the picture display screen a mask is available in which a desirable distance of the mask holes is present, the distance of the mask holes in a mask for a smaller picture display screen should be smaller in comparison with the former mask, since actually the line distances are smaller. A smaller distance of the mask holes cannot be realized without the dimension of the mask holes being also made smaller, because otherwise the mask cannot fulfil its function of preventing electrons of a given electron beam from reaching one of the other luminescent substances. A mask having small holes and in addition a large number of such small holes meets with technological problems in manufacturing the mask. Difficulties may present themselves in addition when providing the luminescent substanves of the picture display screen. Moreover, for a ready functioning of the tube, the distance between the mask and the picture display screen should also be reduced when the distance of the mask holes is reduced. In connection with the tolerances occurring upon securing the mask, a reduction of said distance presents difficulties. For quite different considerations the value of the maximum deflection angle has a limiting effect upon the choice of the distance of the mask holes. When the maximum deflection angle of the beam increases, the angle at which the beam impinges on the mask at a particular place increases. During operation of the tube the mask is heated by the electrons and the suspension of the mask usually is such that under the influence of said heating it experiences an axial displacement. Moreover, in places where the brightness in the displayed picture is great, as a result of which the mask can be locally heated comparatively strongly, an axial displacement of the mask may occur locally. As a result of the axial displacement of the mask, the electron spot which is formed by an electron beam behind a certain mask hole is displaced and said displacement is larger according as the deflection angle is larger. This displacement of the spot on the picture display screen presents the possibility for the passed electrons to impinge upon a luminescent substance other than the one intended, so that a so-called mislanding will occur. With a given transmission of the mask the extent of the mislanding is the larger according as the mutual distance of the mask holes is smaller. In tubes in which a large deflection angle, for example 110°, occurs, it is therefore desirable that the mutual distance of the mask holes be large. This impedes a given choice of the mutual distance of the mask holes for checking the occurrence of moire patterns.

It is therefore of importance to check the occurrence of moire patterns in a different manner. This can be done in a simple manner by using an astigmatic beam when it is ensured that the line structure on the picture display screen is less visible. As already noted, the line structure in the case of a large beam current is already less visible than in the case of a small beam current. It should therefore be ensured that the line structure becomes less visible also in the case of a small beam current without a measure taken for that purpose resulting in undesirable effects in another respect. The line structure can be made less visible in the case of a small beam current by increasing the dimension of the spot at right angles to the scanning lines, in which case the spots of two successive lines will start overlapping each other increasingly also in the case of a small beam current. However, the dimension of the spot in the direction of the scanning lines must not be increased in order that the definition of the displayed picture in that direction be not adversely influenced. In the case of a larger beam current the dimension of the spot is already larger and it should be seen that it is not additionally increased in the direction at right angles to the scanning line, in order that the definition of the displayed picture in the direction at right angles to the scanning lines be not unfavourably influenced. In addition, the dimension of the electron beam at the area of the deflection plane in the case of a large beam current may not be noteworthily increased because otherwise the half-shadow effect of the electron beam occurring behind the mask holes would be inadmissibly increased. On the basis of these considerations the beam in the case of low beam currents should be influenced more than in the case of high beam currents and said influence must mainly result in an increase of the spot at right angles to the scanning lines.

According to the invention, an astigmatic lens element is present in the electron gun in the region of the second and the third grid. The region of the second and the third grid is to be understood to mean herein the part of the electron beam between on the one hand the part of the second grid located on the side of the first grid and on the other hand the part of the third grid which is farthest remote from the cathode. The operation of said astigmatic lens element is as follows. A cross-over of the beam is formed by the cathode. the first and the second grid which serves as an accelerating electrode. The location of the cross-over usually lies in the region from the first grid to the third grid dependent upon the current strength of the beam and the configuration of the grids. In the case of low beam currents the cross-over lies between the first and the second grid, while with increasing beam current it moves in the direction away from the cathode towards the space between the second and the third grid. As a result of the fact that the cross-over in the case of low beam current lies between the first and the second grid and as astigmatic lens element is present in the region of the second and the third grid, the beam, in the case of a low beam current, is influenced by said astigmatic lens element so that an increase of a spot at right angles to this scanning line is obtained. When the beam current increases, the cross-over moves in the direction of the astigmatic lens element so that the influence hereof on the beam is reduced. When the cross-over coincides with the optical centre of the astigmatic lens element, the beam is substantially not influenced by this. So, in general, the location where the astigmatic element is provided will depend on the location of the cross-over and the extent of cross-over displacement upon variation of the beam current in the absence of said element.

The astigmatic lens element may be constructed in various mannners: a non-rotationally symmetric aperture in the second grid through which the beam passes; a non-rotationally symmetric aperture in the third grid through which the beam passes; an extra plate having a non-rotationally symmetric aperture through which the beam passes; this extra plate is present either between the first and the second grid, namely on the side of the second grid, or between the second and the third grid; a non-rotationally symmetric profile of the plate-shaped part of the second grid at right angles to the axis of the gun in which a rotationally symmetric aperture is present through which the beam passes; a non-rotationally symmetric profile of the plate-shaped part of the third grid at right angles to the axis of the gun in which a rotationally symmetric aperture is present through which the beam passes; in the case of a third grid having a tubular part parallel to the axis of the gun and a second grid having likewise a tubular part parallel to the axis of the gun, which part is present around the tubular part of the third grid, the presence of axial apertures in said part of the third grid; in the case of a second grid having a tubular part parallel to the axis of theggun, the presence of axial apertures in said part which are covered by plates located farther away from the axis of the gun and which form an extra electrode. A non-rotationally symmetric profile of a plate-shaped part of a grid can be realised by securing one or more plates to the plate-shaped part so that actually certain parts have a larger thickness or, when the thickness remains the same, giving the plate itself a non-rotationally symmetric profile.

The astigmatic lens element is present in particular in one of the grids. This has the advantage that no separate extra elements need be provided in the gun and/or extra voltages be applied. Since it is not so easy to realize with great precision a small non-rotationally symmetric aperture in a grid, it is to be preferred to use a circular aperture. The grid therefore comprises preferably an astigmatic lens element and a circular aperture.

In order that the invention may be readily carried into effect, it will now be described in greater detail, by way of example, with reference to the accompanying drawings, in which:

FIG. 1 is a cross-sectional view of a cathode-ray tube,

FIG. 2 is a cross-sectional view of an electron gun,

FIG. 3 is a cross-sectional view of the electron gun shown in FIG. 2,

FIG. 4 shows the dimension of the spot on the picture display screen in two mutually perpendicular directions as a function of the beam current,

FIGS. 5a and 5b show diagramatically the path of rays in a part of the cathode-ray tube,

FIG. 6 shows another embodiment of the second grid, and

FIGS. 7a, 7b and 7c show another embodiment of the second grid.

Referring now to FIG. 1, the cathode-ray tube 1 comprises a system 2 which is shown diagrammatically and comprises three electron guns for generating three electron beams. The electron beams are converged onto a shadow mask 3 by means of a convergence device (not shown) present partly inside and partly outside the tube, after which they each impinge on certain parts of a picture display screen 4. The scanning of the picture display screen is carried out by a deflection device 5 which is shown diagrammatically.

FIG. 2 is a cross-sectional view of one of the three electron guns. The gun comprises a cathode 6, a first grid 7, a second grid 8, a third grid 9 and a fourth grid 10. The first grid 7 comprises a plate-shaped part 11 having a circular aperture 12. The second grid 8 comprises a plate-shaped part 13 having a circular aperture 14 and furthermore a circular cylindrical part 15. On the side of the plate-shaped part 13 facing the third grid 9 two plates 16 and 17 in the form of a segment of a circle are present. The third grid 9 comprises a plate-shaped part 18 having a circular aperture 19 and furthermore two circular cylindrical parts 20 and 21. The fourth grid 10 is in the form of a circular cylindrical sleeve.




FIG. 3 is a cross-sectional view of the gun shown in FIG. 2 taken on the line III -- III of FIG. 2. The two plates 16 and 17 in the form of segments of a circle are present on the plate-shaped part 13 of the second grid which comprises a circular aperture 14. The plates 16 and 17 constitute an astigmatic lens element.

In a particular case, the distance between the cathode 6 and the first grid 7 is 0.12 mm, between the first grid 7 and the second grid 8 at the area of the apertures it is 0.47 mm, between the second grid 8 and the third grid 9 at the area of the apertures it is 2.25 mm and between the third grid 9 and the fourth grid 10 it is 1.5 mm. The first grid 7 at the area of the aperture is 0.13 mm thick, the second grid 8 at the area of the aperture is 0.25 mm thick, the thickness of the plates 16 and 17 is 1 mm, and the third grid 9 at the area of the aperture is 0.25 mm thick. The length of the circular cylindrical part 20 is 6 mm, of the circular cylindrical part 21 it is 16.5 mm and of the circular cylindrical sleeve 10 it is 10.0 mm. The inside diameter of the part 20 is 5.5 mm, of the part 21 it is 9.5 mm. The diameter of the aperture 12 is 0.75 mm, of the aperture 14 it is 0.75 mm and of the aperture 19 it is 1.5 mm. The distance between the plates 16 and 17 is 2 mm. In the cathode-ray tube the distance from the cathode to the picture display screen along the axis furthermore is 467 mm. This gun can be operated with the following voltages:

cathode O V first grid between O V and 165 V second grid 500 V third grid between 4400 V and 4600 V fourth grid 25,000 V

The variable voltage at the first grid serves to control the beam. The ratio of the voltages at the third and the fourth grid is chosen to be so that the beam is focused as readily as possible in the spot on the screen.

Due to the presence of the astigmatic lens element constituted by the plates 16 and 17, the shape of the beam in the case of low beam current is influenced and that in such manner that an increase is obtained in a vertical direction of the spot on the picture display screen. This is shown in the graph of FIG. 4 where the beam current expressed in μA is plotted on the horizontal axis and the dimensions of the spot expressed in millimetres in the centre of the picture display screen is plotted on the vertical axis. A solid line 22 relates to the vertical dimension of the spot and a broken line 23 relates to the horizontal dimension of the spot. For comparison a dot-and-dash line 24 is shown in addition which relates to a gun which is the same as the above-described gun with the only difference that the plates 16 and 17 are lacking so that no astigmatic lens element is pressed. Since the lines 22 and 23 substantially coincide in the case of high currents, a substantially circular spot is obtained; in the case of low currents a vertical spot is obtained. It appears from the position of the line 23 relative to the line 24 that, compared with the case in which no astigmatic lens element is present, the horizontal dimension of the spot has remained approximately the same or has been reduced so that the horizontal definition has remained the same or has improved. This result can be explained as follows. With a low beam current, a real cross-over is formed in the space between the first and the second grid by the cathode 6, the first grid 7, and the second grid 8. Not counting aberrations, space charge and transverse speeds upon emission, this real cross-over is substantially punctiform. As a result of the astigmatic element on the second grid, the beam in the space between the second and the third grid becomes astigmatic. Viewed from the equipotential space within the cylindrical part of the third grid, this gives rise to two virtual cross-overs, one in a vertical line and one in a horizontal line. With low beam current, said virtual cross-overs lie at some distance from each other and whent the beam current increases, said distance decreases until the virtual cross-overs substantially coincide. Then they are again substantially punctiform. This is a result of the fact that the real cross-over which, in the case of low beam current, is formed between the first and the second grid, moves in the direction of the second grid when the beam current increases and hence towards the optic centre of the astigmatic lens element. The vertical virtual line cross-over is focused onto the picture display screen by the main lens of the gun constituted by the third grid 9 and the fourth grid 10, while the horizontal virtual cross-over is focused in the area between the gun and the picture display screen. The gap formed by the plates 16 and 17 is horizontal. As a result of this, with the voltages stated and with low beam current, a virtual cross-over of the rays of the beam situated in a vertical plane is formed (horizontal line cross-over) which is situated farther away from the picture display screen than the virtual cross-over formed of the rays of the beam situated in a horizontal plane (vertical line cross-over).

FIGS. 5a and 5b show diagrammatically the path of rays in the tube from said virtual cross-over to the picture display screen. FIG. 5a shows the path of rays in a vertical plane and FIG. 5b in a horizontal plane. In FIG. 5a, 25 is the location of the virtual cross-over in the case of low beam current of the rays of the beam situated in a vertical plane and 27 is the location thereof in the case of a high beam current. In FIG. 5b, 26 is the location of the virtual cross-over in the case of a low beam current of the rays of the beam situated in a horizontal plane and 28 is the location thereof in the case of a high beam current. Since the locations 27 and 28 substantially coincide, they are shown at the same distance from the picture display screen 29. The centre of the lens constituted by the third and the fourth grid is denoted by 30. The virtual cross-over 26 is focused on the picture display screen at 31 by the lens. The virtual cross-over 25 is focused in a point 32 which is situated nearer so that a vertical extension 33-34 is formed of this on the picture display screen. In the case of high beam current focusing on the display screen in 31 takes place both in the horizontal plane and in the vertical plane. In a cathode-ray tube in which the gun does not comprise the plates 16 and 17, a virtual cross-over is formed with the same voltages by the lens effect of the second and the third grid, of which cross-over in the case of low beam current the location 35 is situated between 25 and 26 and in the case of high beam current the location is situated in 27 and 28. When the focusing is considered in a horizontal plane (FIG. 5b) it is obvious that in the absence of the plates 16 and 17 the cross-over, in the case of variation of the beam current, moves over a larger distance than in the presence of said plates so that in the latter case a smaller variation of the voltage of the third grid will be sufficient for optimum focusing. This is a favourable property, because in practice the voltage of the third grid is adjusted at a constant value.

FIG. 6 shows another embodiment of the astigmatic lens element of the second grid. In this case a plate 36 having an elongate aperture, namely a rectangular aperture 37, is present on the side of the third grid 9 on the plate-shaped part 13 comprising a circular aperture 14 of the second grid 8.

FIGs. 7a, 7b and 7c show another embodiment of the astigmatic lens element of the second grid. Figure 7a is a plan view; FIG. 7b is a cross-sectional view taken on the line VIIb -- VIIb of FIG. 7a; FIG. 7c is a cross-sectional view taken on the line VIIc -- VIIc of FIG. 7a. The second grid consists of a circular cylindrical part 38 and a plate-shaped part 39 in which an elongate bulge 40 is provided on the side of the first grid. A circular aperture 41 is present in the centre of the plate-shaped part 39. This embodiment has the advantage that the astigmatic lens element can be realized with a simple mechanical operation.


Convergence unit having three identical V-shape bent plates for shielding pole shoes:PHILIPS CRT TUBE A66-410X ELECTRON GUN TECHNOLOGY,

A color cathode-ray tube wherein a convergence unit comprises a symmetric shielding assembly in the form of three identical V-shaped bent plates, secured to each other by only three welds.

What is claimed is: 1. A color cathode-ray tube having, in an evacuated envelope, means to generate three electron beams, a display screen comprising a large number of regions luminescing in three different colors, a color selection electrode having a large number of apertures and means to converge the electron beams on the display screen, said electron beams being each associated with luminescent regions of one color by means of the color selection electrode, said convergence means comprising a cylindrical sleeve to which three pairs of ferromagnetic pole shoes extending radially inwardly are secured and in which shielding means are present to shield the magnetic fields of the said pole shoes, said shielding means including three identical V-shaped bent plates the apexes of which join each other along the axis of the said cylindrical sleeve and the first and second ends of which extend from said axis towards the wall of the said cylindrical sleeve in such manner that the space in said cylindrical sleeve is divided into three symmetrical compartments in which the said pairs of pole shoes are present, said V-shaped bent plates having respectively at their first end a lug which projects through an aperture in the cylindrical sleeve and is welded to the outer wall thereof, and have at their second ends a short projection which extends in a corresponding other aperture in the cylindrical sleeve for fixation in the tangential direction and is not welded, and further comprising means for fixation of the adjoining plates in the radial and the axial directions, said means including a projecting part at one arm of one plate and a recess in the engaging arm of the other plate. 2. A cathode-ray tube as claimed in claim 1, wherein said projection is a local bulge and the recess is a hole in which the said bulge extends. 3. A cathode-ray tube as claimed in claim 1, wherein said projection is a lug and the said recess is a slot through which the said lug extends.

Description:

The invention relates to a cathode-ray tube for displaying coloured pictures and comprising in an evacuated envelope means to generate three electron beams, a display screen comprising a large number of regions luminescing in three different colours, a colour selection electrode having a large number of apertures and means to converge the electron beams on the display screen, which electron beams are each associated with luminescent regions of one colour by means of the colour selection electrode, which convergence means comprise a cylindrical sleeve to which three pairs of ferromagnetic pole shoes extending radially inwardly are secured and in which means are present to shield the magnetic fields from the said pole shoes which consists of three V-shaped bent plates the tops of which join each other along the axis of the said cylindrical sleeve and of which the first and second ends extend from said axis towards the wall of the said cylindrical sleeve in such manner that the space in said cylindrical sleeve is divided into three compartments in which the said pairs of pole shoes are present.

Such a cathode-ray tube is described in the U.S. Pat. No. 3,689,791. In such a cathode-ray tube, three electron beams are generated which are converged on the display screen by means of three pairs of ferromagnetic pole shoes. This is carried out by means of the field between the pole shoes of each pair, which field is generated by means of an external convergence unit which is present outside the said envelope and cooperates with the pole shoes. The magnetic field in each of the pole shoes can deflect the relevant beam radially in such manner that the beams intersect each other substantially in a point at the area of the display screen.

The convergence unit in the cathode-ray tube consists of a cylindrical sleeve of non-magnetic material which has an apertured bottom. The top side of the cylindrical sleeve is open. The said pole shoes extend radially inwardly in said cylindrical sleeve. The cylindrical sleeve furthermore comprises means of a ferromagnetic material to shield the magnetic fields between the said pole shoes which generally consist of three V-shaped bent plates the tops of which join each other along the axis of the cylindrical sleeve and the ends extend in such manner towards the wall that the space in the cylindrical sleeve is divided into three identical compartments in which the said pole shoes are present.

Lugs of the ends of said V-shaped bent plates project through apertures in the cylindrical sleeve and are then welded. Hence six welds are necessary for said three plates.

The said United States Patent Specification describes a construction which reduces said number of welds from six to three, but said construction has the drawback that the systems of means to screen the magnetic fields is not symmetrical and is manufactured from two different parts.

It is the object of the invention to provide a construction which does not consist of different parts and which is symmetrical while the number of welds is restricted to three.

According to the invention, a cathode-ray tube of the type mentioned in the first paragraph is characterized in that the said V-shaped bent plates have at their first end a lug which projects through an aperture in the cylindrical sleeve and, after bending, is welded to the outer wall thereof, and have at their second end (a) means for fixing relative to the first end of the adjoining V-shaped plate in the radial and axial directions of the cylindrical sleeve, (b) a short projection which extends in an aperture in the cylindrical sleeve for fixation in a tangential direction and is not welded.

The fixation means mentioned sub (a) may consist of a projecting part at one of the said ends (for example a bulge, a lug) and a recess in the engaging end (for example a hole or slot).

It is an advantage in the manufacture that three identical V-shaped bent plates can be manufactured. Another advantage is the smaller number of welds with a smaller risk of the occurrence of welding sputters which cause separate parts in the tube, which may result in short-circuit in the means to generate electron beams.

The invention will be described in greater details with reference to the accompanying drawing, of which:

FIG. 1 shows a cathode-ray tube for displaying coloured pictures according to the invention,

FIG. 2 is an elevation of the internal convergence unit of the tube,

FIG. 3 is a sectional view according to a plane at right angles to the cylinder axis and taken on the line III--III of FIG. 5,

FIG. 4 is a sectional view analogous to FIG. 3 of another embodiment,

FIG. 5 is a sectional view taken on the line V--V of FIG. 2.

The cathode-ray tube for displaying coloured pictures shown in FIG. 1 is of the shadow mask type and comprises an evacuated glass envelope 1, a set of electron guns 2 in triangular arrangement to generate three electron beams 3, 4 and 5, a display screen 6 comprises a large number of regions luminescing in red, green and blue, respectively, a colour selection electrode 7 comprising a large number of apertures 8, and a set of deflection coils 9 for scanning the display screen 6 with the electron beams 3, 4 and 5. The electron beams 3, 4 and 5 are converged on the display screen 6 by means of an internal convergence unit 10 and an external convergence unit 11. The internal convergence unit 10 comprises a cylindrical sleeve 12 which is coaxial with the axis 13 of the cathode-ray tube.

FIG. 2 is an elevation of the internal convergence unit 10 viewed from the side of the shadow mask 7. The co-operation of the internal convergence unit 10 with the external convergence unit 11 is shown diagrammatically with an external magnetic circuit 14 having a coil 15. A current through coil 15 causes a magnetic field between the pole shoes 16 and 17 which causes a radial displacement of the electron beam 5. In a similar manner the electron beam 3 is influenced by the field between the pole shoes 18 and 19 and the electron beam 4 by the field between the pole shoes 20 and 21. In this manner the electron beams 3, 4, and 5 can be converged in one point on the display screen 6. The pairs of pole shoes are shielded relative to each other by ferromagnetic V-shaped bent plates 22, 23 and 24. It is obvious that V-shaped bent plates the top of which is more or less circular may also be used. The cylindrical sleeve 12 has a bottom plate 25 which has three apertures for passing the electron beams 3, 4 and 5. The ferromagnetic V-shaped bent plates are fixed in a tangential direction by a short projection 26 which passes through the hole 27 and bent lug 32 which also passes through hole 27 and is moreover welded.

FIG. 3 is a sectional view at right angles to the cylinder axis of the cylindrical sleeve 12 and the ferromagnetic V-shaped bent plates 22, 23 and 24 and taken on the line III--III of FIG. 5. The bulge 28 extends in hole 29 and fixes the said plate in the axial and radial directions.

In FIG. 4 the lug 30 extends through slot 31, which represents another embodiment of the fixation.

FIG. 5 is a sectional view taken on the line V--V of FIG. 2. In this case the short projection 26 extends through the hole 27.

PHILIPS A66-410X Cathode structure comprising a heating element:

A cathode structure comprises at an end portion (1) an electron-emitting material (4) and a heating element (5) of wire (7), said cathode structure having a plurality of primary helical turns (8). These primary turns are used to form a first series of secondary turns (9) which are wound in a first direction with a pitch and which extend towards the end portion (1), and to form a second series of secondary turns (10) which extend from the end portion (1) and which have the opposite direction of winding yet the same pitch. Near the end portion (1), the first and second series of turns (9, 10) are interconnected by an arc-shaped connecting portion (12) having primary turns (8). This arc-shaped connecting portion (12) has a span S_a and a rise r_a, the ratio r_a /S_a preferably ranging from 0.3 to 0.5.

1. A cathode structure which comprises an electron-emitting material at an end portion, and in which there is a filamentary heating element comprising a plurality of primary, helical turns with which a first series of secondary turns is formed, which are wound in a first direction with a pitch and which extend in the direction of said end portion, and with which a second series of secondary turns is formed which extend from said end portion and which are wound in the opposite direction yet with the same pitch, said first and second series of turns being interconnected at the end portion by a connecting portion with primary turns, characterized in that the connecting portion is arc-shaped with a span S_a and a rise r_a, the ratio r_a /S_a ranging from 0.1 to 1.0.

2. A cathode structure as claimed in claim 1, characterized in that the ratio r_a /S_a ranges from 0.1 to 0.5.

3. A cathode structure as claimed in claim 2, characterized in that the ratio r_a /S_a ranges from 0.3 to 0.5.

4. A cathode structure as claimed in any one of claims 1 to 3, characterized in that, in the connecting portion, a diameter d_w of the wire, an internal diameter d_p of the primary turns and an average pitch P_p of the primary turns satisfy the relationship: ##EQU6## in which the argument of the sine is expressed in radials.

5. A cathode structure as claimed in claim 4, characterized in that: ##EQU7##.

6. A cathode structure as claimed in claim 5, characterized in that: ##EQU8##.

7. A cathode structure as claimed in any one of claims 1 to 3, characterized in that the diameter d_w of the wire exceeds 20 μm.

8. A cathode structure as claimed in any one of claims 1 to 3, characterized in that the internal diameter d_p of the primary turns exceeds 100 μm.

9. A cathode structure as claimed in any one of claims 1 to 3, characterized in that the span S_a of the connecting portion is smaller than 500 μm.

10. A cathode ray tube comprising an electron source which includes a cathode structure having a heating element as claimed in any one of claims 1 to 3.

Description:

BACKGROUND OF THE INVENTION

The invention relates to a cathode structure which comprises an electron-emitting material at an end portion, and in which there is a filamentary heating element comprising a plurality of primary, helical turns with which a first series of secondary turns is formed, which are wound in a first direction with a pitch and which extend in the direction of said end portion, and with which a second series of secondary turns is formed which extend from said end portion and which are wound in the opposite direction yet with the same pitch, said first and second series of turns being interconnected at the end portion by a connecting portion with primary turns.

The invention further relates to a cathode ray tube comprising an electron source which includes a cathode structure which is provided with a heating element.

Cathode structures comprising heating elements are used in electron sources for cathode ray tubes, for example, in display devices for displaying monochromatic or colour images, camera tubes, video amplifiers and oscilloscopes.

Such a cathode structure is known from the brochure "Quick-Vision CTV Picture Tube A66-410X" by L. J. G. Beriere and A. J. van IJzeren (Philips Product Note, 1973). In said document, a description is given of a tubular cathode structure in an electron gun for use in a cathode ray tube, which cathode structure comprises at an end portion a layer of an electron-emitting material to emit electrons. The cathode structure comprises a heating element which serves to heat the electron-emitting material. Said heating element comprises a wire having primary and secondary turns which is bifilarly wound in the form of a double helix. The secondary turns are built up from a first series of turns, which are wound in a first direction with a pitch and which extend in the direction of the end portion, and from a second series of turns which extend from the end portion and which are wound in the opposite direction yet with the same pitch. The first and second series of secondary turns are interconnected near to the end portion of the cathode structure by a connecting portion.

A drawback of the known cathode structure is that a number of the primary turns in the connecting portion may be short-circuited. These short-circuits occur, particularly in the primary turns in the transitions from the connecting portion to the first and second series of secondary turns. Due to the fact that the connecting portion is nearest to the electron-emitting material, the efficiency with which the heating element heats the electron-emitting material is adversely affected by these short-circuits.

SUMMARY OF THE INVENTION

It is an object of the invention to provide a cathode structure in which a short-circuit in the primary turns of the heating element near the end portion of the cathode structure is precluded and/or an improved distribution of the primary turns of the heating element near the end portion of the cathode structure is realised, so that the efficiency of the heating element is improved.

To this end, the heating element in accordance with the invention is characterized in that the connecting portion is arc-shaped with a span S _a and a rise r _a , the ratio r _a /S _a ranging from 0.1 to 1.0.

The advantage of an arc-shaped connecting portion is that, in said connecting portion, the primary turns are arranged in a flowing line relative to each other, that is, the distances between the primary turns in the connecting portion change gradually. In addition, an arc-shaped connecting portion causes the primary turns to be flowing, particularly in the transitions from the connecting portion to the first and second series of secondary turns, so that the risk of short-circuits between the primary turns is precluded.

The known cathode structure has a so-called "flat head" (r _a /S _a ≉0), which is to be understood to mean that the connecting portion between the two transitions to the first and second series of secondary turns is situated in a plane transverse to a longitudinal axis of the cathode structure. As a result, in the known cathode structure the transitions from the connecting portion to the first and second series of secondary turns in the heating element are curved substantially.

In order to heat the electron-emitting material as effectively as possible, it is desirable that the turns of the heating element, particularly near the end portion, should be used as efficiently as possible. This can be achieved by using a wire with primary and secondary turns which is bifilarly wound in the form of a double helix instead of a single heating wire to build up the heating element. An efficient heating element is obtained if the density of the primary turns is relatively high, while electrical contact between the primary turns, particularly also in the connecting portion and in the transitions from the connecting portions to the first and the second series of secondary turns, is avoided. Given the span S _a and a rise r _a of the arc-shaped connecting portion, the primary turns can be closely spaced, while preventing a short-circuit between said primary turns by choosing the ratio r _a /S _a to be in the range from 0.1 to 1.0.

An embodiment of the heating element in accordance with the invention is characterized in that the ratio r _a /S _a ranges from 0.1 to 0.5.

If the ratio of the span to the rise of the arc-shaped connecting portion r _a /S _a is 0.5, then the connecting portion is semi-circular. If the ratio r _a /S _a is above 0.5, then the connecting portion has a pointed shape relative to the longitudinal axis of the cathode structure, so that the average distance from the primary turns of the connecting portion to the end portion of the cathode structure increases and the efficiency of the heating element decreases. If the ratio r _a /S _a is chosen in the range from 0.1 to 0.5, the connecting portion obtains a (somewhat) flattened shape, as compared to a circular connecting portion which is obtained if r _a /S _a =0.5, and the primary turns are relatively closely spaced. By flattening the connecting portion, a more efficient heat dissipation is brought about, which is necessary to heat the electron-emitting material.

An embodiment of the heating element in accordance with the invention is characterized in that the ratio r _a /S _a ranges from 0.3 to 0.5.

An optimum density of primary turns in the connecting portion of the heating element is obtained by choosing the ratio of the span to the rise of the arc-shaped connecting portion r _a /S _a to be in the range from 0.3 to 0.5.

A preferred embodiment of the heating element in accordance with the invention is characterized in that, in the connecting portion, a diameter d _w of the wire, an internal diameter d _p of the primary turns and an average pitch P _p of the primary turns satisfy the relationship: ##EQU1## in which the argument of the sine is expressed in radials.

Given a number of easily measurable parameters of the wire and the various winding ratios, the criterion which must be satisfied to preclude electrical contact between the primary turns, to be indicated by f _w ≤1.5 d _w , is contained in the above formula.

A further embodiment of the heating element in accordance with the invention is characterized in that ##EQU2##

By observing a lower limit in the formula (f _w ≥0.3 d _w ) a safe margin for the minimum distance between the primary turns in the heating element is obtained, so that, also when the heating element is in operation to heat the electron-emitting material, the primary turns do not contact each other as a result of possible thermal expansion, and hence do not cause a short-circuit between the primary turns.

A further embodiment of the heating element in accordance with the invention is characterized in that ##EQU3##

In the known cathode structure having the so-called "flat head" (r _a /S _a ≉0) short-circuits between the primary turns cannot be precluded if the various parameters of the heating wire in the formula f _w are chosen to be such that the outcome of the formula does not exceed the upper limit to be observed by the formula (f _w ≤1.0 d _w ). However, the combination of an arc-shaped connecting portion with pre-conditions for the ratio r _a /S _a as defined hereinabove, that is, in the range from 0.1 to 1.0, preferably from 0.1 to 0.5, in particular from 0.3 to 0.5, enables such values to be selected for the parameters of the heating wire (diameter d _w of the wire, internal diameter d _p of the primary turn, average pitch P _p of the primary turn and span S _a of the (arc-shaped) connecting portion) that the upper limit (f _w ≤1.0 d _w ) is not exceeded. In this manner, a low-power heating element without short-circuits is obtained which can attain high temperatures.

Within the framework of the invention, a thickness d _w for the wire in excess of 20 μm, or an internal diameter d _p of the primary turns in excess of 100 μm, or an average pitch P _p of the primary turns below 50 μm, or a span S _a of the connecting portion below 500 μm can now advantageously be used.

The above relations between the dimensions of the wire and the data about the winding ratios in the heating element enable those skilled in the art to design and use, in a simple manner, an efficient heating element for a cathode structure and hence to preclude short-circuits in the primary turns of the heating element near the end portion of the cathode structure and/or to achieve an improved distribution of the primary turns of the heating element near the end portion of the cathode structure. The inventors have recognized that, given a suitably chosen ratio of the span S _a to the rise r _a , the use of an arc-shaped connecting portion enables wire parameters and winding ratios (diameter d _w of the wire, internal diameter d _p of the primary turn and the average pitch P _p of the primary turn) in the connecting portion of the heating element to be chosen which, when r _a /S _a is chosen to be 0, i.e. a so-called "flat" head, always cause a substantial number of short-circuits between the primary turns in the connecting portion. The formula f _w provides those skilled in the art with a simple "tool" for choosing suitable wire parameters and winding ratios of the wire to be used for the connecting portion.

FIG. 2A is a schematic view, partly in cross-section, of a cathode structure in accordance with the prior art. This cathode structure comprises an end portion 1 and a cathode shaft 2 which is closed by means of a cover 3 which is partly covered by an electron-emitting material 4. In this embodiment, said cover and the part of the cathode structure cooperating with said cover form the end portion 1 of the cathode structure. The cathode shaft 2 accommodates a heating element 5 which serves to heat the electron-emitting material 4. Said heating element 5 comprises a wire 7 having primary turns 8 and secondary turns 9, 10 which is bifilarly wound in the form of a double helix and which is covered by an electrically insulating layer 6. Said secondary turns are built up of a first series of turns 9 which are wound in a first direction (i.e. counterclockwise) with a pitch and which extend towards the end portion 1, and of a second series of turns 10 which extend from the end portion 1 and which are wound in the opposite direction yet with the same pitch. The first and second series of secondary turns 9, 10 are interconnected close to the end portion 1 of the cathode structure by a connecting portion 11 having primary turns 8. This connecting portion 11 has a flat shape with respect to a longitudinal axis of the cathode structure. Above the cathode structure, there are a number of electrodes, one of which is shown in FIG. 2. The electrode 18 is commonly referred to as g ₁ -electrode and comprises an aperture 19.

CRT TUBE PHILIPS A66-410X DEFLECTION COIL SYSTEM FOR COLOUR TELEVISION


A deflection coil system for colour television, comprising deflection coils which are subdivided into sections, an adjustable impedance being connected parallel to some of these sections for compensation of differences in the distribution of turns of the coils.

1. A deflection coil system for a colour television display tube, comprising deflection coils for electron beam deflection in the horizontal and the vertical direction, respectively, each coil being a single coil and consisting of a number of turns of a conductive wire, each of the coils for at least one of the two deflection directions being divided into a number of series-connected sections in that at least one turn which is situated between the first and the last turn of the coil is provided with a tapping, characterized in that at least one of the sections of each of the coils for at least one deflection direction, is connected in parallel with a circuit means of adjustable impedance for adjusting the dynamic convergence. 2. A deflection coil system as claimed in claim 1, characterized in that at least the first (45) or the last section (53) of the relevant coils is provided with a parallel circuit (21). 3. A deflection coil system as claimed in claim 1, characterized in that a coil section (49) which is provided with a parallel circuit extends on both sides of a mean turn (27), the angular distance α_g between said mean turn (27) and the deflection direction associated with this coil satisfying the formula: ##SPC3## 4. A deflection coil system as claimed in claim 1, characterized in that each of the sections provided with a parallel circuit comprises a number of turns substantially equal to one tenth of the total number of turns of the relevant coil. 5. A deflection coil system as claimed in claim 1, characterized in that each of the parallel circuits (21) of the horizontal deflection coils consists of a coil (59, 61, 63) of adjustable inductance, each of the parallel circuits (21) of the vertical deflection coils consisting of a variable resistor (65, 67, 69). 6. A deflection coil system as claimed in claim 1, characterized in that sections (45, 53'; 49, 49'; 53, 45') which form part of different coils are pair-wise connected in parallel. 7. A deflection coil system for a color television display tube, comprising a pair of single deflection coils for electron beam deflection in the horizontal and the vertical directions respectively, each coil comprising a plurality of turns of a conductive wire, one of said coils having a tap dividing said one coil into a plurality of series-connected sections, and means parallel coupled to at least one of the sections of each of the coils for adjusting the dynamic convergence of the tube comprising an adjustable impedance element. 8. A deflection coil system as claimed in claim 7, wherein at least an end section of the relevant coils is provided with said parallel coupled impedance element. 9. A deflection coil system as claimed in claim 7, wherein a coil section which is provided with a parallel circuit extends on both sides of a mean turn, the angular distance α_g between said mean turn and the deflection direction associated with this coil satisfying the formula: ##SPC4## 10. A deflection coil system for a color television display tube, comprising a pair of single deflection coils for electron beam deflection in the horizontal and the vertical directions respectively, each coil comprising a plurality of turns of a conductive wire, one of said coils having a tap dividing said one coil into a plurality of series-connected sections, means parallel coupled to at least one of the sections of each of the coils for adjusting the dynamic convergence of the tube comprising an adjustable impedance element, and wherein one of said sections which is provided with a parallel circuit extends on both sides of a mean turn the angular distance α_g between said mean turn and the deflection direction associated with this coil satisfying the formula: ##SPC5##

Description:

The invention relates to a deflection coil system for a colour television display tube, comprising deflection coils for electron beam deflection in the horizontal and the vertical direction, respectively, each coil consisting of a number of turns of a conductive wire, each of the coils for at least one of the two deflection directions being divided into a number of series-connected sections in that at least one turn which is situated between the first and the last turn of the coil is provided with a tapping.

Contemporary colour television display tubes usually comprise three electron guns which are preferably arranged at the corners of an equilateral triangle or are adjacently arranged on one line. Each of these electron guns generates an electron beam which, after having passed through one of the apertures in a shadow mask, impinges upon a phosphor dot on a rectangular display screen, the said phosphor dot thereby luminescing a given colour. By means of magnetic fields generated in a deflection coil system, the electron beams can be deflected such that they scan the entire display screen.

The dynamic convergence of the beams is of major importance in this respect. This means that, regardless of the angle through which the beams have been deflected, the three beams must always intersect each other at the area of the shadow mask so as to cause luminescence of the correct phosphor dots. This dynamic convergence greatly depends on the distribution of the turns of the wires constituting the deflection coils. During the manufacture of the coils, deviations occur in the turn distribution which cause a difference in the dynamic convergence in individual coil systems. Such differences are not acceptable, particularly in the case of colour television display tubes having large deflection angles.

A proposal has already been made for correcting such deviations, see U.S. Pat. No. 3,169,207. According to this proposal, each of the coils is wound with a number of parallel wires, the coil being divided into a number of series-connected sections by means of tappings. The parallel wires of some of these sections are connected in parallel, and the parallel wires of other sections are connected in series. The effective turn distribution of the finished product can be influenced by varying the number of turns per section when the coil is being wound.

A drawback of this method is that the correction must be performed during winding, whilst the correction must also be rather coarse because a difference of at least one complete turn must each time be introduced. The invention has for its object to eliminate these drawbacks by providing a deflection coil system in which the effective turn distribution of the finished product can be changed while it has already been mounted on the display tube, arbitrarily small variations also being possible.

To this end a deflection coil system according to the invention is characterized in that at least one of the sections of each of the coils for at least one deflection direction is connected in parallel with a circuit of adjustable impedance.

One embodiment of the system according to the invention by means of which the convergence can be influenced on the entire display screen is characterized in that at least the first or the last section of the relevant coils is provided with a parallel circuit.

For each winding of a deflection coil the angular distance α _n between this winding and the deflection direction can be determined. From this distance the angular distance α _g of a mean turn can be calculated by means of the formula ##SPC1##

Therein, n(α ) represents the number of turns the angular distance of which amounts to α _n . It was found that the convergence in the corners of the display screen can be influenced, whilst the convergence on the axes remains substantially the same if a deflection coil system according to the invention is used which is characterized in that a coil section which is provided with a parallel circuit extends on both sides of the mean turn.

The invention will be described in detail with reference to the drawings, in which

FIG. 1 is a perspective view of a saddle-shaped deflection coil which forms part of a deflection coil system according to the invention,

FIG. 2 is a diagrammatic cross-sectional view of a ferromagnetic core having vertical saddle-shaped deflection coils,

FIG. 3 is a diagrammatic cross-sectional view of a ferromagnetic core having vertical toroidal deflection coils,

FIGS. 4 and 5 are diagrammatic views of two methods of dividing the coils of FIGS. 2 and 3 into sections in accordance with the invention, and

FIGS. 6a, 6b and 7a, 7b show a number of practical circuits for a deflection coil system according to the invention.

The saddle-shaped deflection coil shown in FIG. 1 is intended to be arranged on the outside of a display tube (not shown) so as to deflect an electron beam which is generated in the tube and whose direction of travel is denoted by an arrow 1. The coil consists of a number of turns of a conductive wire which is composed of one or more strands, preferably copper wire, which envelope a window 3. The coil comprises active parts 5 and 7 in which the wire extends mainly parallel to the direction of travel of the electrons, and a foremost coil end 9 and a rearmost coil end 11 where the wire extends substantially at right angles to the said direction of travel. The electrons are deflected substantially exclusively by the magnetic field which is generated in the active parts 5 and 7. The current required for the deflection can be supplied via connection wires 13 and 15 at the area of the rearmost coil end 11. Two turns are provided with tappings 17 and 19 that are also located at the rearmost coil end 11. As a result, the coil is divided into three sections, the first section consisting of the turns between the connection wire 13 and the first tapping 17, the second section consisting of the turns between the two tappings 17 and 19, and the third section consisting of the turns between the second tapping 19 and the connection wire 15. Connected in parallel to each of the three sections is a circuit 21 which has an adjustable impedance. Part of the deflection current which is applied to the coil via the supply wires 13 and 15 passes through a coil section while the remainder passes through the parallel circuit 21 which is associated with this section. If the impedance of the parallel circuit 21 is reduced, a larger part of the deflection current will pass through the parallel circuit. This has the same effect on the deflection of the electron beam as a reduction of the number of turns in the relevant section. In this manner continuous variation of the number of turns per section and hence of the turn distribution in the coil is possible.

FIG. 2 is a diagrammatic cross-sectional view of an angular ferromagnetic core 23 which surrounds a pair of saddle-coils, only the active parts 5 and 7 and 5' and 7' thereof being visible in FIG. 2. The coils are arranged with respect to each other such that the connection line between the two windows 3 and 3' extends horizontally and intersects the axis of the core 23. The magnetic field which generates a current which flows through these coils causes a vertical deflection of an electron beam (not shown) travelling at right angles to the plane of the drawing. The deflection direction is denoted in the FIGS. 2 to 5 by a double arrow 25. For the deflection in the horizontal direction, a complete deflection coil system is also provided with a second pair of saddle coils (not shown for the sake of clarity), which are arranged to be coaxial with the first pair and which have been turned through an angle of 90° with respect to the first pair so that the connection line between the windows of the second coil pair extends in the vertical direction.

The effect of the magnetic field which is generated by each turn of the coil on the deflection of the electrons is dependent on the location of this turn. This location is determined by measuring, in a plane perpendicular to the direction of travel of the electron beam, for example, the plane of the drawing in the FIGS. 2 to 5, the angle between the deflection direction of the coil on the one side and the connection line from the axis of the core 23 to the relevant turn on the other side.

FIG. 2 shows the angular distance α _o of the turns which are arranged nearest to the deflection direction, and the angular distance α _N of the turns which are arranged furthest from the deflection direction.

The mean angular distance α _g can be calculated from the angular distances of the turns by means of the formula: ##SPC2##

Therein, n(α ) represents the number of turns the angular distance of which amounts to α _n . The turn whose angular distance is equal to α _g is called the mean turn. In FIG. 2, this mean turn is denoted by 27 for the right-hand coil, and by 27' for the left-hand coil.

FIG. 3 is a cross-sectional view of a toroidal ferromagnetic core 23 which corresponds to FIG. 2. However, in this case the deflection coils consist of four coils 29, 31, 33 and 35 which are toroidally wound about the core. The various angular distances α _n can be determined with reference to FIG. 3 in the same manner as for saddle-coils with reference to the FIG. 2. The calculation of the mean angular distance α _g is also the same. The mean turns are denoted in FIG. 3 by 37, 39, 41 and 43.

In FIG. 4 a subdivision into sections of the saddle coils of FIG. 2 is shown diagrammatically. By means of four tappings (not shown) each of the two coils is divided into five sections 45, 47, 49, 51, 53 and 45', 47', 49', 51', 53', respectively. Each of these sections can be provided with a parallel circuit (not shown in FIG. 4). However, it appears to be particularly advantageous to provide one or more of the sections 45, 49 and 53 with a parallel circuit. The parallel circuit of the first section 45 and of the last section 53 can be utilized to influence the location of the mean turn 27. By reducing the impedance of the parallel circuit of section 45, the current flowing through this section is reduced. As regards the magnetic field which is generated by the coil this has the same effect as a reduction of the number of turns in this section and hence of the factors n(α) in the formula for α _g .

As these factors remain the same in the other sections, it is obvious that the value α _g increases as a result thereof. This means that the location of the mean turn is shifted in the direction of the last section 53. Similarly, a reduction of an impedance which is connected in parallel with section 53 will cause a shift of the mean turn 27 in the direction of the first section 45. The location of the mean turn 27 is of importance for the convergence on the entire display screen, whilst the convergence on the axes of the display screen is even determined substantially exclusively by this location. An optimum adjustment of the latter convergence can thus be obtained in the manner described above.

The convergence in the corners of the display screen is also dependent of the location of the mean turn 27. However, this convergence is also dependent of the manner in which the turns are distributed over the width of the coil. By influencing this distribution without changing the location of the mean turn 27 to any significant extent, optimum adjustment can be achieved of the convergence in the corners of the display screen without the already adjusted convergence on the axes being disturbed. To this end the section 49, extending on both sides of the mean turn, is also provided with a parallel circuit. If the impedance of this parallel circuit is varied, the factors n(α) in the formula for cos α _g vary for values of α _n which are slightly larger or smaller than α _g . The overall effect of these variations on the value of α _g is very slight, so that the location of the mean turn remains substantially the same. The turn distribution over the width of the coil, however, does change which can be clearly demonstrated by completely short-circuiting section 49. In that case the section no longer contributes to the magnetic field generated by the coil so that at this area an interruption is produced in the turns as if it were.

In a practical embodiment of saddle coils for a colour television display tube comprising three electron guns arranged on one line and having a deflection angle of 110°, each of the coils comprising 100 turns, each coil was divided by means of tappings such that each of the sections 45, 49 and 53 comprises ten turns. By providing these sections with parallel circuits and by varying the impedance thereof from zero to a value which is very large with respect to the impedance of each of the sections, it was found to be possible to correct convergence errors of 3 mm on the axes and of 5 mm in the corners of the display screen of the tube.

During the winding of the coil it can be ensured that the mean turn 27 of the uncorrected coil is always shifted in one direction with respect to the desired location. In that case the mean turn 27 must always be displaced in the same direction upon correction, so that for this purpose only section 45 or only section 53 must be provided with a parallel circuit. That one of these two sections which is not used need not even exist as a separate section in that case, so that the relevant tapping can also be omitted.

So as to achieve a saving as regards the number of tappings, the sections 47 and 51 which are not provided with a parallel circuit can also be omitted. In that case the situation shown in FIG. 5 is obtained where the coil, in accordance with FIG. 1, is divided into only three sections 45, 49 and 53.

In the simplest case the coil is wound such that the mean turn is always shifted in the direction of, for example, section 53. In that case only one tapping is made, i.e. the tapping which limits section 53, and an impedance is connected parallel to section 53. Subsequently, the convergence in the corners and on the axes of the display screen is simultaneously adjusted by variation of this impedance. Obviously, this is possible only if this combined convergence adjustment constituted an acceptable compromise.

Using FIG. 2 as a basis, the division into sections has been described for saddle coils with reference to the FIGS. 4 and 5. It is obvious that on the basis of FIG. 3 analogous figures for toroidal coils can be drawn which are subject to the same considerations. It is also possible to incorporate in one deflection coil system saddle coils for the deflection in the one direction and toroidal coils for the deflection in the other direction. This does not affect the above-mentioned considerations either.

The circuits which are connected parallel to the various sections preferably consist of variable resistors for the vertical deflection coils which are supplied with a deflection voltage of low frequency. For the horizontal deflection coils which are supplied with a deflection voltage of a much higher frequency, coils having a variable inductance are preferably used as the parallel circuit.

FIGS. 6 and 7 show some examples of circuits which can be used for deflection coil systems according to the invention.

FIG. 6a shows a pair of saddle-shaped horizontal deflection coils which are divided into sections 45, 47, 49, 51 and 53 and 45', 47', 49', 61' and 53', respectively. The coils are connected in series via a variable coil 55 which serves for compensation of mutual differences, and they are earthed in the centre of the series network. They are supplied from a known sawtooth generator 57. Connected in parallel to each of the sections 45, 49, 53, 45', 49' and 53' is a coil having a variable inductance. These coils are denoted by 59, 61, 63, 59', 61' and 63', respectively.

FIG. 6b shows an analogous circuit for saddle-shaped vertical deflection coils. The sections are numbered in the same manner as in FIG. 6a. Connected in parallel with each of the sections 45, 49, 53, 45', 49' and 53' is a variable resistor. These resistors are denoted by 65, 67, 69, 65', 67' and 69', respectively. The assembly is supplied from a known sawtooth generator 71. The voltage at the area of the centre of the series network can be adjusted by means of a potentiometer 73 which constitutes, on conjunction with the two fixed resistors 75 and 77, a voltage divider which is connected in parallel with the series network.

It appears from FIGS. 6a and 6b that, if the two deflection coils for one deflection direction are connected in series, a separate variable resistor or coil is required for each section provided with a parallel circuit. It appears that the number of variable resistors and coils can be reduced to one half by parallel connection of the deflection coils. This not only represents a saving as regards the number of components required for the circuit, but also the number of operations to be performed for adjustment is reduced.

FIGS. 7a and b show how such a circuit can be constructed for the horizontal and the vertical deflection coils, respectively. In both Figures each coil is again divided into five sections, the numbering of which corresponds to that in FIGS. 6a and b. Sections forming part of different coils are pairwise connected in parallel. As a result, the following pairs of sections are obtained: 45 and 53', 47 and 51', 49 and 49', 51 and 47', 53 and 45'. FIG. 7a shows that a coil having a variable inductance 79 is connected parallel to the first pair of sections 45, 53' of the horizontal deflection coils. Similarly, a variable coil 81 is connected parallel to the third pair of sections 49, 49', and a variable coil 83 is connected parallel to the fifth pair 53, 45'. The complete parallel network is again supplied from the sawtooth generator 57 via a variable coil 85 for compensating differences between the two deflection coils.

FIG. 7b shows that a variable resistor 87 is connected parallel to the first pair of sections 45, 53' of the vertical deflection coils, a variable resistor 89 being connected parallel to the third pair 49, 49', and a variable resistor 91 being connected parallel to the fifth pair 53, 45'. Via a variable resistor 93 for the elimination of irregularities in the two deflection coils, the complete parallel network is again supplied by the sawtooth generator 71.

On the basis of the same considerations as described with reference to FIGS. 6 and 7 for saddle coils, it is also possible to demonstrate the desirability of pairwise parallel connection of sections forming part of different coils of the toroidal type.

PHILIPS Device for radial, static and dynamic convergence of electron beams in a colour television display tube with CRT DELTA GUN:

Thus far a substantial amount of energy was required for the two coils for the dynamic line frequency convergence in a device for radial, static and dynamic convergence. So as to improve this situation, a magnetic bridge is arranged between the two legs of a U-shaped electromagnet forming part of the convergence device, the magnetic resistance of this bridge being comparatively small in the magnetic circuit of the coils for the dynamic line frequency convergence and comparatively large in the magnetic circuit of the coils for the static and field frequency convergence.

1. A television convergence device comprising a substantially U shaped ferromagnetic core have two substantially parallel legs and a yoke coupled to said legs; a pair of static convergence coils disposed about said legs respectively proximate said yoke; a pair of dynamic line frequency convergence coils disposed about said legs respectively proximate the free ends thereof; and means for providing a small magnetic resistance with respect to the line frequency magnetic circuit and a large resistance with respect to the internal magnetic resistance of the static magnetic circuit, said providing means comprising a ferro magnetic bridge disposed proximate said two legs and between said static coils on one side thereof and said dynamic coils on the other side thereof. 2. A device as claimed in claim 1 wherein said bridge comprises a rectangular soft iron body disposed between said legs substantially parallel to said yoke and defining an air gap between at least one of said legs and the nearest end of said body. 3. A device as claimed in claim 1 wherein said bridge comprises a rectangular soft iron body disposed adjacent said legs substantially parallel to said yoke and having a length at least equal to the distance between said legs and defining at least one air gap between said body and said core. 4. A device as claimed in claim 1 wherein said bridge comprises an adjustable bridge. 5. A device as claimed in claim 1 wherein said static convergence coil also comprises a field frequency dynamic convergence coil. 6. A device as claimed in claim 1 wherein said line frequency convergence coil also comprises a field frequency convergence coil. 7. A device as claimed in claim 1 wherein said bridge comprises a magnetically soft material.

Description:

The invention relates to a device for radial, static and dynamic convergence of electron beams in a colour television display tube, adapted to be mounted about the neck of the display tube approximately at the level of the foremost end of the electron guns provided in the display tube for generating the electron beams, the said device comprising U-shaped electromagnets, each of which is capable of generating a magnetic field in the area of one of the electron beams via associated pole shoes provided in the display tube, variation of said field causing a radial displacement of the relevant electron beam, i.e. towards the axis of the display tube or away from that axis, said electromagnets consisting of a ferromagnetic U-shaped core having two substantially parallel legs which are connected by a yoke, each of said legs being enveloped by a coil, provided near the yoke, for static and possibly field frequency dynamic convergence, and also by a coil, provided near the free end of the leg, for dynamic line frequency and possibly field frequency convergence.

The German Auslegeschrift 1,290,174 describes which different convergence devices are required for correctly positioning the electron beams in a colour television display tube. The present invention relates to the device for the radial, static and dynamic convergence as set forth. In devices commonly used thus far, the magnetic resistance in the magnetic circuit of both coils for the dynamic line frequency convergence was found to be very large. As a result, a large magnetic energy (apparent power) is required for an effective adjustment of the convergence by means of these two coils. This is because the magnetic circuit for these two coils is mainly closed only by the stray field between the two legs of the U-shaped core.

In order to eliminate this drawback, it is known to construct the legs of the U-shaped core to be longer, so that the relevant magnetic circuit is somewhat improved in that the magnetic stray flux behind the coils is increased. However, this step also increases the stray field in the magnetic circuit for the two coils for the static and field frequency convergence, which also increases the cross-talk between the magnetic circuits associated with different electron guns.

The invention has for its object to provide a substantial reduction of the largest resistance in the magnetic circuit of the coils for the dynamic line frequency convergence, without the magnetic circuit of the coils for the static and field frequency convergence being essentially affected, and without the cross-talk being increased.

In order to achieve this object, the device according to the invention is characterized in that a magnetic bridge is provided between the portions of the two legs which are bound by the coils for the static and field frequency convergence on the one side and by the coils for the line frequency convergence on the other side, said bridge forming a comparatively small magnetic resistance in the magnetic circuit of the coils for the dynamic line frequency convergence, but having a comparatively large magnetic resistance with respect to the internal magnetic resistance of the coil system for the static and field frequency convergence.

To this end, the magnetic bridge may consist of an oblong soft-iron body which is provided between the two legs of the core, approximately parallel to the yoke, an air gap being present between at least one of the ends of the body and the nearest leg.

The magnetic bridge may also consist of at least one oblong soft-iron body which is provided adjacent to the two legs, approximately parallel to the yoke, the length of the body being at least equal to the distance between the two legs, at least one air gap being present between the body and the core. In both cases the position of the soft-iron body is preferably adjustable.

An advantage of the device according to the invention is that, due to the reduction of the magnetic resistance in the magnetic circuit of the coils for the dynamic line frequency convergence, the sensitivity of these coils is substantially increased. For the same magnetic deflection of an electron beam, now only half the magnetic energy (apparent power) is required. Consequently, the steps to be taken in the associated electronic circuit are substantially simplified and use can possibly be made of components with a lower permissible load. Due to the increased sensitivity of the coils on the ends of the legs of the U-core, used thus far only for the line frequency radial convergence, these coils can also be used for the field frequency radial convergence, for which purpose a current, obtained by additive combining of line and field frequency convergence currents, is fed through these coils, for example, each time from a transistor output stage for the red, the green and the blue radial convergence system.

In the foregoing some preferred constructions for the magnetic bridge were described. Within the scope of the invention, however, bridge bodies can also be used which are made, for example, of a mixture of soft-iron and a synthetic resin material.

The adjustability of the bridge body offers, for example, the possibility of varying the inductance of the coils for the static and field frequency convergence, the resistance of the magnetic circuit for line frequency convergence remaining substantially the same, so that in given circuits adjustment of the desired shape of the (usually more or less parabolic, 50-Hz) convergence current is possible since the relation between inductance and resistance determines the shape of the curve of, for example, a left-hand parabolic branch which decreases exponentially. When these steps are taken, however, it is to be taken into account that the magnetic resistance in the magnetic circuit for the coils of the static and field frequency convergence must remain large at the area of the bridge in relation to the internal magnetic resistance of this coil system, i.e. that the magnetic bridge resistance between the two U-shaped legs must be large with respect to the magnetic resistance which is formed by the two legs and the yoke of the U-shaped core of the electromagnet.

In order that the invention may be readily carried into effect, some embodiments thereof will now be described in detail, by way of example, with reference to the accompanying diagrammatic drawings, in which:

FIG. 1 shows an electromagnet for a device according to the invention, having a soft-iron body between the two legs of the U-shaped core of the electromagnet,

FIG. 2 shows an electromagnet for another device according to the invention, having a soft-iron bridge body which is provided adjacent to the two legs of the U-shaped core of the electromagnet,

FIG. 3 shows an electromagnet resembling that shown in FIG. 2 and having two soft-iron bridge bodies arranged adjacent to the two legs of the core,

FIG. 4 shows the relative position of three U-shaped cores of electromagnets which together form a convergence unit for the radial, static and dynamic convergence, in conjunction with the associated circuit for the static convergence, and

FIG. 5 shows a transistor output stage for supplying convergence currents to a pair of coils.

FIG. 1 shows an electromagnet having a U-shaped core, consisting of a yoke 2 and two parallel legs 3 and 4 which are enveloped in the vicinity of the yoke 2 by coils 5 and 6 for static convergence, it being possible to use said coils also for field frequency dynamic convergence for which purpose they are than supplied with parabolic currents having a frequency of 50 Hz. In the vicinity of the ends 7 and 8 of the legs 3 and 4, coils 9 and 10 are arranged about the legs for the dynamic line frequency convergence. If desired, it is also possible to supply the 50-Hz parabolic currents for the field frequency convergence via these two coils 9 and 10. Situated between the two legs 3 and 4 is a magnetic bridge consisting of an oblong soft-iron body 11. For adjusting purposes, this body is preferably arranged to be pivotable about a pivot 12. If desired, it may also be arranged to be displaceable in its longitudinal direction. Provided between the body 11 and the leg 3 is an air gap 13, a similar air gap being also present between the leg 4 and the body 11. The magnetic circuit 14 (denoted by a broken line) of the coils 9 and 10 is closed over an important portion by the body 11, because this body 11 reduces (at the area where it is used) the magnetic tension drop which would occur if this body were not provided.

The flux generated by the coils 5 and 6, denoted in FIG. 1 by broken lines at 15, flows partly through the body 11 which also acts as a magnetic shunt for this flux, and partly to the area of an electron beam (not shown) via the ends 7 and 8 of the legs 3 and 4. The magnetic resistance which is formed by the body 11 and the two air gaps 13 is large with respect to the magnetic resistance in the path along the leg 4, the yoke 2 and the leg 3, so that the magnetic field generated by the coils 5 and 6 for static and field frequency convergence is still strong enough at the ends of the legs.

The drawbacks of the devices known thus far were that the magnetic flux, generated due to the current through the coils 9 and 10, was also embraced by the coils 5 and 6. As a result, in accordance with Lenz's law a counter-field was formed which had the effect that, due to the very low frequency of the current in the coils 5 and 6 with respect to the line frequency, the magnetic circuit of the coils 9 and 10 was interrupted at this area. Consequently, this circuit was virtually not loaded from a magnetic point of view. The effect was the same as if the legs of the core were cut off before the beginning of the coils 5 and 6, viewed from the coils 9 and 10. Consequently, according to the insight on which the invention is based, the body 11 had to be introduced before these "interruptions" .

FIG. 2 shows an electromagnet which resembles that shown in FIG. 1, but in this case the soft-iron body acting as the magnetic bridge is formed by a bar 19, the length of which is larger than the distance between the two legs 3 and 4 of the U-shaped core 1. The bar 19 is arranged to be adjacent to the legs, parallel to the yoke 2. The magnetic circuits are subject to the same conditions as are applicable for providing the body 11 shown in FIG. 1. The remarks as regards adjustability of the position of the body of FIG. 1 also apply to that shown in FIG. 2.

FIG. 3 is a side view of a U-shaped core 1. In this case two soft-iron bars 19 are provided to form the bridge, the said bars being arranged at a small distance from and adjacent to the legs 3 and 4 and corresponding to the bar 19 shown in FIG. 2.

FIG. 4 is a front view of three electromagnets having U-shaped cores 1, i.e. viewed from the screen side of a colour television display tube, said electromagnets together forming a device for radial convergence. The upper electromagnet is for the colour blue (B), the right-hand magnet is for green (G) and the left-hand electromagnet is for red (R). The deflections for the static convergence and also for the dynamic convergence are in principle effected in the radial direction, as is indicated by the arrows 16. The upper portion of the Figure shows a circuit diagram of a circuit for the static convergence. The circuit is composed of three parallel-connected 470-ohm resistors R ₁ , R ₂ , R ₃ , each resistor having an adjustable tapping so as to supply a direct voltage to the convergence device.

The circuit is special in that the coils 5 and 6 of the U-shaped cores 1 for red and green are divided into the sections 5a, 5b, 6a and 6b. The tapping of the resistor R ₁ is connected to a series-connection of the sections 6a (red), 5a (red), 5a (green) and 6a (green), and the tapping of the resistor R ₂ is connected to a series-connection of the sections 5b (red), 6b (red), 5b (green) and 6b (green). The magnetic circuits of these coil sections extend similarly to those of the coils 5 and 6. The connection of the coils shown here offers the advantage that the convergence can be very readily adjusted. When using an orthogonal raster pattern on the display screen for the convergence adjustment, an apparent horizontal and vertical mutual adjustment of the red and the green raster pattern is observed instead of radial electron beam deflections. In order to make the red and the green raster pattern coincide in the vertical direction, the tapping of R ₁ is adjusted such that the current through the sections 6a and 5a of the red system and 5a and 6a of the green system varies. The adjustment in the horizontal direction is effected in an analogous manner by means of R ₂ .

Like the coils 5 and 6 for the static (and field frequency) convergence, the coils 9 and 10 for the line frequency convergence can also be subdivided into sections, For the electrical compensation of the cross-talk, the blue and red convergence units are connected via a 4.7 kohm resistor R ₄ . For the current supply of the coils 9 and 10, it is very advantagous to use an output stage comprising a transistor 20 the coils being connected in the collector lead thereof, (see FIG. 5). An output stage transistor can each time be used for red, green and blue, it being possible to use an output stage of this kind not only for the line frequency convergence but also for the field frequency convergence. To this end, the line frequency and field frequency convergence currents may be added by means of two 220-ohm resistors R ₅ and R ₆ .

METHOD AND APPARATUS FOR STATIC AND DYNAMIC CONVERGENCE IN A DELTA GUN CRT COLOR TUBE:

The three electron beams in a color television cathode ray tube are statically converged at the beginning and end of vertical and horizontal scan lines, and are dynamically converged in the center of the raster. Each external convergence assembly, mounted adjacent a pair of internal pole pieces for a corresponding electron beam, includes a single coil passing both vertical and horizontal convergence currents which have been combined by a semiconductor circuit. The convergence coil for the blue beam is split into two sections which are separately excited to independently control lateral movement on opposite sides of the raster. The convergence method uses active circuits, and simplified passive circuits which require a reduced number of input waveforms from the receiver scanning stages, such as the waveform across the S shaping capacitor.

1. In a color television receiver having a cathode ray tube with plural electron beams and scanning means for deflecting said plural electron beams to produce scanning lines, a convergence system, comprising: 2. The convergence system of claim 1 wherein said static convergence means statically converges said plural electron beams at opposite edges of said scanning lines, said dynamic convergence means causes said plural electron beams when not coincident to have an arcuate path with a maximum deviation from a straight path in the vicinity of said center portion. 3. The convergence system of claim 2 wherein said adjustable means includes vertical line adjustable means for laterally moving vertical scanning lines and horizontal line adjustable means for laterally moving horizontal scanning lines, the pair of line adjustable means producing maximum correction deflection during said center portion of scan and progressively lesser amounts of correction deflection in opposite directions away from said center portion of scan. 4. The convergence system of claim 1 wherein said scanning means comprises vertical deflection means for producing vertical signals synchronized with the vertical deflection of said electron beams and horizontal deflection means for producing horizontal signals synchronized with the horizontal deflection of said electron beams, said dynamic convergence means including vertical generator means having an input coupled to said vertical deflection means for converting the vertical signals into a generally parabolic waveform having a maximum deviation at the center of each vertical scanning line and vertical adjustable means for varying the amount of said maximum deviation, horizontal generator means having an input coupled to said horizontal deflection means for converting the horizontal signals into a generally parabolic waveform having a maximum deviation at the center of each horizontal scanning line and horizontal adjustable means for varying the amount of said maximum deviation, and exciter means responsive to said parabolic waveforms for producing corresponding deflections in the scanning lines. 5. The convergence system of claim 4 wherein at least one of said generator means includes a pair of active devices each having an input and an output and connected as amplifiers, input means coupled to the inputs for driving said pair of devices oppositely by a generally sawtooth shaped waveform occurring in synchronism with said synchronized signal, and output means coupled to the outputs of said pair of devices for deriving said generally parabolic waveform. 6. The convergence system of claim 5 wherein said amplifiers include capacitive feedback means coupled between the output and the input of said pair of devices to cause said devices to convert the generally sawtooth shaped waveform into the generally parabolic waveform. 7. The convergence system of claim 4 wherein said exciter means comprises a convergence assembly external to said cathode ray tube and adjacent one of said plural electron beams for applying convergence correction thereto, said assembly includes a coil for generating a magnetic flux field which produces said correction, and said dynamic convergence means includes mixer means for combining the generally parabolic waveform from said vertical generator means and the generally parabolic waveform from said horizontal generator means to produce a combined convergence waveform coupled to said coil. 8. The convergence system of claim 1 wherein the cathode ray tube has two internal pole pieces defining an axis equidistant therebetween through which passes one of said plural electron beams, said dynamic convergence means includes core means having a first leg adjacent one of said two pole pieces and a second leg adjacent the other of said two pole pieces, convergence coil means wound about said core means for generating a magnetic flux field which produces the deflection of the scanning lines in the vicinity of the center portion, and unbalance means for producing a more concentrated magnetic flux field in one of said legs to cause said electron beam to be deflected at a skew with respect to the equidistant axis. 9. The convergence system of claim 8 wherein said unbalance means comprises means mounting said coil means about only one of said first and second legs. 10. The convergence system of claim 1 wherein said dynamic convergence means includes initial convergence means for producing an initial correction waveform occurring during an initial portion of a scanning period and final convergence means for producing a final correction waveform occurring during a final portion of a scanning period, a convergence coil for one of said electron beams having a first winding section and a second winding section, initial adjustable means coupled to said initial convergence means for adjustably dividing said final correction waveform between said first and second winding sections. 11. The convergence system of claim 10 wherein each of said adjustable means comprises a potentiometer having a pair of end terminals with a fixed resistance located therebetween and a wiper movable across said fixed resistance, means separately coupling the end terminals to said first and second winding sections, and said wiper being coupled to the corresponding convergence means. 12. In a color television receiver having a cathode ray tube with at least one electron beam which is angularly deflected both vertically and horizontally by vertical deflection means and horizontal deflection means, respectively, a dynamic convergence system, comprising: 13. The dynamic convergence system of claim 12 wherein said combining means includes a summing junction for combining the horizontal and vertical correction signals, and active means coupled between said summing junction and said single coil. 14. The dynamic convergence system of claim 12 in which said cathode ray tube includes a second electron beam which is angularly deflected both vertically and horizontally by said vertical deflection means and said horizontal deflection means, respectively, a second convergence assembly external to said cathode ray tube and adjacent said second electron beam and including second core means having a pair of second core legs and a second single coil wound about only one of said pair of second core legs for generating a magnetic flux field which produces convergence correction for said second electron beam, said combining means includes a summing junction for combining the horizontal and vertical correction signals, and divider means coupled between said summing junction and said first and second coils for adjusting the ratio of the combined signals which are impressed on said first and second coils. 15. The dynamic convergence system of claim 14 wherein said divider means comprises potentiometer means having a fixed resistance between first and second end terminals and a wiper movable across said fixed resistance, said first end terminal being coupled to said first coil and said second end terminal being coupled to said second coil, and said wiper being coupled to said summing junction whereby the position of said wiper varies said ratio. 16. The dynamic convergence system of claim 12 wherein said horizontal correction means comprises parabolic generator means for generating a generally parabolic horizontal correction signal having a maximum deviation at the center of the horizontal scanning period, said vertical correction means comprises a parabolic generator means for generating a generally parabolic vertical correction signal having a maximum deviation at the center of the vertical scanning period, and said combining means is responsive to the horizontal and vertical parabolic signals for producing dynamic convergence correction which is adjustable for the center portion of each scanning period and fixed at the beginning and end of each scanning period. 17. In a color television receiver having a cathode ray tube with at least one electron beam passing between two internal pole pieces and scanning means for deflecting said electron beam to produce scanning lines, a dynamic convergence system, comprising: 18. The dynamic convergence system of claim 17 wherein said scanning means deflects an electron beam during an initial scanning period and a final scanning period to produce a single scanning line, said generator means produces an initial convergence waveform coincident with said initial scanning period and a final convergence waveform coincident with said final scanning period, said adjustable means includes initial adjustable means for selectively varying the division of said initial convergence waveform between said first and second coil sections and a final adjustable means for selectively varying the division of said final correction waveform between said first and second coil sections. 19. The dynamic convergence system of claim 17 wherein said adjustable means comprises a potentiometer having a pair of end terminals with a fixed resistance therebetween and a wiper movable across said fixed resistance, one end terminal being coupled to one side of said first coil section, the other end terminal being coupled to one side of said second coil section, the opposite sides of said first and second coil sections being coupled to a reference source, and said wiper being coupled to said generator means. 20. The dynamic convergence system of claim 17 wherein said scanning means comprises horizontal scanning means and vertical scanning means, said generator means being coupled to one of said horizontal and vertical scanning means to cause said adjustable means to selectively divide the correction waveform corresponding to said one scanning means, second generator means coupled to the remaining one of said horizontal and vertical scanning means for generating a corresponding convergence correction waveform, and combining means for causing both horizontal and vertical convergence waveforms to flow through said first and second coil sections. 21. In a color television receiver having a cathode ray tube with at least one electron beam which is horizontally deflected by horizontal scan current through a horizontal yoke winding, and an "S" shaping capacitor in series with said horizontal yoke winding to cause the horizontal scan current to flow through both the horizontal yoke winding and the "S" shaping capacitor, a dynamic convergence system, comprising: 22. The dynamic convergence system of claim 21 wherein said horizontal correction means develops said horizontal correction signal with a maximum deviation at the center of the horizontal scanning period, and adjustment means for varying said maximum deviation to dynamically adjust the center portion of each scanning period while statically adjusting the ends of each scanning period. 23. The dynamic convergence system of claim 21 wherein said cathode ray tube includes three electron beams, three convergence assemblies including three coil means each associated with a different one of said three electron beams, said horizontal correction means includes circuit means locating one of said three coil means in series with a parallel combination of the remaining two of said three coil means. 24. The dynamic convergence system of claim 23 wherein said horizontal correction means includes potentiometer means having a fixed resistance and a movable wiper, the ends of said fixed resistance being coupled to said remaining two coil means which are in parallel, and the wiper being coupled to said one coil means in series. 25. The dynamic convergence system of claim 21 wherein said cathode ray tube includes three electron beams, three convergence assemblies including three coil means each associated with a different one of said three electron beams, said horizontal correction means includes circuit means locating said three coil means in series with said "S" shaping capacitor. 26. The dynamic convergence system of claim 25 wherein said horizontal correction means includes potentiometer means having a fixed resistance and a movable wiper, the ends of said fixed resistance being coupled across two of said series connected coil means, and the wiper being coupled to a junction between said two series connected coil means.

Description:

BACKGROUND OF THE INVENTION

This invention relates to an improved method and apparatus for statically and dynamically converging the electron beams of a cathode ray tube in a color television receiver.

In prior convergence systems for television receivers, the convergence procedure has consisted of two parts. First, permanent magnets associated with the convergence exciter assemblies are pre-set to statically converge the red (R), blue (B), and green (G) beams in the center of the picture tube. Following static convergence at the center of the picture tube, the currents and coils associated with the convergence exciter assemblies are adjusted to cause the R, B and G beams to coincide at the edges of the picture tube.

Dynamic convergence correction is thus required at the beginning and end of the horizontal and vertical scan periods. Thus, the maximum amplitude of convergence correction current occurs at the edges of the CRT screen. Since the convergence yoke is a current integrator, and the rate of change of current level with time causes a deterioration in the current waveform, this prior approach creates problems. Since the convergence current typically increases at a parabolic rate, the integration effect degrades convergence performance. Furthermore, the convergence exciter assembly must also perform as a transducer in that it must generate a magnetic field proportional to the drive currents in its exciter coils. This creates a considerable problem since a ferrite core is a poor magnetic conductor at high flux densities which are required to deflect the electron beam for proper convergence at the edge of the picture tube.

In prior systems, maximum correction for dynamic convergence is required at the edges of the picture tube. If a considerable amount of correction is needed at the right side of the tube, for example, and only a small amount of correction is needed at the left side, the convergence exciter assembly is forced to change radically in output during the retrace period. The memory of the ferrite core combined with the current integration effect of the exciter coil often prevents this rapid change from taking place. In order to make such a rapid change, an extreme amount of driving power may be necessary. This requires not only high currents, but also forces an interrelationship between the convergence controls on opposite sides of the tube. Thus a change in convergence level at the right side of a picture tube may affect the convergence effect on the left side. Such an interrelationship adds many steps to the convergence process since correcting one area may degrade another area.

The prior art has recognized in general that initial beam convergence may be at any desired deflected position, and thus the beams could be initially converged at one corner of the raster. Such a general recognition is contained, for example, in U.S. Pat. No. 3,048,740 to Nelson, issued Aug. 7, 1962, and entitled "Electron Beam Convergence Apparatus." However, no apparatus has been provided for effecting static convergence at the edges, and dynamic convergence at the center of a scan, nor has the unexpected advantages of such a method of convergence been recognized, as discussed in the following section.

Generally, the convergence exciter assembly for each beam has included a horizontal convergence coil, and a separate vertical convergence coil which usually was of a greater number of turns. A parabolic or similar non-linear current signal for coupling to the horizontal convergence coil is developed by a horizontal convergence circuit, and similarly a vertical convergence circuit develops a parabolic or similar non-linear current for coupling to the separate vertical convergence coil. In addition to requiring two pairs of coils for each convergence exciter assembly, this arrangement has the further disadvantage of increasing the length of the ferrite core, thereby increasing the overall dimensions of the convergence yoke assembly. The physical size of the yoke is an important consideration in commercial television receivers since the amount of space available for CRT neck components is of a limited nature. The size of the ferrite core is also controlled by the amount of power necessary to effect convergence. Both of these considerations have resulted in a convergence yoke assembly of substantial size.

Independent lateral adjustment of the blue beam over the entire length of the raster has not been possible. In a typical convergence procedure, a single dynamic control adjusts the lateral positions of the blue vertical lines. If the vertical blue lines are outside of the converged red and green vertical lines, which condition is known as a wide blue field, then a known adjustment moves the blue lines inward on both sides of the raster in order to converge with the red and green vertical lines. Conversely, if the blue vertical lines are inside the converged red and green vertical lines, which condition is known as a narrow blue field, a known adjustment laterally moves the lines into coincidence. However, if one half of the raster has a wide blue field while the other half exhibits a narrow blue field, it has been necessary to reject the cathode ray tube or deflection yoke since known convergence apparatus have not been capable of providing this type of correction.

SUMMARY OF THE INVENTION

In accordance with the present invention, the problems noted above with prior convergence methods and apparatus have been overcome. Initial static convergence correction for both the vertical and horizontal scanning lines is effectuated at the edges of the picture tube. Thereafter, dynamic convergence correction is applied to cause the beams to coincide in the center of the picture tube. Using this method, the convergence exciter assembly output is never forced to make a radical change during the retrace period. Also, with dynamic convergence applied in the center of the picture tube, a considerable time period is provided for the correction current to build up in the exciter coils. As a result, the convergence system requires much lower energy levels. A further advantage is the elimination of interaction between convergence controls for the right and left side of the picture tube, simplifying the convergence procedure.

Unlike many prior convergence systems, it is unnecessary to utilize in the exciter assembly separate coils for the vertical and horizontal convergence functions. A reduced number of parabolic or similar non-linear waveform generators supply currents to matrixing circuitry which feed a single combined output to a common coil associated with each color beam, which common coil can be mounted on only one of two core legs of a generally U-shaped convergence core. The circuitry includes separate excitation and adjustment for two sections of the blue coil, allowing independent lateral control over the beginning and end of a horizontal scanning period. Thus, a CRT and deflection yoke which exhibits a wide blue field on one half of the picture tube, and a narrow blue field on the other half, can be corrected. The circuitry necessary to implement the present invention can be either active or passive.

A comparison between the applicant's convergence method and apparatus and prior convergence systems reveals a substantial savings in components and a substantial simplification in the convergence procedure. For example, for a prior color television receiver which used a conventional convergence system, 40 steps were required in the alignment procedure. With the applicant's present design, only 18 steps were required. A substantial savings can also be effected in the design of a convergence exciter assembly. A prior design required a total of 12 coils and 12 bobbins, whereas the present apparatus permits a total of four coils and four bobbins. This represents a significant savings in material and labor costs, and also significantly reduces the physical size of the convergence yoke.

One object of the present invention is the provision of an improved convergence method and apparatus for statically converging electron beams at the edges of a scan line and for dynamically converging the beams at the center of scan.

Another object of the present invention is the provision of a convergence exciter assembly which includes a single coil on a single core leg of a U-shaped core assembly, which coil is coupled to matrixing circuitry which combines horizontal correction current and vertical correction current.

A further object of the present invention is the provision of independent convergence controls for separately correcting the lateral position of a vertical line for initial and final horizontal scanning periods, allowing for example a CRT and deflection yoke having a wide blue field and a narrow blue field on opposite halves of a raster to be corrected.

Yet another object of this invention is the provision of improved convergence methods and apparatus which are applicable to both active and passive circuits, and which reduce the drive requirements and the number of waveform generators which must be provided. The input waveform can be taken from across the S shaping capacitor in series with the yoke coil.

Further features and advantages of the invention will be apparent from the following description and from the drawings. While illustrative embodiments of the invention are shown in the drawings and will be described in detail herein, the invention is susceptible of embodiment in many different forms and it should be understood that the present disclosure is to be considered as an exemplification of the principles of the invention and is not intended to limit the invention to the embodiments illustrated.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of one embodiment of the applicant's convergence apparatus as incorporated in a conventional color television receiver;

FIG. 2 illustrates the face of a picture tube after vertical and horizontal static convergence has been completed, and depicts the dynamic vertical correction provided by the present invention;

FIG. 3 shows the face of the picture tube of FIG. 2 and depicts the dynamic horizontal correction provided by the present invention;

FIG. 4 illustrates the face of a picture tube after vertical and horizontal static convergence has been completed, and depicts the dynamic vertical correction provided heretofore by the prior art;

FIG. 5 shows the face of the picture tube of FIG. 4 and depicts the dynamic horizontal correction provided heretofore by the prior art;

FIG. 6 is a schematic diagram showing the circuit of FIG. 1 in detail;

FIG. 7 illustrates the face of a picture tube during dynamic vertical correction for a wide blue field occurring in the initial horizontal scan period and a narrow blue field occurring in the final horizontal scan period;

FIG. 8 is a schematic diagram of a modified portion of the circuit of FIG. 6, and illustrates a simplified convergence circuit for the blue color beam;

FIG. 9 is a schematic diagram of a passive convergence system in accordance with the present invention;

FIG. 10 is a schematic diagram of another embodiment of a passive convergence system in accordance with the present invention;

FIG. 11 is a schematic diagram of a horizontal convergence section and showing a novel source of waveform, consisting of the S shaping capacitor, for supplying current to the convergence section;

FIG. 12 is a schematic diagram of a horizontal convergence section incorporating a series-parallel connection for the convergence coils;

FIG. 13 is a schematic diagram of a horizontal convergence section incorporating a series connection for the convergence coils; and

FIG. 14 is a schematic diagram of a simplified horizontal convergence section made possible by optimum selection of yoke, picture tube and neck component parameters.

GENERAL OPERATION

FIG. 1 in conjunction with FIGS. 2 and 3 shows in an otherwise conventional color television receiver, the applicant's static and dynamic convergence method and apparatus for converging the plural color beams of a trigun color cathode ray tube (CRT) 20. The CRT has three identical gun structures for generating three electron beams which impinge blue (B), green (G) and red (R) phosphors on the face of the CRT. For convenience, similar structures will be identified by the same reference numeral, followed by the designation B, G or R to indicate whether the same is associated with the blue, green or red electron beam guns, respectively.

Within the glass tube envelope, a magnetic shield divides the neck of the CRT into three compartments associated with the B, G and R electron guns. In the absence of a convergence magnetic flux field, the beam travels undeflected along an axis perpendicular to the plane of FIG. 1 and equidistance from spaced internal pole pieces 23, 24 which each are of generally L-shaped cross-section. Associated with each pair of internal pole pieces 23, 24 is a convergence exciter assembly 26 mounted external to the CRT.

To adjust for static convergence, a permanent magnet 28 spans an air gap between a pair of abutting L-shaped legs 30 and 31 which form a U-shaped ferrite core. The permanent magnet 28 may be rotated, or otherwise suitably moved with respect to the orientation of its magnetic North and South fields, in order to alter the direction and magnitude of the steady magnetic flux field which circulates through the legs 30, 31.

To effect dynamic convergence, each of the exciter assemblies 26 includes a coil which is coupled to the convergence circuitry, to be explained. Dynamic convergence of the B electron beam is effected by a coil formed of two identical coil sections 35 and 36 wound on the legs 30B and 31B, respectively. Dynamic vertical and horizontal convergence of the R beam is effected by a single coil 38, wound on the leg 31R of the red assembly 26R. Finally, dynamic vertical and horizontal G beam is provided by a single coil 39 wound on the leg 30G of the green assembly 26G.

The static and dynamic convergence procedure, using the circuit of FIG. 1, may be understood with reference to FIGS. 2 and 3 which show a face 42 of the CRT 20. The corresponding convergence procedure of prior apparatus is demonstrated by FIGS. 4 and 5. In the applicant's convergence method, the static convergence magnets 28B, 28R and 28G are pre-set such that the R, B and G scanning lines converge at the edges of the face of the picture tube, namely, at the beginning and end of vertical scan (FIG. 2) which occurs at the center of the horizontal period, and the beginning and end of horizontal scan (FIG. 3) which occurs at the center of the vertical period. Following static convergence at the edges of the picture tube, the currents in coil 38 of the red assembly 26R and coil 39 of the green assembly 26G are adjusted to vary the R and G vertical lines until they coincide. The maximum extent of convergence deflection occurs in the center of the picture tube, as represented by arrow 44 in FIG. 2. Next, the currents in coils 35, 36 of the blue assembly 26B are adjusted such that the B horizontal line is caused to be coincident with the G and R horizontal lines. The maximum extent of deflection of the B horizontal line occurs along the center of the face of the CRT, as represented by arrow 46 of FIG. 3.

By statically converging the edges of the scanning lines, and dynamically converging the center of the scanning lines, the driving currents in the convergence exciter assemblies 26 are never forced to make a radical change or shift during the retrace period. With dynamic convergence applied in the center of the picture tube, as illustrated, there is a considerable time period for the correction current to build up in the exciter coils, resulting in a lower energy level requirement. Also, the convergence procedure is simpler due to the independence of the convergence controls in the various quadrants of the picture tube.

The above convergence procedure should be contrasted with the prior convergence procedure, as illustrated in FIGS. 4 and 5. First the permanent magnets associated with the convergence exciter assembly are pre-set during a static convergence phase to cause the R, B and G beams to converge in the center of the picture tube, as shown in FIGS. 4 and 5. Next, the currents in the coils associated with the red and green exciter assemblies are adjusted to cause the R and G vertical lines to coincide at the top and bottom edges of the vertical scanning period, which results in maximum convergence deflection in the vicinity of arrows 48 of FIG. 4. Finally, the coils associated with the blue exciter assembly are fed currents which cause the B horizontal line to coincide with the G and R horizontal lines, creating a maximum convergence deflection along the arrows49 of FIG. 5.

Returning to FIG. 1, the structure for the red assembly 26R and green assembly 26G is not identical with the structure for the blue assembly 26B. The structure of the blue assembly 26B causes the B electron beam to be deflected in a radial direction 51. While radial deflection is desirable for the blue beam, due to the fact that the blue gun structure in a conventional CRT is orientated in a vertical plane, radial deflection of the G and R electron beams would produce vector components in both the vertical and horizontal planes. A vertical component of movement for the R and G beams is frequently unnecessary.

To overcome this problem, the green dynamic convergence assembly 26G and the red dynamic convergence assembly 26R are modified so as to produce a non-symmetrical or unbalanced flux field between the pole pieces 23, 24 associated therewith. This field skews the path of deflection of the associated electron beam, and produces essentially lateral G and R beam movement, as illustrated by the arrows through the G and R beams. To accomplish this result, a single convergence coil is wound over only one leg of the U-shaped core 30, 31. The green convergence coil 39 is wound about core leg 30G, while the red convergence coil 38 is wound on the core leg 31R.

If desired or necessary, the core legs carrying the single coils may abut a pole shoe (not illustrated) sandwiched between the core leg and the glass envelope of the CRT. The net result is to change the magnetic flux pattern produced in the vicinity of the electron beams, because the magnetic flux passing through the core legs 30G and 31R is considerably greater or more concentrated than the flux passing through the opposed core legs 31G and 30R. For a more complete description of such apparatus and the resulting operation, reference should be made to the applicant's before identified co-pending application, Ser. No. 263,632 filed June 16, 1972.

OPERATION OF DYNAMIC CONVERGENCE CIRCUIT

A description will now be given of the dynamic convergence circuitry utilized in conjunction with the applicant's convergence method, which may be characterized as an "inverse" convergence approach. It should be noted that while active circuitry is disclosed, passive type circuitry can also be used, as explained later. In the inverse convergence approach, the main objective is to supply the convergence exciter coils 35, 36, 38 and 39 with both horizontal and vertical parabolic currents synchronized with the television scan circuits. Unlike conventional convergence approaches, it is unnecessary to utilize separate coils for the vertical and horizontal convergence functions, thereby allowing the length of the core legs 30, 31 to be shortened.

Generally speaking, the inverse conversion circuitry requires four parabolic waveform generators. One generator provides the vertical convergence current, and three generators provide the horizontal convergence current. The output from all four waveform generators are combined by appropriate matrixing circuitry fed from output amplifiers.

Considering FIG. 1 in detail, the color television receiver includes a horizontal deflection circuit 60 (shown twice in FIG. 1) and a vertical deflection circuit 62, both of which may be conventional. A horizontal sawtooth waveform 64 from the horizontal deflection circuit 60 is applied to the input terminal of a parabolic waveform generator A1. An inverted version 65, the same sawtooth waveform is applied to the input terminal of a parabolic waveform generator A2. Generator Al provides at its output an initial half 67 of a parabolic waveform, which is coupled to coil 36 in the left channel of the blue exciter assembly 26B. The other half of the dynamic correction parabola is supplied by parabolic waveform generator A2, which generates a terminating half 68 of the parabolic waveform.

The output from generator Al is supplied through a variable resistor 72 to the input of an amplifier A3 which forms a part of the left driving channel for assembly 26B. The output from generator A2 is supplied through a variable resistor 74 to the input of an amplifier A4 located in the right channel. The reason for separate correction currents supplied through separate channels to a split blue convergence coil is to provide separate and independent lateral correction over the beginning and terminating portions of a horizontal scan. This allows a wide blue field/narrow blue field on the face of a CRT to be corrected, as will be explained in detail later.

A vertical frequency sawtooth waveform 76 is supplied to a parabolic waveform generator A5. An inverted version 77 of the sawtooth waveform is also provided to generator A5. The resultant parabolic correction waveform appears at an output terminal, and is supplied to the blue conversion exciter assembly through a variable resistor 80 in series with the wiper 82 of a potentiometer 83 having one end of its fixed resistance coupled to the input of amplifier A3 and the other end coupled to the input of amplifier A4. The position of wiper 82 determines the ratio of the vertical deflection correction which is applied to the left and right channels of the blue exciter assembly. Variable resistor 80 allows the strength or amplitude of the vertical correction signal passing to the blue conversion exciter assembly to be preset. Wiper 82 of potentiometer 83 allows the amount of correction applied to the coils 35 and 36 to be varied as desired.

The vertical correction signal from parabolic generator A5 is also passed through an adjustable resistor 86, a fixed resistor 87, and a wiper 89 of a potentiometer 90 to the red and green exciter assemblies. One end of the fixed resistance forming potentiometer 90 is coupled through an output amplifier A6 to coil 38, whereas the opposite end of the fixed resistance is coupled through an output amplifier A7 to the coil 39. The undriven ends of coils 38 and 39 are coupled to a source of positive DC voltage, or B+. Variable resistor 86 thus controls the amount or level of the vertical correction current which is fed to the red and green convergence exciter assemblies, whereas the position of wiper 89 controls the ratio of vertical correction current which divides between the red and green assemblies.

For horizontal convergence of the R and G electron beams, a parabolic waveform generator A8 has a pair of inputs coupled to a horizontal sawtooth waveform 94 and an inverted form 95 thereof, which may be similar to the waveforms 64 and 65 associated with the blue exciter assembly. The resulting parabolic correction signal from generator 88 is passed through a variable resistor 97 and the fixed resistor 87 to the wiper 89 of potentiometer 90.

A detailed description of the circuitry shown in block form in FIG. 1 will now be given with reference to FIG. 6. The vertical parabolic generator A5 includes amplifier transistors 110 and 111. Voltage from the B+ supply is coupled to each transistor through a common load resistor 113. Transistor 110 is driven into conduction during the initial portion of the vertical sawtooth waveform 76, whereas transistor 111 is driven into conduction during the second half of the vertical scan period, due to the positive going inverted sawtooth 77 which rises above zero volts.

Negative feedback is provided from the collector of the transistors 110, 111 to their respective bases through capacitors 115 and 117, respectively. This results in the generation of a parabolic waveform 120 which is coupled to a buffer amplifier stage 122. The generator A5 is extremely flexible, and allows the shape of the parabolic slope to be modified separately for each half of the vertical scan period by appropriate adjustment of the time constants within the generator A5. Without feedback capacitors 115 and 117, a tiangular waveshape would appear in place of the parabolic waveshape 120. Potentiometers 124 and 126 provide separate adjustment for the top and bottom sections of the CRT screen.

Many variations are possible in the design of the parabolic generator A5. The output shape and amplitude of the waveform 120 is a function of the input DC offset, and the amplitude and linearity of the sawtooth. By driving the transistors 110 and 111 into saturation for various portions of the scan period, the resulting output waveform can be varied between a large number of shaped tailored for the convergence correction which is necessary for a particular CRT 20.

Buffer amplifier 122 provides impedance matching for the output amplifiers. The parabolic correction waveform is fed from potentiometer 80 to the blue output amplifiers A3 and A4 via the balancing potentiometer 83. The blue section is different in operation from the red and green sections since two separate output amplifiers A3 and A4 are used to drive separate sections 35 and 36 of the blue convergence coil. Since the blue beam moves on a vertical axis, due to the geometry of the delta gun configuration of the CRT 20, it is generally desired to provide balanced currents in the output amplifiers A3 and A4, which may be accomplished by appropriate adjustment of potentiometer 83. The vertical parabolic correction signal from buffer amplifier 122 is also applied to the red and green circuitry via the adjustable potentiometer 90.

The red/green horizontal parabolic generator A8 operates in a similar fashion to generator A5. A pair of transistors 130 and 132 are driven from two anti phase horizontal sawtooth waveforms 94 and 95. The two transistors share a common load resistor 134, connected with B+. A pair of negative feedback capacitors 136 and 137 shunt the collector to base electrodes of the pair of transistors. Thus, the output waveform 140 is of parabolic shape for the same reasons described with reference to generator A5, but occurring at the horizontal frequency. Furthermore, the same modifications can be made to the generator in order to tailor the waveform 140 for a particular CRT 20.

The major difference between generators A8 and A5 arises from the fact that in most television receivers, the necessary sawtooth waveforms 94, 95 cannot be obtained directly from the horizontal scan circuit. Therefore, it is derived from the flyback pulse which is integrated by an RC network. In particular, a pair of anti phase horizontal flyback pulses 144 and 145 are applied to input terminals for the generator A8. The waveform 144 is integrated by a resistor 147 and a capacitor 148, the capacitor having a potentiometer 149 coupled in shunt thereacross. Similarly, waveform 145 is supplied to an integrator consisting of a resistor 152 and a capacitor 153 which has a potentiometer 154 coupled in shunt thereacross. Thus, the wipers of potentiometers 149 and 154 may be adjusted in a manner similar to the wipers of potentiometers 124 and 126 in generator A5, and provide separate and independent control over the left and right hand portions of the screen. It is not essential to produce a parabolic waveform 140, in that current integration produced by the convergence exciter may be utilized to convert the triangular waveshape into a desired parabolic correction waveshape, if desired.

The waveform 140 is coupled to a buffer amplifier 160 which performs impedance matching functions and has an output coupled to a summing junction 162 which feeds wiper 89 of potentiometer 90. Thus, the buffer amplifiers 122 and 160 form a mixer circuit for combining the horizontal parabolic wave with the vertical parabolic wave. The composite waveform is coupled to the convergence exciter coils through the differential control potentiometer 90 and output transistors 164 and 165, which form amplifiers A6 and A7, respectively. The output transistors 164 and 165 are connected in common emitter fashion and isolate both the vertical and horizontal parabolic generators from the exciter assemblies 26R and 26G.

The blue horizontal parabolic generator A1 and A2 function in a manner somewhat similar to the red/green horizontal parabolic generator A8. That is, a pair of transistors 170 and 172 are connected in common emitter fashion, and have sawtooth waveforms 64 and 65 coupled thereto via RC networks which include adjustable potentiometers 174 and 175. The basic dissimilarity with generator A8 is that transistors 170 and 172 are split, by use of separate load resistors 178 and 179, to provide two independent outputs. As a result, the first half of the parabolic correction signal 67 is on one output line, and the terminating half of the parabolic correction signal is present at a different output line.

The pair of output lines are fed to a buffer amplifier 180 which includes separate transistors 181 and 182 with the potentiometers 72 and 74 forming the emitter resistors for the respective transistors. Amplitude correction is accomplished by adjusting potentiometer 72 for the left half of the CRT face. Both potentiometers generate the maximum amplitude of convergence correction at the middle of the scan period, corresponding to the center of the screen as illustrated in FIG. 3.

The output of potentiometer 72 is coupled to the wiper 190 of a potentiometer 192 having its end terminals coupled to the base of transistors forming blue output amplifiers A3 and A4. Similarly, the output of potentiometer 74 is coupled to the wiper 194 of a potentiometer 195 having its end terminals in shunt with the end terminals of potentiometer 192, and hence connected to the base electrodes of the transistors forming blue output amplifiers A3 and A4.

Amplifier A3 is formed by a transistor 200 having its collector coupled directly to one end of winding 35, with its opposite end being coupled to B+. Similarly, amplifier A4 is formed by a transistor 202 having its collector coupled to one end of winding 36, the opposite end of which is connected to B+. The pair of windings 35 and 36 are shunted by resistors 204 and 206, respectively. When the wipers 190 and 194 are set to their center positions, transistors 200 and 202 supply equal currents to the windings 35 and 36, and the magnetic field generated for convergence is balanced. The balanced flux state results in a straight vertical electron beam displacement at the center of the CRT screen.

Wipers 190 and 194 allow correction of what is termed wide blue lateral field and narrow blue lateral field, which conditions are illustrated in FIG. 7. As noted in FIG. 7, the red and green guns have been properly converged, causing the vertical R and vertical G lines to lie on top of one another. Red/green convergenced can be obtained through the convergence circuitry, since these beams move toward one another in the convergence correction procedure. The blue beam, however, emanates from a gun which is not adjusted simultaneously with the red and green guns.

Thus, the blue gun may create a blue vertical line offset from the red/green vertical lines at the edges of the screen. As seen in FIG. 7, the blue vertical line B is outside of the R/G lines for the left hand portion of the screen. The amount of offset, designated by the arrow 210, is known in the art as a wide blue field. It is also possible that the blue vertical line B may be inside the R/G line at the opposite edge of the screen, as represented by the offset 212. This latter condition is known as a narrow blue field. Heretofore, it has been possible to correct for a wide blue field occurring on both sides of the screen, or a narrow blue field existing on both halves of the screen, but not a mixed condition as shown in FIG. 7. The apparatus of FIG. 6 allows correction for this mixed condition.

Returning to FIG. 6, amplifier A3 and coil 35 provide left channel correction, while amplifier A4 and coil 36 provide right channel correction. The right channel blue signal applied to coil 36 rotates the magnetic vector across the pair of internal pole pieces in the direction 214, causing beam displacement along a transverse direction 216. The left channel produces the reverse operation, in that flux rotation occurs along the direction 216 and hence beam displacement is along the axis 214. The horizontal components of vectors 214 and 216 provide lateral correction for wide/narrow blue vertical lines.

Proper adjustment of the potentiometers 192 and 195 results in correction of either a wide or a narrow blue field, which correction is independent for opposite halves of the screen. For example, the left side can be adjusted for a wide blue field by moving the wiper 190 toward the upper position, while the right side can be corrected for wide blue field by adjusting the wiper 194 towards the lower end of its range. A narrow blue field can be corrected by reversing the above procedure. Or, the two controls can be separately adjusted to correct for a wide blue field on one half, and a narrow blue field on the other half of the screen. If there is no lateral blue error which requires correction, then the wipers 190 and 194 should be set to their center positions. The above described network also provides the proper waveform necessary to correct for blue droop in the deflection yoke and CRT.

For completeness, the steps taken to converge the three beams are set forth below. It should be noted that independent correction exists in each quadrant of the CRT screen, due to the minimum interraction between the various magnetic fields. A typical convergence procedure would involve the following steps:

1. Adjust the static convergence magnets to apply static correction at the edges or periphery of the CRT face. Completion of this step results in vertical and horizontal scanning lines as shown in FIGS. 2 and 3.

2. Apply dynamic correction to adjust the red and green vertical lines such that they become parallel. At the right and left side, the red and green lines will also be coincident, but may be separated at the center of the screen.

3. Apply dynamic correction to the red and green horizontal lines, which lines will now coincide from top to bottom.

4. Apply dynamic correction to the red and green vertical lines at the right side of the CRT face. The red and green vertical lines will then be coincident from approximately the center of the screen to the right side.

5. Apply dynamic correction to the red and green vertical lines at the left side of the CRT face. The red and green lines will now be coincident at all points on the CRT face.

6. Apply dynamic correction to the blue horizontal lines in the central portion of the CRT face. The blue lines will now be coincident with the red and green lines in the central portion of the CRT, but elsewhere the blue lines will tend to fall below the coincident red and green horizontal lines, depending on the yoke field.

7. Apply dynamic correction to the blue horizontal lines at the top of the CRT face.

8. Apply dynamic correction to the blue horizontal lines at the bottom of the CRT face. This completes the convergence procedure, and all three beams will converge properly for all points on the CRT face.

While the inverse convergence method has been described with reference to an active convergence system, as illustrated in FIGS. 1 and 6, various modifications and simplifications can be made as shown in the remaining figures.

MODIFIED EMBODIMENTS

The blue correction section shown in FIG. 6 may be simplified, if desired, to eliminate the separate lateral controls for the left and right hand portions of the screen. As shown in FIG. 8, the parabolic generator formed by differential transistors 170 and 172 is integrated into a single unit by use of a common load resistor 230 coupled between the collectors of the differential transistors and B+. A single parabolic output, which corresponds to combined waveforms 67 and 68 of FIG. 6, is coupled to a single stage buffer amplifier formed by a transistor 232 connected as an emitter follower. Potentiometer 234 forms the emitter resistance, and suplies a signal to a summing junction 236 which is coupled to the output from potentiometer 80 in the vertical buffer amplifier 122 of FIG. 6. The combined output drives a single output transistor 240 having a collector coupled to B+ through the series connection of blue winding sections 35 and 36. A load resistor 242 shunts the series connected coils 35 and 36.

Another alternate design (not illustrated) for the circuit of FIG. 6 is to provide a separate pair of output transistors, corresponding to transistors 164 and 165, for the vertical channel. Since there is a ratio of 262.5:1 between the horizontal and vertical scan frequencies, the output power can be reduced by providing separate load impedances. That is, a set of amplifiers, similar to A6 and A7, would be fed from the wiper of potentiometer 86 of the vertical parabolic generator. The red coil 38 and green coil 39 each would be split into separate sections by means of a tap on the winding. The output amplifiers for the vertical stage would be coupled between one side of the windings 38 and 39 and the B+ connection taps thereto, whereas the output amplifiers A6 and A7 for the horizontal generator would be coupled to the taps and the B+ side of the coils 38 and 39.

The inverse convergence method is equally applicable to passive systems of the type illustrated in FIGS. 9 and 10. The inverse convergence method can be implemented in general by a conventional blue convergence circuit since maximum correction currents occur in such a circuit at the middle of the horizontal scan. The green and red convergence circuits can also be implemented with the same basic design, because in the inverse method, all three convergence fields require maximum correction at the middle of the scan period. A passive system incorporating these concepts is illustrated in FIG. 9. In this circuit, separate horizontal convergence coils 250, each associated with a different beam, are provided as is conventional in the art. A conventional blue horizontal convergence circuit 252 supplies horizontal convergence correction current to horizontal convergence coil 250B, and is powered from the horizontal flyback pulse 144. A coil 254 provides a shaping function.

A novel red/green horizontal convergence section is coupled directly across the horizontal convergence coil 250B. One end of the coil 250B is coupled to a variable inductor 256 in series with a capacitor 257 terminating at a junction of the red 250R and green 250G coils. The opposite sides of coils 250R and 250G are coupled to end terminals of a potentiometer 260 having a wiper 262 connected in series with a variable resistor 264 connected to a source of reference potential, or ground 265. The junction between capacitor 257 and the coils 250R and 250G is shunted to ground 265 through a resistor 268 in series with a diode 270.

The red/green section thus derives its parabolic current from the same source which feeds the blue coil 250B. Inductor 256 and capacitor 257 provide right side amplitude and current slope shaping. The amplitudes of the left side are adjusted by the setting of variable resistor 264. The left and right R/G horizontal lines are adjusted simultaneously by movement of wiper 262 of potentiometer 260. Since the red and green exciter coils are driven from a common source, a single clamping circuit, formed by resistor 268 and diode 270, is sufficient. A capacitor 272 provides the proper current profile for the convergence exciter coils.

The above described circuit can be simplified if the inductances of all the horizontal exciter coils 250 are reduced sufficiently to total the original inductance of the blue horizontal coil 250B. Various parameters can also be modified, both with respect to component values and exciter coil inductances, to produce the simplified design shown in FIG. 10. The function of variable resistor 264 is provided by a resistor 280 in series with a variable resistor 282 coupled in shunt between the junction of all three horizontal coils 250 and ground 265. The conventional horizontal blue circuit 252 can drive all three horizontal coils 250 provided that the equivalent inductances of the exciter coils equal the total inductance of the original blue coil.

The necessity for converting the horizontal flyback pulse into a parabolic waveform for driving the inverse horizontal convergence circuit can be eliminated entirely in many solid state television receivers. In FIG. 11, a portion of the horizontal deflection circuit 60 for a solid state television receiver is illustrated. The horizontal output amplifier 290 drives a flyback winding 292 connected between the output amplifier and B+. The horizontal deflection yoke windings 294 are connected in series with an "S" shaping capacitor 296, which improves linearity of the scanning current as is well known. A diode 300 in series with a centering variable resistor 302 is connected between B+ and the "S" shaping capacitor 296. The above described horizontal deflection circuit is conventional, and provides approximately 30 to 40 volts of AC signal which rides on a 110 volt DC level. This situation should be contrasted with many vacuum tube designs in which the "S" shaping circuit impedance is too high to be used for convergence applications, and the boost voltage is too high for economic coupling of the waveform across the "S" capacitor to a convergence circuit.

With solid state television receivers of the type described above, this situation is changed. The voltage across the "S" shaping capacitor can be coupled through a capacitor 306 to a simplified horizontal convergence control. The single source for driving the horizontal convergence control is a parabolic current already available in the television receiver, namely, the signal across the "S" shaping capacitor 296. This eliminates the flyback pulse converting techniques previously used with passive circuits.

The passive inverse circuit, which is simplified by using the parabola developed across the "S" correction capacitor 296 of the horizontal output stage, operates as follows. The horizontal output transistor controls the current through the flyback winding 292 and deflection yoke windings 294. The horizontal centering circuit consists of the variable resistor 302 in series with the diode 300 to provide a differential voltage between flyback and yoke winding.

The resulting horizontal parabolic waveform 317 is developed across the "S" capacitor 296 by the sawtooth yoke current. The "S" parabola is coupled to the convergence circuit by capacitor 306. The capacitor 306 performs a dual function by blocking the DC potential from the scan circuit, and providing a reactive component for the convergence circuit. The output of the capacitor 306 is connected to the R-G-B color circuits.

The blue circuit amplitude, phase, and waveform profile are adjusted by the combination of the inductor 316 and resistance control 314. Capacitors 315 and 318 are selected to provide the proper phase and current waveform parameters for the blue convergence exciter coil 250B.

The red and green circuit is matrixed with a differential potentiometer 310 to provide adjustment of the horizontal lines over the center of scan. As previously described, static convergence occurs at the ends of the scanning lines. The vertical lines are adjusted by a resistance control 309 and a variable inductance 307. Capacitors 308, 311 and 312 are selected to provide the proper current waveform for the convergence exciter coils 250R and 250G. The ampere turns of the convergence exciter coils are also selected to provide the optimum magnetic characteristics and circuit inductances for the system.

The active inverse design can also be simplified by using the parabolic waveform derived from the horizontal "S" correction capacitor. The parabolic generators A1, A2, A5 and A6 of FIG. 1 can be eliminated by the application of the "S" parabola.

The inverse concept permits additional simplification of the R-G-B circuit since the maximum dynamic correction is required at the CRT center for all three color beams. A simplified approach is shown in FIG. 12. The blue coil 325B is in series with both the horizontal parabola source, which desirably is the "S" shaping capacitor although other sources can also be used, and a parallel combination of the red 325R and green 325G coils. The series-parallel exciter circuit design shown in FIG. 12 requires fewer components than FIG. 11.

Another design simplification occurs by using all series connections for the exciter coils. This configuration is shown in FIG. 13. The matrixed exciter design permits great simplicity in the convergence system. In both FIGS. 12 and 13, a horizontal parabola 320 is derived from the "S" capacitor, or circuitry that will convert the flyback pulse to a parabolic waveform. The coil 322 is adjusted to correct for the blue field. The blue section is adjusted first since it requires the maximum drive. The red and green section is adjusted with two resistance controls 326 and 329. The inductances of the red and green exciter coils will be different in the series-parallel configuration of FIG. 12, than for the series design of FIG. 13.

The deflection yoke and picture tube design can also be simplified by use of my invention. The present combination has been developed over the years to be compatible with the present passive nonlinear convergence system. However, the disclosed inverse concept makes it possible to modify present design and improve the optical system performance. For example, the picture tube fabrication can be made easier. Presently, the R-G-B electron gun mount is tilted to converge the beams at the CRT center. A tilted gun is more difficult to align at a precise angle. However, my inverse convergence system permits all three guns to be constructed on a parallel axis. This allows the free fall beam landing geometry to be arranged in an underconverged pattern.

Since parallel gun design permits greater accuracy and better uniformity of the beam landing pattern, it is possible to eliminate or greatly reduce the static magnet flux required from the exciter assembly. This static flux reduction reduces the misconvergence of the op ical system. Improved optical uniformity is possible due to the reduction of the random leakage flux surrounding the static permanent magnets.

By selecting the optimum design parameters of the deflection yoke, picture tube, and neck components, it is possible to reduce the convergence circuit to the design shown in FIG. 14. The simplified circuit of FIG. 14 requires a horizontal parabola 320 which supplies the convergence exciter current. A capacitor 321 isolates the DC potential from the scan circuit, and provides current shaping for the remaining circuit. The exciter coils 325B, 325G and 325R have the required ampere turns needed to compensate for symmetrical or asymmetrical R-G-B fields. Capacitors 323, 327 and 328, in conjunction with the exciter coil inductances, are used for current waveform shaping. The current amplitude and phase is adjusted by a variable inductance 322. While certain modifications and simplified circuits have been described, it will be appreciated that many other variations are equally possible.

quadripolar field system of deflection coils Color television display device including a cathode-ray tube

A color televison display device including a cathode-ray tube whose neck supports a system of deflection coils which causes isotropic astigmatic errors upon deflection. This is eliminated with the aid of a quadripolar field to which end four windings are wound preferably as four toroid windings on the core of the deflection coil system at the area of the deflection plane. The windings are arranged pairwise opposite to each other and this in such a manner that two windings are exactly located in the gaps of the field deflection coils and the two other windings are shifted 90° in the tangential direction. Parabola currents which may have the line frequency and/or the field frequency must flow through the four windings. However, it is alternatively possible to replace the four windings by two each (thus a total of 8) in which a parabola current proportional to the line frequency flows through one pair of four windings and a parabola current proportional to the field frequency flows through the other pair of four windings.

1. A circuit for a color television display tube, said circuit comprising first and second deflection coil units adapted to receive respective deflection currents, each unit having two symmetrical coil halves which are arranged opposite to each other and defining a pair of places therebetween wherein said windings have at least a reduced winding density, the first unit being disposed 90° in the tangential direction relative to the second unit, said units being disposed around the neck of the cathode-ray tube for deflecting at least one electron beam generated in the cathode-ray tube into two orthogonal directions when the respective deflection current flows through each coil unit; means for correcting distortion including at least four windings disposed tangentially relative to one another at an angle of approximately 90° near said places respectively, switching means coupled to said windings for applying a correction current through the four windings, means coupled to said switching means for generating said correction current in accordance with the deflection current flowing through at least the first deflection coil unit, whereby the windings generate a quadripolar field which is in accordance with said deflection current at the area of the deflection plane of the electron beam. 2. A circuit as claimed in claim 1, wherein said units further comprise a core, said four windings comprising toroid coils wound on the core. 3. A circuit as claimed in claim 1, wherein said tube has three beams located on the vertices of an equilateral triangle at the area of the deflection plane, and wherein said correction current comprises the difference between a current which is substantially proportional to the square of the current flowing through the first deflection coil unit and a current which is substantially proportional to the square of the current flowing through the second deflection coil unit said difference being substantially zero for those instantaneous values of the two deflection currents at which the three beams are located on the diagonals of the tube display screen. 4. A circuit as claimed in claim 1 wherein said tube has at least one coplanar disposed beams and said correction current comprises the sum of a current which is substantially proportional to the square of the current flowing through the first deflection coil unit and a current which is substantially proportional to the square of the current flowing through the second deflection coil unit in accordance with the equation (c₁ - C₃) x² + (c₂ + c₄) Y².

in which the constants C₃ and c₄ are chosen to satisfy the equation c₄ Y² - c₃ x² = O

along the diagonals of the screen, while furthermore c_i - c₃ ≥ 0.

5. A circuit as claimed in claim 1 wherein the four windings are coupled in series and said switching means for supplying the correction current comprises a mixer circuit having an input adapted to receive a parabola signal of the frequency of the current flowing through at least one of the deflection coil units,and an output coupled to said windings. 6. A circuit as claimed in claim 5 wherein said input is adpated to receive a parabola signal of the frequency of the current flowing through the remaining deflection coil unit. 7. A circuit as claimed in claim 5, further comprising means for the elimination of asymmetries including means for applying a polarity reversible sawtooth signal of the frequency of the deflection current flowing through at least one of the deflection units to the mixer circuit input. 8. A circuit as claimed in claim 7 further comprising means for applying a polarity reversible sawtooth signal of the frequency of the deflection current flowing through the remaining deflection unit to the mixer input. 9. A system of deflection coils for use in a circuit that eliminates distortion comprising a core, and a first and a second deflection coil unit, each unit having two symmetrical coil halves which are arranged opposite to one another, the first unit being disposed 90° in the tangential direction relative to the second unit, and defining a pair of places therebetween wherein said windings having at least a reduced winding density and also defining two positions where electron beam deflection directions cross said core four toroid windings disposed on the core at said positions tangentially relative to one another at an angle of approximately 90°, the two toroid windings located opposite to each other are situated near the two places between the symmetrical coil halves of one of the two deflection coil units. 10. An in-line gun type color television picture tube apparatus comprising:

a. an in-line gun type color cathode-ray tube in which three electron guns are arranged in a line along a first axis and the spots of electron beams from these electron guns appear along the first axis on the screen of the cathode-ray tube and are converged at the center of the screen,

b. a deflection yoke which deflects and scans the electron beams so that both side spots of the three electron beams appear on the screen at an equal distance on both sides of the center spot when the three electron beams are not converged,

c. a convergence magnetic field generating means, said convergence magnetic field generating means consisting essentially of at least one pair of convergence windings which are respectively provided externally on the neck of said cathode-ray tube and are opposite from each other in reference to the axis of the cathode-ray tube and a convergence power supply which supplies the convergence current to said windings which forms said convergence magnetic field generating means forming a convergence magnetic field which shifts only said side electron beams and has a field distribution which is symetrical and linear in reference to a second and third axis, said second and third axes being positioned 90° with respect to each other and 45° with respect to said first axis, the plane formed by said second and third axes being perpendicular to the axis of the cathode-ray tube, wherein both side electron beams are symmetrically shifted in reference to the center electron beam, as a function of the deflection angle, in order to complete convergence of the beam spot trio through said convergence magnetic field.

11. An apparatus according to claim 10, wherein a deflection yoke deflects and scans the electron beams so that the distances between side spots and center spots appearing at two symmetrical positions in reference to the center of the screen are equal when three electron beams are not converged. 12. An apparatus according to claim 10, wherein said convergence magnetic field generating means is positioned such that the convergence magnetic field is induced at the deflecting position of the deflection yoke to shift symmetrically both side electron beams in reference to the center electron beam at the center of deflection. 13. An apparatus according to claim 10, wherein said at least one pair of convergence windings comprises two sets of said convergence windings arranged at positions at 90° from each other around the axis of said cathode-ray tube. 14. An apparatus according to claim 10, wherein said convergence windings are toroidally wound on a core made of a ring-shaped highly magnetic material which surrounds the neck of said cathode-ray tube about the axis of said cathode-ray tube. 15. An apparatus according to claim 14, wherein said deflection yoke is comprised of a ring-shaped yoke core surrounding the neck of the cathode-ray tube and a pair of coils which are opposed to each other and mounted on said yoke core, and wherein said yoke core is employed as the core for said convergence windings.

Description:

The invention relates to a colour television display device including a cathode-ray tube having a display screen and a system of deflection coils comprising a first and a second deflection coil unit, each unit having two symmetrical coil halves which are arranged opposite to each other, the first unit being shifted 90° in the tangential direction relative to the second unit, said system of deflection coils being slid around the neck of the cathode-ray tube for deflecting at least one electron beam generated in the cathode-ray tube into two orthogonal directions when the relevant deflection current flows through each coil unit.

The U.S. Pat. No. 3,440,483 describes a display device provided with means to correct the anistropic astigmatism. In case of anisotropic astigmatism substantially no errors in the picture formation occur along the axes of the display screen, that is to say, the horizontal and the vertical axis in the middle of the display screen which means that the system is substantially anastigmatic along the axes. On the other hand picture errors do occur along the diagonals and on either side thereof on the display screen which errors are greatest in its corners. To eliminate this astigmatism, which assumes inadmissible proportions especially in a 110° deflection colour display tube, said patent proposes to pass unequal currents through the two symmetrical coil halves of one of the two deflection units, the inequality being determined by a correction current which is the product of the instantaneous value of the two deflection currents.

This is an eminent solution. However, a drawback thereof is that a deflection coil system which is free from isotropic astigmatism is more difficult to manufacture than a deflection coil system which is free from anisotropic astigmatism.

A deflection coil system which is free from anisotropic astigmatism shows picture errors along the axes, but does not show substantially any picture errors on the diagonals. However, the difficulty to eliminate the picture errors along the axes is that there is no possibility of unequal control of the deflection currents in the two symmetrical coil halves of one of the two deflection coil units. In fact, errors along the diagonals are corrected with this kind of control, whereas exactly errors along the axes have to be corrected.

In order to be able to eliminate the errors along the axes for this isotropic astigmatism the display device according to the invention is characterized in that for the purpose of correcting the isotropic astigmatism the deflection coil system is furthermore provided with at least four windings which are provided tangentially relative to one another at an angle of approximately 90° and this in such a manner that two windings thereof, which are located opposite to each other, are situated near the two gaps between the symmetrical coil halves of one of the two deflection coil units, switching means being present to pass a correction current through the four windings, which current is dependent on a current which is mainly proportional to the square of the deflection current flowing through the first and/or on a current which is mainly proportional to the square of the current flowing through the second deflection coil unit so that the four windings generate a quadripolar field which is proportional to said currents at the area of the deflection plane of the electron beam.

The step according to the invention is based on two recognitions to be explained hereinafter.

(1) To obtain a satisfactory electron landing on the screen the correction is to be performed in the deflection plane itself. This argument particularly applies to a colour display tube of the shadow mask type.

(2) The correction is to be performed with a quadripolar field.

As will be described hereinafter, no interaction varying proportionally with the deflection occurs between correction and deflection fields when using a quadripolar field. This means that the operation of the correction field remains the same independent of the fact whether or not the deflection field is active.

Both above-mentioned requirements are satisfied with the aid of the four windings provided, according to the invention at the area of the deflection plane.

In order that the invention may be readily carried into effect, a few embodiments thereof will now be described in detail by way of example with reference to the accompanying diagrammatic drawings, in which:

FIG. 1 shows a display device provided with a three-gun cathode-ray tube and switching means for sypplying the correction current through the four windings.

FIG. 2 is an elevational side view of a deflection coil system according to the invention in which the four windings are provided as toroid windings on the core of the deflection coil system itself.

FIG. 3 is an elevational front view of the deflection coil system according to FIG. 2.

FIG. 4 is a simplified elevational front view of the system as is arranged on the neck of the cathode-ray tube, in which only the four toroid windings with their series connection and the position of the three beams on the vertices of an equilateral triangle are indicated at the area of the deflection plane in the neck of the cathode-ray tube.

FIG. 5 shows the same elevational front view as that in FIG. 4 in which, however, the three beams are co-planar.

FIG. 6 serves to explain the fact that when the correction is not performed in the deflection plane itself, a mislanding of the electrons on the screen is the result thereof.

FIG. 7 shows the area of the screen S of a cathode-ray tube of the shadow mask type when the three sources of the electron beams are placed on the vertices of an equilateral triangle according to FIG. 4, and in which the picture errors occurring as a result of the isotropic astigmatism are shown.

FIG. 8 illustrates the influence of the correcting quadripolar field along the x-axis for errors as shown in FIG. 7.

FIG. 9 illustrates the influence of this quadripolar field for the errors as shown in FIG. 7 along the y-axis.

FIG. 10 shows the area of the screen S of the cathode-ray tube when the three electron sources are placed on one line as is shown in FIG. 5, and in which the occurring errors are shown.

FIG. 11 is a detailed circuit arrangement for supplying the various currents to the four windings and

FIG. 12 is a further detailed circuit arrangement when only parabolic currents are applied to the four windings.

It is to be noted that picture errors are understood to mean those deflection errors, which are referred to as astigmatism and coma. Predominantly important in this respect are the astigmatic errors since it has been attempted as much as possible in the design of the coils to eliminate the coma errors. In addition the present invention allows the designer of the deflection coil system more freedom because he may admit the astigmatic errors to a greater extent because these errors can be corrected with the aid of the quadripolar field.

FIG. 1 shows a block diagram of a colour television display device including a three-gun display tube 1 of the shadow mask type. For convenience' sake it is assumed in this Figure that the cathode-ray tube is controlled on its three cathodes by the three chrominance signals red (R), green (G) and blue (B). However, it is alternatively possible to apply the luminance signal Y to these three cathodes and to apply the three chrominance signals to separate Wehnelt cylinders not shown in FIG. 1. The display tube 1 is provided with a deflection coil system 2 which is diagrammatically shown in FIG. 1 as two flaps but will be shown in greater detail in the following Figures. Sawtooth deflection currents of line frequency are applied to this deflection coil system 2 from a line generator 3 and the vertical deflection currents are applied from a generator 4. The EHT of approximately 25 kV, denoted in FIG. 1 by V _EHT , is also applied from the generator 3, which EHT provides for the final anode voltage of the cathode-ray tube 1.

As already stated in the preamble, the deflection coil system 2 may be of the isotropic astigmatic type which means that such a coil system is free from anisotropic astigmatism. This means that picture errors will occur along the axes of the display screen and to a certain extent beyond these axes and substantially no errors will occur along the diagonals of the system of axes. Alternatively, it is of course possible to eliminate the errors as a result of the anisotropic astigmatism in a different manner, for example, with the aid of the correction method described in said prior metioned U.S. patent. The still remaining errors as a result of the isotropic astigmatism may then be corrected with the aid of the quadripolar field generated by the windings which are provided in accordance with the present invention.

To be able to correct said errors correction currents must be applied to the four windings to be described hereinafter from switching means 5 through the lines 6 and 7. The nature of these correction currents will be further described hereinafter. In this respect it is only to be noted that these correction currents must be proportional to the square of the horizontal deflection current, in this case denoted by x ² and/or the square of the vertical deflection current, in this case denoted by y ² . Particularly, when both the errors in the direction of the x-axis, which is the horizontal axis in the centre of the screen, and in the direction of the y-axis, which is the vertical axis on the centre of the screen, are to be eliminated and yet not to disturb the faultless deflection along the diagonals, the correction currents are to be chosen in accordance with the equation c ₁ x ² - c ₂ y ² ,

wherein c ₁ and c ₂ are constants which are to be chosen in such a manner that c ₁ x ² - c ₂ y ² = 0

in the vicinity of the diagonals, that is to say, the influences of the correction currents along the diagonals eliminate each other.

In the foregoing reference is made to a correction current which is proportional to the square of the horizontal and/or to the square of the vertical deflection current. In connection with non-linearities, either in the circuit arrangements used or as a result of the shape of the display screen, it may be necessary that this correction current is more proportional or less proportional to the said squares. Thus, it may be necessary under certain circumstances that also terms of, for example, x ⁴ , x ⁶ and y ⁴ , y ⁶ , etc., are included in the correction current. In practice, this means that current waveforms are used which are not purely parabolic but slightly deviate therefrom.

FIG. 1 further shows the video amplifier 8 which supplies the three chrominance signals for the cathodes of the display tube 1, the intermediate frequency amplifier 9 incorporating detectors and amplifiers which apply the video signal to the line 10 from which the three chrominance signals can be derived and which apply the synchronizing signal to the line 11 which applies field synchronizing signals through the line 12 to generator 4, and the line synchronizing signals through the line 13 to generator 3. Furthermore, FIG. 1 shows the RF-amplifier 14, which receives colour television signals from the aerial 15. Finally a line 16 leads from generator 3 to generator 5 for supplying parabolic signals of line frequency to generator 5. These parabolic signals are therefore to be assumed as control signals for the ultimate supply of the correction current proportional to x ² which is the square proportional to line or horizontal deflection current. Sawtooth signals of line frequency are applied through line 17 which, as will be further described with reference to FIG. 11, serve for possible correction of asymmetries in the horizontal deflection coil unit or of a slant arrangement of the guns in the tube 1.

Similarly parabolic signals of the frequency of the vertical deflection current are obtained from the vertical or field deflection generator 4 through the line 18. The last-mentioned parabolic signals are therefore responsible for the ultimate correction signal to be formed which is proportional to y ² being proportional to the square of the vertical deflection current. Sawtooth signals of the vertical frequency are applied through the line 19 from the generator 4, which signals likewise serve to correct asymmetries, if any, in the vertical deflection coil unit or a slant arrangement of the guns.

In order to be able to perform the desired correction as a result of the isotropic astigmatism, four windings are included in the deflection coil system 2 in accordance with the principle of the invention. These four windings may be provided directly on the neck of the display tube 1, namely below the deflection coils so that the quadripolar field generated by these four windings is indeed active at the area of the deflection plane which is the area from which the deflection of the electron beams commences. These four windings may be provided on the neck pairwise facing each other and shifted 90° in the tangential direction relative to each other. However, it is better to work with four toroid windings 20, 21, 22 and 23 as is shown in FIGS. 2, 3, 4 and 5 which coils are wound on a core 24 associated with the deflection coil system. In that case these four toroid windings are to be provided in such a manner that they are located pairwise facing each other and are shifted 90° relative to each other in the tangential direction. In addition they must be located in a certain manner relative to the deflection directions x and y so that the axes of the quadripolar field which is generated by the four toroid windings coincide with the diagonals of these x and y directions. This is shown in FIGS. 4 and 5 in which both the x- and y axes and the diagonals positioned at an angle of 45° relative thereto are shown. In this case it is to be noted that the x-y system of axes of FIGS. 4 and 5 is assumed to be in the deflection plane D, while the x-system of axes of FIGS. 7 and 10 is assumed to be at the area of the screen S.

Said locations may be obtained by winding the windings 21 and 22 at the area of the gaps between the two coil halves of the vertical deflection coil unit 25 and by winding the windings 20 and 23 opposite to each other at the area of the windows of the two coil halves of the vertical deflection coils 25 around the core 24. This means that the windings 21 and 22 are located at the area of the windows of the two coil halves of the horizontal deflection unit 26 and that the windings 20 and 23 are located at the area of the gaps between the two coil halves of this horizontal deflection unit 26. Furthermore, FIGS. 2 and 3 show that each deflection coil unit consists of two deflection coil halves, to wit the vertical deflection unit 25 consists of a deflection coil half 27 and a deflection coil half 28 while the horizontal deflection unit 26 consists of a first deflection coil half 29 and a second deflection coil half 30. It is to be noted that although only four windings 20 to 23 are mentioned above, each of the four windings may alternatively be replaced by two windings (that is to say, winding 20 may be replaced by two windings, winding 21 may be replaced by two windings, etc.) so that an overall number of 2 pairs of windings each constituting four windings is obtained. A current proportional to the line frequency is then to flow through one pair while a current proportional to the field frequency is to flow through the other pair.

The deflection coil units themselves receive their sawtooth currents from the generators 3 and 4, respectively, and will not be further referred to because they operate in known manner.

The four toroid windings are arranged in series in the Examples according to FIGS. 4, 5 and 11, so that the desired correction currents can be applied from the generator 5 to their ends 6 and 7. However, it will be evident that these coils may alternatively be arranged in parallel or pairwise in series and each further pair in parallel. The choice thereof will depend on the number of turns for each toroid winding and therefore the current required in various cases.

FIGS. 4 and 5 show magnetic lines of force which are generated by the field of the four toroid windings 20 to 23 if a current flowing in a certain direction flows through these windings. The arrows in these lines of force show clearly that a quadripolar field is used whose axes constitute the diagonals of the x-y axis system. FIGS. 4 and 5 also show by way of the characters R, G and B the positions of the red (R), the green (G) and the blue (B) electron beams, respectively. Since in FIGS. 4 and 5 this is assumed to be at the area of the deflection plane D, the points R, G and B should be considered as fictitious points, since actually the electron beams are deflected by the action of the deflection field. In that case the deflection takes place gradually over a certain distance. Actually, it is therefore not possible to speak of a deflection plane. For the sake of simplicity, reference is always made to a deflection plane D in this respect because this does not detract from the nature of the description. In the Example of FIG. 4, the three beams are considered to be situated on vertices of an equilateral triangle, while in the Example of FIG. 5 they are considered to be situated in a plane which passes through the x-axis and the axis of tube 1. In the case of FIG. 4, forces on the electron beams R, G an B are exerted by the quadripolar field which forces are denoted by the arrows at each of the points R, G and B. It will be explained with reference to FIGS. 8 and 9 how these forces cause the desired correction to take place. The direction of the arrows as shown in FIG. 4 then corresponds to a correction as will be described with reference to FIG. 8, which correction applies for the x-direction on the screen. In that case the current which then flows through lines 6 and 7 may be considered as a positive current proportional to x ² . For the correction required along the y -axis as will be described with reference to FIG. 9, the forces should reverse their direction. This is evident from the arrows shown in FIG. 9 at the points R, G and B. Consequently, the current then flowing through the lines 6 and 7 must be proportional to - y ² which in the foregoing is expressed by the formula c ₁ x ² - c ₂ y ² . Thus FIG. 4 corresponds to FIG. 8 and FIG. 9 corresponds to a quadripolar field according to FIG. 4 in which the current which is applied through the lines 6 and 7 is of opposite sign and consequently the arrows shown in the magnetic lines of force and the forces caused thereby at the points R, G and B denoted by the arrows shown at said points must all be reversed in their direction. Furthermore FIGS. 8 and 9 diagrammatically show the quadripolar fields by means of four magnetic poles 31, 32, 33 and 34 which are shown at the ends of the diagonals and which symbolize the action of the quadripolar field at the area of the screen S. However, it is to be noted that this quadripolar field at the area of the deflection field is generated by the four toroid windings 21 to 23 and is therefore actually active in that area.

Furthermore it is to be stated that in FIG. 4 the positions of the beams R and G are located on the lower side of the x-axis while the positions of the beams R', G' and R and G in FIGS. 8 and 9 are located above the x" and x' axes, respectively. This results from the fact that reversal takes place at deflection because the beams cross each other before they impinge upon the screen so that the situation at the area of the screen S is reversed relative to the situation at the area of the deflection plane D. These points of intersection are substantially located on a sphere whose radius of curvature is determined by the picture field curvature of the deflection coil system 2. However, since the action of force by the quadripolar field is effected at the area of the deflection plane D, the direction of the arrows as shown in FIG. 4, associated with each of the points R, G and B, must be the same as those in FIG. 8 for the corresponding points. Consequently, the electron beams are indeed displaced in a manner as is shown in FIG. 8, if this Figure corresponds to FIG. 4. The same applies of course to FIG. 9 if the current direction in FIG. 4 is reversed.

Furthermore, it is to be noted that a centre C ₀ is shown in FIGS. 4, 5, 7 and 10, which centre corresponds to the axis z of the display tube 1. It is true that the FIGS. 4 and 5 are assumed to be in the deflection plane D and FIGS. 7 and 10 are assumed to be at the area of the screen S, but since the axis z passes both through the centre C ₀₁ of the deflection plane D and the centre C ₀₂ of the screen S, these centres may be considered to be corresponding. Centres C ₀ " and C ₀ ' in FIGS. 8 and 9 must, however, be considered as transformed centres, because FIGS. 8 and 9 apply to a deflected position on the screen S in the direction of the x-axis and the direction of the y-axis, respectively.

A consideration of FIGS. 7 and 8 shows that the desired correction can be obtained with the aid of the generated quadripolar field. In fact, in FIG. 7 the error along the x and y-axes is indicated for a display device in which the deflection coil system 2 exclusively has an isotropic astigmatic error. It can be seen that the desired circular form for which the three electron beams at the area of the screen S are always located on the vertices of an equilateral triangle is distorted along the axes and remains intact along the diagonals. Thus, the vertices of FIG. 7 include four circles from which it is clearly evident that the electron beams R, G and B at the area of the screen S remain located on the vertices of an equilateral triangle in spite of the deflection. This means that the dynamic radial convergence which is established in known manner by means of a separate Convergence unit with convergence currents, which have substantially the same amplitude for each of the three beams can combine the three beams in one point so that they actually cross one another at the area of the screen S.

FIG. 7 shows that as a result of the isotropic astigmatism along the x-axis the circle is extended to form an ellipse, the long axis of the ellipse being located in the y-direction which corresponds to FIG. 8. This is exactly reversed along the y-axis and then likewise the circle is extended to form an ellipse the long axis of which is, however, located in the x-direction which corresponds to FIG. 9. As already described hereinbefore, FIG. 8 corresponds to a correction in the x-direction and this correction is performed with the aid of quadripolar fields which are proportional to x ² being the square of the horizontal deflection current. In fact, this horizontal deflection current increases in the horizontal direction on either side of the y-axis passing through the centre C ₀₂ . Since the errors on the left and right-hand sides of this y-axis are the same, it follows that a current must be passed through the four windings 20 to 23 which current is mainly proportional to the square of the horizontal deflection current, that is to say, proportional to x ² . FIG. 8 shows that the points R, G and B on the ellipse are displaced along the arrows as a result of the action of force in the quadripolar field to the points R', G' and B', respectively, which are located on a circle. Due to the normally active dynamic convergence the electron beams brought to the points R', G' and B' may then be combined in the centre C ₀ , at substantially the same convergence currents, so that a satisfactory colour display is ensured. It will be evident that the desired correction is obtained because the correction current is proportional to the square of the horizontal deflection current at any point of the screen on either side of the y-axis. As will be further described with reference to FIG. 11, the extreme value of the parabolic current of line frequency used for this purpose must then be set at a zero value in the middle of the line scan period, because no correction proportional to x ² is required for this middle. This means that the extreme value of the parabola must be clamped at a zero level, or, when adaptation to the static convergence in the centre C ₀₂ is desired, to an adapted level.

The same applies to FIG. 9 because in this Figure the ellipse with its long axis is located in the x-direction and therefore FIG. 9 corresponds to the errors which occur along the y-axis in the vertical direction. Since the errors above and below the x-axis are the same, this explain the necessity to have the correction current to be proportional to y ² . Also in this case the correction will proceed smoothly when a parabolic current of field frequency is used whose extreme value is set to a zero value in the middle of the field scan period, or to a level adapted to the static convergence. Then again it is ensured that the desired correction is performed for any point on the display screen above and below the x-axis because the correction current is proportional to the square of the vertical deflection current. It follows from FIG. 9 that the points R, G and B located on an ellipse are displayed along the arrows to the points R', G' and B' due to the action of force of the quadripolar field, which points are again located on a circle and which can be centered in the centre C ₀ ' due to the normally active dynamic convergence. This again ensures that the three electron beams can be combined at substantially the same convergence currents at any point of the screen.

The aforementioned choice of C ₁ x ² -C ₂ y ² = 0 on the diagonals ensures that the situation on the diagonals is not disturbed by the correction. Said choice of C ₁ x ² -C ₁ y ² = 0 on the diagonals is, however, only necessary when both the isotropic astigmatism on the x-axis and that on the y-axis are to be eliminated. On the other hand, if, for example, for the indexing tube or the three-gun chromatron tube the errors as a result of the isotropic astigmatism are to be eliminated only on the x-axis or only on the y-axis, then it is sufficient to pass through the four toroid windings only a current which is proportional to x ² or only a current which is proportional to y ² .

It has already been stated in the preamble that the action of the correcting quadripolar field must take place in the deflection plane D. The reason for this will be described with reference to FIG. 6. This Figure diagrammatically illustrates a cross-section through the display tube 1 in which the plane K denotes the plane where the cathodes of the display tube 1 are situated, the plane D denotes the deflection plane at the area of the deflection coil system 2, M denotes the position of the mask and S denotes the position of the screen on which the phosphors have been provided. Furthermore the z-axis in FIG. 6 indicates the axis of the display tube and the point C ₀₁ corresponds to the centre C ₀₁ in FIG. 4 and FIG. 5, respectively, and the centre C ₀₂ corresponds to the centre C ₀₂ in FIGS. 7 and 10, respectively. FIG. 6 furthermore only shows one electron beam, for example, the blue (B) beam which emanates from the plane K and which normally passes through the point P so as to pass through the point C ₀₂ at the area of the screen S which point is the point of intersection of the screen S and the axis z of the display tube 1. If the electron beam were deflected in the plane D from the point P, it would meet at point Q on the screen S which point Q is the correct point because this corresponds to the blue phosphor dot which is provided on this spot on the screen S. However, the foregoing shows that a correction field is necessary to eliminate the error due to isotropic astigmatism and picture field curvature. If this correction as seen in the direction of displacement of the electrons were performed before the deflection plane D, then this means that the electron beam, starting from the point B in the plane K undergoes a displacement in advance so that it would not pass the deflection plane D at the point P, but at the point P' and would undergo a deflection in situ, so that ultimately this electron beam impinges upon the screen S at the point Q'. However, since the points Q and Q' on the screen S do not coincide, this means that there is actually a mislanding of the electrons on the screen S. In fact the landing is displaced and consequently this may result in a less saturated colour or a faulty colour rendition, since only a correct landing in the point Q on the blue phosphor dot provided in situ ensures a saturated and faultless colour rendition. This can be achieved by performing the required correction not before the deflection plane but in the deflection plane and this in such a manner that the deflection point remains the point P, but that the deflection of the electron is corrected by the quadripolar field. It is then achieved that a correct landing of the electrons on the screen S is ensured under all circumstances. Consequently, the quadripolar field produced by the four toroid windings should engage the deflection plane D. By winding in accordance with the principle of the invention the four toroid windings on the core 24 this condition is satisfied and it has been achieved that the plane of the re-adjustment of the electron beams coincides or at least approximately coincides with the deflection plane D of the deflection coil system 2, so that the landing on the screen S is not influenced. It may be noted that the normal dynamic convergence which is still required displaces the electron beams before they reach the deflection plane D, but this displacement does not result in a mislanding. The reason is that it is ensured with the aid of the principle according to the invention that the three convergence currents for the separate convergence unit have the same mutual amplitude for any point on the screen. It is true that a displacement in the landing spot is obtained (because the separate convergence unit is active between the planed K and D) but this may be adapted by a varied screen mask distance. For the described correction with the quadripolar field the displacement of the three beams R, G and B is, however, unequal. In summary it may therefore be stated that: the required displacement is split up into

(a) a part which is mutually equal for the three beams and which is formed with the aid of the separate convergence unit which, as is known, is active between the plane of the cathodes K and the deflection plane D; and

(b) a part which is mutually unequal for the three beams and which is formed with the aid of the quadripolar field active in the deflection plane D.

The four windings are always indicated hereinbefore as four toroid windings wound on the core 24. However, it is alternatively possible to adhere the four windings separately on the neck of the tube under the deflection coil system 2. Then the adjustment is, however, much more critical because the position of these four windings must exactly be determined in advance relative to the deflection coil system 2 to be provided later on. In addition there must be a possibility of shifting the deflection coil system 2 somewhat in the axial direction so as to be able to adjust the correct position of the deflection coil system 2. Winding on the core is therefore preferred because upon shifting the windings 20 to 23 are also shifted.

Although the Example of FIG. 1 always described a three-beam cathode-ray tube of the shadow mask type, it will be evident that the principle of the invention is not limited to this kind of tube along. Thus, it is alternatively possible to use a three-beam display tube of the chromatron type in which the electron beams are located in a plane passing through the x-axis and through the axis of the tube 1. (See FIG. 5). The same is alternatively possible for a three-beam display tube of the shadow mask type. In that case errors as shown in FIG. 10 will likewise occur as a result of isotropic astigmatism. This means that the electron beam B always remains in the deflection centre or either side of the x-axis, but that the beams R and G are further moved away from this centre. By generating a field of force as shown in FIG. 5 the beams G and R are displaced in accordance with the arrows shown in this Figure. This means that the beam G is displaced to the right-hand side as a result of the quadripolar field and the beam R is displaced to the left-hand side. These displacements are necessary to eliminate the errors in either side of the x-axis. Consequently the quadripolar field according to FIG. 5 is produced by a current which is proportional to the square of the vertical deflection current, that is to say, proportional to y ² .

Unlike the Examples described with reference to FIGS. 4 and 7, there is now the possibility to make the three beams R, G and B completely coincide without a separate dynamic convergence unit. In fact, as FIG. 10 shows, the blue beam B on either side of the y-axis is not displaced from the new deflection centre so that also when the same forces are exerted by the quadripolar field on the beams G and R, these beams can be made to coincide with the beam B at the area of the screen S. It follows that also the quadripolar field which is active in the x-direction must be proportionally to x ² which means that it must be generated with the aid of a parabolic current of line frequency.

The same applies to the diagonals. In fact, also in that case beam B always remains in the deflection centre, but the beams G and R are located on either side thereof (see the circles in the vertices of the screen S in FIG. 10). Also in this case a force on beam G to the right-hand side and a force on beam R to the left-hand side may ensure that these two beams are made to coincide with the beam B.

However, the required field of force in the direction of the x-axis is the smallest, it is larger along the diagonals and it is the largest on the direction of the y-axis. If it is assumed that everything on the screen S were located on a circle, the required current for the four windings 20 to 23 would have to be proportional to c ₁ x ² + c ₂ y ² .

However, the correction in the x-direction may be smaller than for the case of the circle because the beams G and R have already been displaced somewhat to the centre as a result of the isotropic astigmatic error. As a result the correction current is to be reduced by a factor of c ₃ x ² relative to the case of the pure circle.

However, in the y-direction the action of the quadripolar field must be stronger because the beams G and R are then farther away from the beam B. Consequently the correction current is to be increased by a factor of c ₄ y ² .

Consequently an overall correction current is obtained which is determined by c ₁ x ² + c ₂ y ² - c ₃ x ² + c ₄ y ²

In that case there must apply that c ₄ y ² - c ₃ x ² = 0

on the diagonals, since exclusively the correction relative to the case of the circle must be performed for these diagonals.

The last equation may be written as (c ₁ - c ₃ )x ² + (c ₂ + c ₄ )y ²

which equation with (c ₁ - c ₃ ) = c ₁ ' and (c ₂ + c ₄ ) = c ₂ '

changes into c ₁ 'x ² + c ₂ 'y ²

and in which c ₁ ' = c ₁ - c ₃ ≥0

(The zero case applies when the isotropic astigmatic error ensures that the beams R, G and B already coincide in the x-direction).

In the case of FIG. 10 this means that as a rule the overall correction current must consist of the sum of a parabolic current of line frequency and a parabolic current of field frequency. The circuit arrangement which is required for generating these currents is shown in FIG. 12 and will be described hereinafter.

It will be evident that the principle described in FIG. 10 is alternatively usable for a colour display tube of the indexing type. In fact, in such tubes the spot may not extend in a horizontal direction because this would lead to the display of unsaturated colours in an indexing tube in which the colour strips are arranged vertically on the screen. A quadripolar field according to the invention may then ensure that the sagittal picture plane for one deflection direction and the meridional picture plane for the other deflection direction coincide with the screen of the tube. Due to this additional degree of freedom in the design of the coils it is possible to start from a deflection coil having smaller residual errors.

The fact that the action of the quadripolar field on the beams is independent of the deflection performed is proved as follows:

From Maxwell's second law there follows that div B = 0 (1)

wherein B is the vectorial representation of the magnetic induction.

Equation (1) may be written as δB _x /δ _x + δB _y /δ _y + δB _z /δ _z = 0 (2)

if a three-dimensional field is supposed calculated relative to a system of axes x, y, z.

Since for the correction field a flat plane D is always used, a system of axes x, y is left as is shown in FIGS. 4 and 5. For such a flat plane δB _z /δ _z = 0

which causes equation (2) to change into δB _x /δ _x + δB _y /δ _y = 0 (3)

For the calculation to be further performed it is simpler to change into polar coordinates r and ψ for which there applies that r = √x ² + y ² and tg ψ = x/y

In this formula ψ is the angle located between the radius r and the y-axis. When furthermore the solution for the field strengths B _x and B _y in pole coordinates is given by some approximation in a field of n-poles by B _x = f (r)  sin (1 - n/2)ψ (4)

and B _y = f (r)  cos (1 - n/2)ψ (5)

then after the change-over into pole coordinates with the aid of (3) it is found that δB _x /δ _x + δB _y /δ _y = cos nψ/2 {f' (r) + (1-n/2) f (r)/r }= 0 (6)

with f' (r) = [df(r)/dr]

From this follows: f' (r) = (n/2 - 1) [f (r) /r]

with the solution f (r) = A.r (n/2 - 1) (7)

wherein A is the integration constant.

Filling in equation (7) in the equations (4) and (5) results in B _x = Ar (n/2 - 1) sin (1-n/2)ψ (8) B _y = Ar (n/2 - 1) cos (1 - n/2)ψ (9)

The values of the field strength B may be assumed to be a vector by expressing it in a complex plane as B = B _y + iB _x = Ar (n/2 - 1).sbsp.e ⁱ (1-n/2) ψ= A(r. e ^-i ψ) (n/2 - 1) (10)

It can be deduced from equation (10) that the chosen solutions in accordance with the equations (4) and (5) are correct. In fact, for n = 2, that is, for a bipolar field equation 10 changes to B = A. (10')

this means that the field strength is constant and real which is correct, if the dispersion losses are ignored a field can be seen in the y-direction (being the real axis) which has the same intensity throughout independent of the coordinates r and ψ and x and y, respectively.

In order to check the influence of the deflection for these multipolar fields to a complex auxiliary magnitude v is introduced so as to simplify equation (10) which magnitude may be expressed in the coordinates x and y in accordance with v = y - ix.

If the latter equation is written in polar coordinates it changes into V = r cosψ - r.i. sinψ - re ⁱ ψ

Filling in equation (11) in equation (10) the result is B = A (v) (n/2 - 1) (12)

In case of deflection the influence of the field must be checked in accordance with equation (12) which may be effected by assuming that a transformation equation v' = v - v _o (13)

can be set up after deflection to a point v _o which expresses the value of a new complex magnitude v' relative to the new origin v _o .

Introducing the transformation equation (13) into equation (12) yields B = A(v' + v _o ) (n/2 - 1) (14)

The influence of the multipolar field can now be checked with the aid of equation (14) for different values of n. For a quadripolar field n = 4 and equation 14 changes into B = A (v' + v _o ) (15)

Both Av' and Av _o may be considered as a pure quadripolar field. In fact, the factor n/2 - 1 which determines the nature of the field changes for n = 4 into n/2 - 1 = 1, that is to say, a quadripolar field is expressed by a power 1. Since in equation (15) both v' and v _o have the power 1 they may both be considered as a quadripolar field. The term Av _o represents a homogeneous field which exerts the same action on the three beams in magnitude and direction. The term Av' represents a quadripolar field which as regard its effect on the three beams is equal to the original quadripolar field Av. This may alternatively be expressed by stating that v _o may be considered as a new centre of a correcting quadripolar field v which varies proportionally with the deflection.

However, if a hexapolar field were chosen as a correction field, then n would have become 6. Introducing this in equation (14) results in B = A(v' + v _o ) ² = A(v' ² + v _o ² + 2v'v _o ) (16)

The factor n/2 - 1 changes into 2, that is to say, a hexapolar field is expressed by a power 2. Consequently it is found from equation (16) that in addition to the active hexapolar field Av ² and the homogenous field Av _o ² , wherein v _o may be considered as a new centre for the correcting field, a quadripolar field 2Av' is produced which is influenced in amplitude and direction by the new centre v _o because, in fact, it occurs as a product term together with v _o in equation (16). From this follows that interaction takes place between correcting hexapolar field and deflection field. A similar reasoning as for a hexapolar field etc. may be maintained for the octapolar field (fill in n = 8 in equation 14) so that this proves that only an additional quadripolar field can be used for the correction of isotropic astigmatism to be described.

FIG. 11 shows a possible embodiment of the generator 5 according to FIG. 1 for generating currents for correcting errors as described with reference to FIGS. 4 and 7. A possible signal 35 of line frequency is applied to the input terminal 16 of the generator 5, which parabola is thus proportional to the square of the horizontal deflection current, that is to say, it is proportional to x ² . This signal 35 is applied to a potentiometer 36 whose wiper is connected through a resistor 37 and a capacitor 38 of high value for the line frequency to a base electrode of a first amplifier 39. At the other end a parabola signal 40 of the vertical frequency is applied to an input terminal 18, whose signal is therefore proportional to y ² , that is to say, to the square of the vertical deflection current. This signal is applied to a potentiometer 41 whose wiper is connected likewise through a resistor 42 and a large isolation capacitor 43 to the base electrode of the amplifier 39. The constants c ₁ and c ₂ may optionally be adjusted with the aid of the wipers on the potentiometers 36 and 41, so that the condition c ₁ x ² - c ₂ y ² = 0 on the diagonals of the screen can be satisfied. The two signals are amplified as a sum signal in the amplifier 39 and are subsequently applied to a push-pull output stage which comprises the complementary pair of transistors 43" and 44 which are connected to a supply voltage of +30 Volts. The interconnected emitters of the transistors 43" and 44 are connected to the terminal 6, while the terminal 7 is connected to earth through a current feedback resistor 48. The series arrangement of the four windings 20, 21, 22 and 23 is arranged between the terminals 6 and 7. Since the same current flows through the four windings in this case, their winding sense, as shown in FIG. 4, should be such that the lines of force have a variation as is shown in this Figure. This means that windings 21 and 22 must be wound in the same sense, but windings 20 and 23 must be wound in the opposite sense on the core 24.

Unlike the afore-described pure series arrangement of the four windings 20, 21, 22 and 23 it is possible to use a series-parallel arrangement in which 21 and 22 are arranged in series and 20 and 23 are arranged in series and in which these two series arrangements are subsequently arranged in parallel, taking into account the desired generation of the quadripolar field. It is of course alternatively possible to reverse the connecting terminals of the windings 21 and 22 in the pure series arrangement as shown in FIG. 11 relative to that of the windings 20 and 23 so as to obtain also a variation of the lines of force as is shown in FIGS. 4 and 5.

FIG. 11 likewise shows clearly that it is possible to use c ₁ x ² - c ₂ y ² because the parabola signal 35 is reversed relative to the parabola signal 40, because their extreme values are directed either positively or negatively. The two signals 35 and 40 are applied through capacitors 38 and 43 to clamping diodes 38' and 43' which clamp the extreme values of these parabola signals at earth potential. These clamped signals are subsequently applied to the base of transistor 39 whose DC-adjustment is ensured by means of a potentiometer comprising a variable resistor 45 and two resistors 46 and 47 of fixed value. The desired direct current can be adjusted by means of the resistor 45.

The feedback resistor 48 which is connected through resistor 47 to the base of transistor 39 provides for the desired linearity of the circuit arrangement. In this manner it is also achieved that the circuit arrangement as seen from the terminal 6 may be considered as a current source.

Furthermore the collector line of transistor 39 includes a resistor 49 which serves to ensure the desired drive of transistors 43" and 44. The series arrangement of a resistor 51 and a capacitor 50 is connected in parallel with the series arrangement of the windings 20, 21, 22 and 23. This series arrangement serves to avoid unwanted ringing phenomena in the circuit arrangement.

FIG. 11 furthermore shows that two sawtooth signals 52 and 53 of line frequency are applied with opposite polarity to the input terminal 17. If the wiper on potentiometer 54 is in the middle, no sawtooth signal is applied to the capacitor 38 through the isolation resistor 55 and consequently no sawtooth signal of line frequency is added the parabola 35. If the wiper on the potentiometer is moved in the direction of the terminal to which the signal 52 is applied, a sawtooth of the polarity of the signal 52 is added to the signal 35. However, if this wiper is moved to the terminal to which the signal 53 is applied, a signal of the polarity of the last-mentioned signal is added to the signal 35. As already described in the preamble these sawtooth signals serve to eliminate asymmetry between the coil halves 29 and 30 for the line deflection. These sawtooth voltages may also correct a possible slant position of the electron guns which must produce the electron beams R, G and B. If there is no question of asymmetry between the horizontal coil halves 29 and 30 and if the guns have no slanted position, wiper 54 may indeed be adjusted precisely in the middle.

The same applies to the sawtooth signals 56 and 57 of vertical frequency which are applied to the terminal 19. If the wiper on potentiometer 58 is adjusted exactly in the middle, no sawtooth signal can be added to the parabola signal 40 through the resistor 59. Dependent on the movement of this wiper, a signal of the polarity of the signal 56 or of the polarity of the signal 57 may be added to the parabola 40. The supply of the sawtooth signals 56 and 57 serves to eliminate asymmetries between the vertical coil halves 27 and 28 or also in this case to correct a possible slanted position of the guns. If the said asymmetries or slanted position of the guns is not present in this case, the wiper on potentiometer 58 may be placed in its central position. If there are two pairs of four windings each, the circuit arrangement according to FIG. 11 is to be split up in two parts. The first part then supplies parabola and/or possibly sawtooth currents of line frequency to the first pair of four windings, the other part supplies parabola currents and possibly sawtooth currents of field frequency to the second pair of four windings.

FIG. 12 shows a circuit arrangement for the case where generator 5 must generate currents in accordance with the equation c ₁ 'x ² + c ₂ 'y ² , that is to say, for the elimination of errors described with reference to FIG. 10. Then the two parabola currents must have the same polarity. This follows from FIG. 12 in which the minimum of the field frequency parabola 40' points in the same direction as the line frequency parabola 35. The coefficients c ₁ ' and c ₂ ' may be adjusted with the wipers on potentiometers 36 and 41, respectively. The circuit arrangement according to FIG. 12 is a so-called magnetic clamping circuit which is extensively described in the U.S. patent application Ser. No. 43,369, filed June 4, 1970. In this case it is only stated that the terminal 6 is not DC coupled, but is coupled through an isolation capacitor 60 to the emitters of the transistors 43" and 44. The required clamping is obtained by means of an additional winding 61 which is also wound on the core 24. The mean current of the pulsatory current flowing through the transistor 43" exclusively flows through the winding 61 because coil 61 is shunted by a capacitor 62 which has a high value for field and line frequencies. Since the correct ratio to be chosen between the number of turns on the winding 61 and the overall number of turns of the four windings 20 to 23, a homogeneous field induced from winding 61 may be added to the alternating field induced from the windings 20 to 23 in the core 24, the extreme values of the parabolas being adjusted at the zero level exactly in the middle of the scan period.

The great advantage of the magnetic clamping circuit according to FIG. 12 is that clamping of the extreme value of the applied signal is exclusively dependent on the shape (in this case the parabola shape) of these signals and not on their amplitude or frequency. In the Example according to FIG. 12, the supply of the sawtooth signals 52, 53 and 56, 57 as shown in FIG. 11 is not shown. If in the Example according to FIG. 12 this is desired, the supply of these sawtooth signals may take place in a similar manner as in the circuit arrangement according to FIG. 11.

MEDIATOR (PHILIPS) 66K365/16Z MULTISTANDARD CHASSIS KM2 CRT TUBE PHILIPS A66-410X. GLASS FOR TELEVISION DISPLAY CATHODE-RAY TUBES:

Glass for envelopes of television display cathode-ray tubes, particularly screen glass for color television, which transmits at most 0.5 mr/h of X-ray radiation at an acceleration voltage of 40 to 45 k. volt, and which has a composition in percent by weight:

what is claimed is

1. Glass for envelopes of television display cathode-ray tubes, particularly intended for the face-plate of the tube, consisting essentially of the following in percent by weight:

2. Glass as claimed in claim 1, consisting essentially of the following in percent by weight:

Description:

The invention relates to glass for an envelope of a television display cathode-ray tube, particularly glass for the face-plate of the tube.

Particular requirements are imposed on glass for envelopes of cathode-ray tubes for the display of colored television images as compared with that for the display of monochrome television images. Such special glasses are known, for example, from the British Pat. specification No. 1,123,857 the composition of which in percent by weight lies within the following range of compositions:

SiO ₂ 62-66 BaO 11-14 Li ₂ O 0-1 MgO 0-3 Na ₂ O 7-8.5 PbO 0-2 K ₂ O 6.5-9 Al ₂ O ₃ 1-4 CaO 2-4.5 As ₂ O ₃ + Sb ₂ O ₃ 0.3-0.7 CeO ₂ 0.05-0.3

the special requirements which, as compared with glass for the envelopes for monochrome display, are imposed on glass for envelopes for color display, are connected with differences in the manufacture and in the use of these tubes. In the first place, the glass components of envelopes for color display unlike those for the monochrome display envelopes cannot be sealed by fusing them together but must be connected together with the aid of an enamel. This is connected with the fact that a shadow mask is provided in these tubes, which mask determines the path of the required three electron beams. Furthermore, an extremely fine grating-like pattern of three different luminescent substances corresponding to the apertures of the shadow mask is provided on the inner side of the screen. The requirements relative to the maximum permissible distortion of the glass are in this case much more stringent in connection therewith than for glass of envelopes for monochrome display. In addition, the temperature at which the tube must be heated during evacuation and sealing must be approximately 20° higher and the heat treatment is of a longer duration than for the tubes for monochrome display.

The glasses within the above-mentioned range are eminently satisfactory in a technological respect relative to the softening point, the quality and the thermal coefficient of expansion. For the acceleration voltages until recently used on the electron guns, the absorption of these glasses for the X-ray radiation generated during operation as a result of the electron bombardment on the glass and on the shadow mask is sufficiently great. This even applies when the tube is built in in a cabinet in direct vision construction, thus without a protective cover glass.

The requirement up till now had been that the intensity of the transmitted X-ray radiation may be at most 0.5 milliroentgen per hour (mr/h) at a maximum thickness of 11 mm of the screen glass, an acceleration voltage of 27.5 k. volt and an anode current of 300 μA in a television display tube.

There is, however, a tendency to still further increase the margin of safety to X-ray radiation transmitted by television display tubes. There is a need of a kind of glass in which at most 0.5 mr/h is transmitted at an acceleration voltage of 35 k.volt. The above-described glasses then no longer have a sufficiently high absorption and do not satisfy the stricter safety requirements. For reasons of a technological nature, the thickness of the screen cannot be increased much further than 11 mm. To obtain a sufficiently high absorption while using a glass within the above-mentioned range of compositions, the screen should be thicker by as much as 2.5 mm.

For a satisfactory processing of the glass and moulding face-plates thereof, it is necessary that the temperature dependence of the viscosity is not too great. In practice this means that the temperature difference between the softening point, which is the temperature at which the viscosity of the glass is 10 ^{7 6} poises, and the annealing point, which is the temperature at which the viscosity of the glass is 10 ^{13 4} poises, must be at least 190° C.

In connection with the conventional manufacturing technique and the very stringent requirements which are imposed on the maximum permissible distortion of the glass components during manufacture of the tube, it is necessary that glass for a color display tube has an annealing point which is not lower than 485° C.

Finally it is of importance that a glass for a color television display tube has approximately the same coefficient of expansion as that of the known glasses (approximately 99 × 10 ^-7 between 30° and 300° C.), so that a better match is obtained with the existing glasses and metal components which must be sealed on or in respectively.

In the kind of glass according to the present invention, a content of PbO is present with an approximately equal BaO content relative to the known glass. It is by no means surprising in itself that the absorption of X-ray radiation is increased as a result thereof. It was, however, not obvious that it was possible to maintain the physical properties of the glass at the same level by means of a few other modifications. Furthermore, it is also known (from U. K. Pat. specification No. 664,769) that no discoloration occurs on the glass of the face-plate due to electron bombardment, provided that the glass contains CeO ₂ and provided that the glass contains no more than 1 percent of readily reducible oxides. However, the glass according to the invention which does not satisfy the last-mentioned requirement owing to its content of PbO, does not discolor under the influence of the electron bombardment.

The range of glass compositions according to the present invention is characterized by the following limits in percent by weight:

SiO ₂ 58-67 PbO 2-7 Li ₂ O0-1 MgO 0-3 Na ₂ O2-3 Al ₂ O ₃ 1-4 K ₂ O11-14 As ₂ O ₃ + 0.3-0.7 CaO 3-4.5 CeO ₂ 0.05-0.6 BaO 11-14

the softening point of these glasses lies between 690° and 710° C.; the annealing point between 485° and 510° C. and the thermal coefficient of expansion is approximately 97 to 100 × 10 ^-7 between 30° and 300° C. The glasses according to the invention amply satisfy the above-mentioned requirement of transmitting at most 0.5 mr/h at an acceleration voltage of 35 K.volt; it was found that this amount was not yet reached at an acceleration voltage of even 44 to 45 k.volt when using these glasses. The electric resistance 9 ^is at least 10 ⁹ 4 ohm.cm and at least 10 ^{7 5} ohm.cm at 250° and 350°, respectively, while these values are 10 ^{8 5} and 10 ^{6 7} ohm.cm for the above-mentioned known glasses.

The following glass is an example of a glass suitable for the relevant purpose. It is obtained in a manner which is common practice in glass technology by melting the relevant oxides or compounds which are converted into the oxides.

SiO ₂ 59% by weight PbO 6.3% by weight. Li ₂ O0.4% by weight Al ₂ O ₃ 2.2% by weight. Na ₂ O2.4% by weight Sb ₂ O ₃ 0.3% by weight. K ₂ O12.7% by weight CeO ₂ 0.5% by weight. CaO 3.6% by weight Softening point 705°C. BaO 12.3% by weight annealing point 502°C. coefficient of expansion (30-300° C.) 97 × 10 ^-7 log 250°C. = 10.3 log 350°C. = 8.2.