CRT Tube ITT/SEL Planigon A66ECF00X01
In-line gun system for a color picture tube:
Super Precision In-Line ITT SEL.
In a color picture tube with an in-line gun system elliptic beam-spot distortion caused by the deflection field is compensated for by pairs of plates in at least one focus electrode. The plates project into the apertures for the electron beams and are located at a distance from the bottom of the focus electrode.
What is claimed is: 1. A color picture tube, comprising:
a screen;
a funnel;
a neck;
a deflection system mounted on said neck at the transition of said neck to said funnel and which contains an inline gun system comprising cathodes and grid and focus electrodes, said focus electrodes having separate apertures each with a continuous edge for guiding electron beams to said screen, at least one of said focus electrodes having plates attached thereto which are located on both sides of the electron beams and are disposed on the screen side of said at least one said focus electrodes; said plates having curved portions which project into said apertures and are arranged in a spaced relationship from the screen side of the aperture of the respective focus electrode; and
one of the grid electrodes contain
s a slit diaphragm.
2. A color picture tube as claimed in claim 1, wherein:
vertices of said curved portions of said plates for the outer electron beams are located beside the center lines of said apertures for these electron beams in the focus electrode.
3. A color picture tube as claimed in claim 1, wherein:
the distances (w) between opposite ones of said plates
are different for the different electron beams.
4. A color picture tube as claimed in claim 1, wherein:
the distances between said plates and the bottom of the respective focus electrode differ for the individual electron beams.
a screen;
a funnel;
a neck;
a deflection system mounted on said neck at the transition of said neck to said funnel and which contains an inline gun system comprising cathodes and grid and focus electrodes, said focus electrodes having separate apertures each with a continuous edge for guiding electron beams to said screen, at least one of said focus electrodes having plates attached thereto which are located on both sides of the electron beams and are disposed on the screen side of said at least one said focus electrodes; said plates having curved portions which project into said apertures and are arranged in a spaced relationship from the screen side of the aperture of the respective focus electrode; and
one of the grid electrodes contain
s a slit diaphragm.2. A color picture tube as claimed in claim 1, wherein:
vertices of said curved portions of said plates for the outer electron beams are located beside the center lines of said apertures for these electron beams in the focus electrode.
3. A color picture tube as claimed in claim 1, wherein:
the distances (w) between opposite ones of said plates
are different for the different electron beams.
4. A color picture tube as claimed in claim 1, wherein:
the distances between said plates and the bottom of the respective focus electrode differ for the individual electron beams.
Description:
The present invention relates to a color picture tube.
U.S. Pat. No. 4,086,513 discloses a color picture tube with an in-line gun system in which parallel plates are attached to a focus electrode on both sides of the beam plane. This parallel pair of plates is directed towards the screen and serves to compensate the elliptic distortion of the beam spots by the deflection field, such distorted beam spots reducing the sharpness of the image reproduced. The pair of plates is attached to the focus electrode nearest to the screen. Alternatively, plates can be attached to a focus electrode near the first-mentioned focus electrode on both sides of the beams directed towards the last focus electrode. These plates are mounted at an angular distance of 90 degrees from the first-mentioned parallel pair of plates.
It is one object of the invention to provide a color picture tube with an in-line gun system causing an improvement in the compensation of the distortion of beam spots.
The embodiments of the invention will now be explained with reference to the accompanying drawings, in which:
FIG. 1 is a side view of a color picture tube;
FIG. 2 is a side view of an in-line gun system;
FIG. 3 is a top view of a focus electrode;
FIG. 4 is a section through the focus electrode of FIG. 3 along line IV--IV.
FIG. 1 shows a color picture 10 tube comprising a screen 11, a funnel 12, and a neck 13. In the funnel 13, an in-line gun system 14 (drawn in broken lines) is located producing three electron beams 1, 2, 3, which are swept across the screen 11 (1', 2', 3'). A magnetic deflection system 15 is located at the transition from the neck 13 to the funnel 12. FIG. 2 is a side view of the in-line gun system 14. It has a molded glass disk 20 with sealed in contact pins 21. The contact pins 21 are conductively connected (not shown) to the electrodes of the in-line gun system 14. The contact pins are followed by grid electrodes 23, 24, focus electrodes 25, 26 and a convergence cup 27. Inside the grid electrode 23, cathodes 22 are arranged which are shown only schematically in broken lines. The first grid electrode 23 is also called control grid, and the second grid electrode 24 is also called screen grid. The cathode together with the control grid and the screen grid is called triode lens. The focus electrodes 25, 26 form a focusing lens. The individual parts of the in-line electrode gun 14 are held together by two glass beads 28.
The focus electrode 25 consists of 4 cup-shaped electrodes 25.1 to 25.4, of which two each are joined together at their free edges and thus form a cup-shaped electrode. In all electrodes of the in-line gun system 14, there are three coplanar aperatures through which the electron beams 1, 2, 3 produced by the three cathodes 22 can pass. Three beams 1, 2, 3 are thus produced in the in-line gun system which strike the Luminescent Layer of the screen 11. In order to change the shape of the beam spot to obtain improved sharpness of the reproduced image, a suitable astigmatism is imparted to the in-line gun system. This effect is obtained by a slit diaphragm in the grid electrode 24 of the triode lens and by plates on both sides of the beam plane or on both sides of the beams in the focus electrode(s).
It is necessary to divide the astigmatism of the beam system between the triode lens and the focusing lens. The triode lens forms a smallest beam section which--in analogy to optics--is imaged on the screen with the following lenses. The astigmatic construction of this triode lens also leads to an astigmatism of the aperture angle of the bundle of rays emerging from the triode lens. A larger aperture angle facilitates defocusing of the image of the smallest beam section and the viewer of the color picture tube focuses on the plane with the larger aperture angle, i.e., the vertical and not the horizontal focal line of the astigmatic beam section of the triode lens is imaged on the screen. On the other hand, the aperture angle must not become too large, because then the bundle of rays moves to the bordering region of the imaging lenses. The large spherical aberration of these rather small electrostatic lenses does not permit a sharp image. Therefore, a sufficient astigmatic deformation of the bundle of rays is possible only if it is partly effected in the last focusing lens of the beam system where the aperture angle of the bundle of rays is no longer influenced.
FIG. 3 is a top view of the cup-shaped focus electrode 26. In the bottom of the focus electrode 26, there are three coplanar apertures 30 for the passage of the electron beams 1, 2, and 3, respectively. At the walls 32 of the focus electrode 26 two plates 31 are attached opposite each other, each of which has three curved portions 33. These curved portions 33 project into the apertures 30. The plates 31 can also consist of three individual curved portions 33. In the embodiment shown in FIG. 3, the curved shape of the portions 33 corresponds to an arc of a circle. The shape of the portions 33 can also be elliptic or parabolic or have a similarly curved shape. The distance w 1 between the opposite vertices of the portions 33 projecting into the central aperture is smaller than the distance w 2 between the opposite vertices of the portions 33 for the outer apertures 30. Furthermore, the vertices of the portions 33 for the outer apertures are not on the center line of the outer apertures 30. In order to make this clear, the distance of the central points of the apertures 30 from each other is designated by the letter S in FIG. 3. The distance of the vertices of the outer portions 33 from the central vertex in the plate 31 is designated by s 1 . It is clear that the value s 1 is smaller than the value S. This makes it possible to influence the angle the outer electron beams 1, 3 make with the central electron beam 2 to achieve static convergence. FIG. 4 is a section of the focus electrode 26 along line IV--IV of FIG. 3. The apertures 30 in the bottom of the focus electrode 26 have burred holes whose height for the individual apertures can be different. The plates 31, which may be attached to the wall 32 of the focus electrode 26 by weld spots 34, are arranged in a defined spaced-apart relation with respect to the inner edge of the burred holes. The distance from the bottom of the focus electrode 26 to the lower edge of the portions 33 of the plates 31 projecting into the apertures 30 is designated by the letter d. The distance
d 1 for the portion 33 projecting into the central aperture 30 is larger than the corresponding distances d 2 of the outer portions 33 from the bottom of the focus electrode 26. By varying the distance d, the astigmatism of the focus electrode can be influenced. It is thus possible to choose the distances d of the various portions 33 from the bottom of the focus electrode individually in order to optimize the adjustment of the astigmatism individually for each electron beam. The height of the portions 33 of the plates 31 is designated by the letter b. By varying this height b, the astigmatism of the focus electrode can also be changed. Here, too, it is possible to determine the height b individually for each portion 33 in order to optimize the adjustment of the astigmatism for each electron beam. In the embodiment shown in FIG. 4, the height b 2 of the outer portions 33 is larger than the height b1 of the inside portion 33. The plates 31 described above do not only influence the astigmatism of the focusing lens, but also the other lens aberrations, i.e., the spherical aberration and the further higher-order aberrations. This influence is different for each of the embodiments described above. The higher-order aberrations can be seen mainly at the edge of the picture. They can be minimized by a suitable combination of the plates at the electrodes of the focusing length. It is possible, for example, to distribute the correction to the two focus electrodes or to impress too strong an astigmatism on one of the two focus electrodes, with partial compensation at the other focus electrode.
By the use of the plates 31 described above, it is possible to adjust the astigmatism very finely, thus producing an improved sharpness across the entire screen. By the fine adjustment of the static convergence, which is possible as well, the sharpness can also be improved. Furthermore, the dynamic convergence is improved, too.
Electron-gun system NOKIA GRAETZ ITT CRT Tube.
In a cathode-ray tube with a thick grid No. 2 (24) in the electron-gun system, current transfer into grid No. 2 (24) may result in a lack of picture sharpness. To avoid this error, the aperture (4) in grid No. 2 (24) has a widening (6) of conical shape or stepped diameter.
1. Electron-gun system for cathode-ray tubes comprising at least one cathode and at least three electrodes, the second of which is a screen grid, which are arranged one behind the other and have apertures through each of which an electron beam can pass, characterized in that the aperture (4) in the screen grid (24) has an unwidened part and a conical widening (6) on its side facing the third electrode (25), whereby current transfer into the screen grid and the third electrode is greatly reduced. 2. An electron gun system for cathode ray tubes, comprising:
at least one cathode;
at least three electrodes, said electrodes and said cathode being arranged one behind the other and having apertures through each of which an electron beam can pass, the aperture of the second electrode having a widening on its side facing the third electrode, said widening being conical in shape and extending over part of the depth of the aperture, and that the other part of the depth satisfies the relationship a divided by d is less than or equal to 0.5, where d is the diameter of the unwidened part of the aperture and a is the depth of the unwidened part of the aperture.
3. An electron-gun system as claimed in claim 2, characterized in that on its side facing the third electrode (25), in the area of the opening (4), the second electrode (24) bears a plate (8) containing the conical widening (6). 4. An electron gun system for cathode ray tubes, comprising:
at least one cathode;.
5. An electron gun system for cathode ray tubes, comprising:
at least one cathode;
at least three electrodes, said electrodes and said cathode being arranged one behind the other and having apertures through each of which an electron beam can pass, the apertures of the second electrode having widenings on sides facing the third electrode, each of said widenings being formed by a step wherein the diameter (d1) of the widened part satisfies the relation d1=d0+2ctanα, where d0 is the diameter of the unwidened part of the aperture (4), c is the depth of the widened part, and α≥10°.
6. Electron-gun system for cathode-ray tubes comprising at least one cathode and at least three electrodes, the second of which is a screen grid, which are arranged one behind the other and have apertures defined by cylindrical surfaces through each of which an electron beam can pass, characterized in that the aperture (4) in the screen grid (24) has a conical widening defined by a conical surface contiguous with the cylindrical surface on its side facing the third electrode (25), whereby current transfer into the screen grid and the third electrode is greatly reduced.
Description:
The present invention relates to an electron-gun system for cathode-ray tubes and more particularly, an electron gun system having at least one cathode and at least three electrodes which are arranged one behind the other and have apertures through each of which an electron beam can pass.
Electron-gun systems for cathode-ray tubes comprising a cathode as well as grid and focusing electrodes are known from (DE-OS 32 12 248) corresponding to U.S. Pat. No. 4,682,073. To achieve a thin electron beam and, thus, a small electron spot on the screen of the cathode-ray tube, it is necessary to make grid No. 2 relatively thick. This means that the aperture in grid No. 2 must have a great depth, it being quite possible that the depth of the aperture is equal to the diameter of the aperture.
With such a design of grid No. 2, it may happen that during the period from the turning on of the cathode-ray tube to the creation of stable space-charge conditions around the cathode, the electron beam expands, touching the wall of the aperture in grid No. 2. The electrons touching the wall of the aperture in grid No. 2 cause the emission of secondary electrons which reach grid No. 3, also called "focusing electrode". Such leakage currents are first unmeasurably small, but with increasing service life, measurable currents in the pA range occur at grid Nos. 2 and 3 for short times because due to deposition of evaporated cathode materials into the aperture of grid No. 2, the secondary-electron yield of initially about 1 multiplies. These leakage currents cause a change in the voltage across grid No. 2 - it becomes more positive - and in the voltage across the focusing electrode, which becomes more negative. Due to these changes in potential, the electron beam is not optimally focused for short periods of time, which leads to a lack of picture sharpness. In unfavorable cases, even self-blocking may be caused by total current transfer into grid Nos. 2 and 3.
It is the object of the present invention to provide an electron-gun system for cathode-ray tubes having a thick grid No. 2 in which no lack of picture sharpness is caused by current transfer into grid Nos. 2 and 3.
This object is attained by making the aperture in grid No. 2 so that it becomes wider at its side facing grid No. 3. Further advantageous features of the invention are achieved by making the aperture widening conical in shape, and in particular, that the conical widening extends over part of the depth of the aperture, and that the other part of the depth satisfies the relation a divided by d is less than or equal to 0.5, where d is the diameter of the aperture and a is the depth of the unwidened part of the aperture. Other features of the invention include the widening of the aperture has an angle of at least 10°, and preferably 15°. In another embodiment, the side of grid No. 2 facing grid No. 3 bears a plate containing the conical widening. The widening may also be in the form of a step, wherein the diameter of the widened part between the step and the side of the grid facing grid No. 3 satisfies the relationship d1=d0+2c tanα, where d0 is the diameter of the unwidened part of the aperture, c is the depth of the widened part, and α is greater than or equal to 10°.
Embodiments of the invention will now be explained with reference to the accompanying drawings, in which:
FIG. 1 is a side view of a cathode-ray tube;
FIG. 2 is a side view of an electron-gun system;
FIG. 3 is a cross-sectional view of a first embodiment of a grid No. 2;
FIG. 4 shows the detail Z of FIG. 3;
FIG. 5 is a cross-sectional view of a second embodiment of a grid No. 2;
FIG. 6 is a cross-sectional view of a third embodiment of a grid No 2;
FIGS. 7a and 7b show the details X and Y of FIG. 6;
FIG. 8 is a cross-sectional view of a further embodiment, and
FIG. 9 shows the detail X of FIG. 8.
FIG. 1 shows a cathode-ray tube 10 comprising a screen 11, a funnel section 12, and a neck 13. Th
ere are singlegun and multigun tubes. In multigun tubes, the electron guns are either separate from each other or combined into one mechanical assembly The present invention relates to all these forms of electron-gun systems even though it will be explained as applied to a multibeam electron-gun system of integrated construction.
The neck 13 of the cathode-ray tube 10 houses an electrongun system 14 (indicated by broken lines) which generates three electron beams 1, 2, 3 These beams are scanned (1', 2', 3') across the screen 11 by a magnetic deflection system 15 located in the junction region of the funnel section 12 with the neck 13.
FIG. 2 shows the electron-gun system 14 in a side view. Seen in the beam direction, the system 14 comprises a grid No 1, designated 23, a grid No. 2, 24, first and second focusing electrodes 25 and 26, and a convergence cup 27. Grid No. 1, 23, contains cathodes 22, which are indicated by dashed lines This grid is also called the "control grid", and grid No. 2, 24, the "screen grid". The cathode, the control grid, and the screen grid are referred to as a "triode lens"The focusing electrodes 25, 26 constitute a focusing lens. The individual parts of the system are held together by two glass rods 28 The electrical connections of the system 14 are not shown for the sake of clarity.
All electrodes of the system 14 contain three apertures which are arranged in a horizontal line and through which can pass the electron beams generated by the three cathodes 22, which later land on the phosphor screen 11.
FIG. 3 shows grid
No. 2, 24, in a sectional view. Indicated above this grid is the first focusing electrode 25. In this embodiment, grid No. 2 has the shape of a cup whose bottom 5 contains the aperture 4 for the electron beam. The other apertures for the other electron beams are not visible in this sectional view. The aperture 4 has a great depth, i.e., its diameter d is approximately equal to the thickness of the bottom 5 of the grid. On the side of the grid facing the first focusing electrode 25, the aperture 4 has a widening 6 which is conical in shape.
FIG. 4 shows the detail Z of FIG. 3. The conical widening 6 need not extend over the entire depth of the aperture 4. In the example shown, the aperture 4 has a depth a over which its sidewalls are parallel to the central axis of the aperture 4. This portion is followed by the conical widening 6. The conical widening has an angle α of at least 10°, preferably 15°. For the relation of the depth a of the aperture 4 to the diameter d, the condition a/b≤0.5 should be satisfied.
FIG. 5 shows a second embodiment of grid No. 2. In this embodiment, grid No. 2 is made from thin metal sheet. Here, too, the conical widening 6 includes an angle α of at least 1O°, and the relation a/d≤0.5 is satisfied.
FIG. 6 shows a third embodiment of grid No. 2. It has the shape of a cup, and the bottom 7 of the cup contains the rectangular aperture 4. A plate 8 resting on the bottom 7 contains an aperture aligned with the aperture 4 and having a conical widening 6. This structure of grid No. 2 permits an astigmatic beamforming element in the grid to be combined in a simple manner with the plate 8 containing the conical widening 6.
FIGS. 7a and 7b show the details X and Y, respectively, of FIG. 6. The details X and Y represent two sections through the grid 24 which are displaced relative to each other by 9O°. The plate 8 contains a rotationally symmetric aperture consisting of a cylindrical portion of depth a and the conical widening 6. The widening again has an angle α of at least 1O°. It does not extend over the entire depth of the aperture but
passes into the portion whose depth is designated a and whose sidewalls are parallel to the central axis of the aperture 4. Here, too, the condition a/d≤=0.5 should be satisfied. The depth of the aperture 4 in the bottom 7 is designated by b, the width by e, and the length by f, and this portion of the aperture acts as an astigmatic beam hole.
FIG. 8 shows a further embodiment of grid No. 2. Here, the widening 6 is formed by a step, and its depth is designated c. In this embodiment, too, the grid can have the shape of a cup whose bottom 7 contains the aperture 4. The bottom 7 then bears the plate S, whose aperture is aligned with the aperture 4 and has the diameter d1 (FIG. 9). This diameter is greater than the diameter dO of the aperture in the bottom 7, so that the step is obtained Here, the condition d1=d0+2ctanα should be satisfied, where α≥10°. FIG. 9 shows the detail X of FIG. 8. In this embodiment, too, the bottom 7 may contain a rectangular aperture which acts as an astigmatic beam hole.
ITT NOKIA DIGIVISION 7170 VT CHASSIS DIGI B-E ITT DIGIT2000 CATHODE RAY TUBE (Kinescope) driver with kinescope current sensing circuit:
A television receiver includes a kinescope and a current sensing transistor for conveying amplified video signals to the kinescope, and for providing at a sensing output terminal an output signal related to the magnitude of kinescope current conducted during given sensing intervals. A clamping circuit clamps the sensing output terminal during normal image intervals, and unclamps the sensing output terminal during the sensing intervals. The clamping circuit facilitates interfacing the sensing transistor with utilization circuits which process the sensed output signal, and assists to maintain a proper operating condition for the sensing transistor.
1. In a video signal processing system including an image reproducing device for displaying video information in response to a video signal applied thereto, apparatus comprising:
a video output driver stage with a video signal input and a video signal output for providing an amplified video signal;
means for conveying said amplified video signal to said image reproducing display device, said conveying means having a sensing output for providing thereat a sensed signal representative of the current conducted by said image reproducing display device;
utilization means responsive to said sensed signal; and
cl
amping means for selectively clamping said sensing output during normal image intervals, and for unclamping said sensing output during intervals when said sensed signal representative of current conducted by said image reproducing display device is subject to processing by said utilization means; wherein
said clamping means comprises clamping transistor means with an output first electrode coupled to said sensing output, a second electrode coupled to an operating potential, and an input third electrode coupled to said sensing output, the conduction of said clamping transistor means being controlled in accordance with the magnitude of said sensed signal as received by said third electrode; and
said clamping transistor means is self-keyed to exhibit clamping and non-clamping states in response to said sensed representative signal.
2. Apparatus according to claim 1, wherein:
said video output stage comprises a video amplifier with a video signal input and a video signal output for providing said amplified video signal; and
said conveying means comprises an active current conducting device with an input first terminal for receiving said amplified video signal, an output second terminal for conveying said amplified video signal to said image reproducing display device, and a third terminal for providing said sensed signal.
3. Apparatus according to claim 2, wherein
said active current conducting device is a transistor with a base input for receiving said amplified video signal, an emitter output for providing said amplified video signal to said image reproducing display device, and a collector output for providing said sensed signal.
4. Apparatus according to claim 1, wherein
said first and second electrodes define a main current conduction path of said clamping transistor means.
5. Apparatus according to claim 4, wherein
said clamping means includes resistive means coupled to said sensing output for providing a voltage in accordance with the magnitude of said sensed signal; and
said third electrode of said clamping transistor me
ans is coupled to said resistive means.
6. Apparatus according to claim 1, and further comprising
filter means for bypassing high frequency signal components at said sensing output.
7. In a video signal processing system including an image reproducing device for displaying video information in response to a video signal applied thereto, apparatus comprising:
a video output driver stage coupled to said image reproducing display device for providing an amplified video signal thereto, and having a sensing output for providing thereat a sensed signal representative of the current conducted by said image reproducing display device;
control means responsive to said sensed signal for developing a control signal;
means for coupling said control signal to said image reproducing display device to maintain a desired conduction characteristic of said image reproducing display device; and
clamping means for selectively clamping said sensing output during normal image intervals, and for unclamping said sensing output during intervals when said control means operates to monitor said sensed signal; wherein
said clamping means comprises clamping transistor means with an output first electrode coupled to said sensing output, a second electrode coupled to an operating potential, and an input third electrode coupled to said sensing output, the conduction of said clamping transistor means being controlled in accordance with the magnitude of said sensed signal as received by said third electrode; and
said clamping transistor means is self-keyed to exhibit clamping and non-clamping states in response to said sensed signal.
8. Apparatus according to claim 7, wherein
said control means includes digital signal processing circuits; and
said control means includes an input analog-to-digital signal converter network.
9. In a video signal processing
system including an image reproducing device for displaying video information in response to a video signal applied thereto, apparatus comprising:
a video amplifier with a video signal input for receiving video signals, and a video signal output for providing an amplified video signal;
a signal coupling transistor with an input first electrode for receiving said amplified video signal from said video amplifier, an output second electrode for providing a further amplified video signal to said image reproducing display device, and a third electrode for providing a sensed signal representative of the magnitude of the current conducted by said image reproducing display device;
utilization means responsive to said sensed signal; and
clamping means for selectively clamping said third electrode of said coupling transistor during normal image intervals, and for unclamping said third electrode during interval when said sensed representative signal is subject to processing by said utilization means, said clamping means comprising clamping transistor means with an output first electrode coupled to said third electrode of said signal coupling transistor, a second electrode coupled to an operating potential, and an input third electrode coupled to said third electrode of said signal coupling transistor, the conduction of said clamping transistor means being controlled in accordance with the magnitude of said sensed signal as received by said input third electrode of said clamping transistor means.
10. Apparatus according to claim 9, wherein
said coupling transistor is an emitter follower transistor with a base input electrode, an emitter output electrode, and a collector output electrode corresponding to said third electrode.
Electron-gun systems for cathode-ray tubes comprising a cathode as well as grid and focusing electrodes are known from (DE-OS 32 12 248) corresponding to U.S. Pat. No. 4,682,073. To achieve a thin electron beam and, thus, a small electron spot on the screen of the cathode-ray tube, it is necessary to make grid No. 2 relatively thick. This means that the aperture in grid No. 2 must have a great depth, it being quite possible that the depth of the aperture is equal to the diameter of the aperture.
With such a design of grid No. 2, it may happen that during the period from the turning on of the cathode-ray tube to the creation of stable space-charge conditions around the cathode, the electron beam expands, touching the wall of the aperture in grid No. 2. The electrons touching the wall of the aperture in grid No. 2 cause the emission of secondary electrons which reach grid No. 3, also called "focusing electrode". Such leakage currents are first unmeasurably small, but with increasing service life, measurable currents in the pA range occur at grid Nos. 2 and 3 for short times because due to deposition of evaporated cathode materials into the aperture of grid No. 2, the secondary-electron yield of initially about 1 multiplies. These leakage currents cause a change in the voltage across grid No. 2 - it becomes more positive - and in the voltage across the focusing electrode, which becomes more negative. Due to these changes in potential, the electron beam is not optimally focused for short periods of time, which leads to a lack of picture sharpness. In unfavorable cases, even self-blocking may be caused by total current transfer into grid Nos. 2 and 3.
It is the object of the present invention to provide an electron-gun system for cathode-ray tubes having a thick grid No. 2 in which no lack of picture sharpness is caused by current transfer into grid Nos. 2 and 3.
This object is attained by making the aperture in grid No. 2 so that it becomes wider at its side facing grid No. 3. Further advantageous features of the invention are achieved by making the aperture widening conical in shape, and in particular, that the conical widening extends over part of the depth of the aperture, and that the other part of the depth satisfies the relation a divided by d is less than or equal to 0.5, where d is the diameter of the aperture and a is the depth of the unwidened part of the aperture. Other features of the invention include the widening of the aperture has an angle of at least 10°, and preferably 15°. In another embodiment, the side of grid No. 2 facing grid No. 3 bears a plate containing the conical widening. The widening may also be in the form of a step, wherein the diameter of the widened part between the step and the side of the grid facing grid No. 3 satisfies the relationship d1=d0+2c tanα, where d0 is the diameter of the unwidened part of the aperture, c is the depth of the widened part, and α is greater than or equal to 10°.
Embodiments of the invention will now be explained with reference to the accompanying drawings, in which:
FIG. 1 is a side view of a cathode-ray tube;
FIG. 2 is a side view of an electron-gun system;
FIG. 3 is a cross-sectional view of a first embodiment of a grid No. 2;
FIG. 4 shows the detail Z of FIG. 3;
FIG. 5 is a cross-sectional view of a second embodiment of a grid No. 2;
FIG. 6 is a cross-sectional view of a third embodiment of a grid No 2;
FIGS. 7a and 7b show the details X and Y of FIG. 6;
FIG. 8 is a cross-sectional view of a further embodiment, and
FIG. 9 shows the detail X of FIG. 8.
FIG. 1 shows a cathode-ray tube 10 comprising a screen 11, a funnel section 12, and a neck 13. Th
ere are singlegun and multigun tubes. In multigun tubes, the electron guns are either separate from each other or combined into one mechanical assembly The present invention relates to all these forms of electron-gun systems even though it will be explained as applied to a multibeam electron-gun system of integrated construction. The neck 13 of the cathode-ray tube 10 houses an electrongun system 14 (indicated by broken lines) which generates three electron beams 1, 2, 3 These beams are scanned (1', 2', 3') across the screen 11 by a magnetic deflection system 15 located in the junction region of the funnel section 12 with the neck 13.
FIG. 2 shows the electron-gun system 14 in a side view. Seen in the beam direction, the system 14 comprises a grid No 1, designated 23, a grid No. 2, 24, first and second focusing electrodes 25 and 26, and a convergence cup 27. Grid No. 1, 23, contains cathodes 22, which are indicated by dashed lines This grid is also called the "control grid", and grid No. 2, 24, the "screen grid". The cathode, the control grid, and the screen grid are referred to as a "triode lens"The focusing electrodes 25, 26 constitute a focusing lens. The individual parts of the system are held together by two glass rods 28 The electrical connections of the system 14 are not shown for the sake of clarity.
All electrodes of the system 14 contain three apertures which are arranged in a horizontal line and through which can pass the electron beams generated by the three cathodes 22, which later land on the phosphor screen 11.
FIG. 3 shows grid
No. 2, 24, in a sectional view. Indicated above this grid is the first focusing electrode 25. In this embodiment, grid No. 2 has the shape of a cup whose bottom 5 contains the aperture 4 for the electron beam. The other apertures for the other electron beams are not visible in this sectional view. The aperture 4 has a great depth, i.e., its diameter d is approximately equal to the thickness of the bottom 5 of the grid. On the side of the grid facing the first focusing electrode 25, the aperture 4 has a widening 6 which is conical in shape. FIG. 4 shows the detail Z of FIG. 3. The conical widening 6 need not extend over the entire depth of the aperture 4. In the example shown, the aperture 4 has a depth a over which its sidewalls are parallel to the central axis of the aperture 4. This portion is followed by the conical widening 6. The conical widening has an angle α of at least 10°, preferably 15°. For the relation of the depth a of the aperture 4 to the diameter d, the condition a/b≤0.5 should be satisfied.
FIG. 5 shows a second embodiment of grid No. 2. In this embodiment, grid No. 2 is made from thin metal sheet. Here, too, the conical widening 6 includes an angle α of at least 1O°, and the relation a/d≤0.5 is satisfied.
FIG. 6 shows a third embodiment of grid No. 2. It has the shape of a cup, and the bottom 7 of the cup contains the rectangular aperture 4. A plate 8 resting on the bottom 7 contains an aperture aligned with the aperture 4 and having a conical widening 6. This structure of grid No. 2 permits an astigmatic beamforming element in the grid to be combined in a simple manner with the plate 8 containing the conical widening 6.
FIGS. 7a and 7b show the details X and Y, respectively, of FIG. 6. The details X and Y represent two sections through the grid 24 which are displaced relative to each other by 9O°. The plate 8 contains a rotationally symmetric aperture consisting of a cylindrical portion of depth a and the conical widening 6. The widening again has an angle α of at least 1O°. It does not extend over the entire depth of the aperture but
passes into the portion whose depth is designated a and whose sidewalls are parallel to the central axis of the aperture 4. Here, too, the condition a/d≤=0.5 should be satisfied. The depth of the aperture 4 in the bottom 7 is designated by b, the width by e, and the length by f, and this portion of the aperture acts as an astigmatic beam hole. FIG. 8 shows a further embodiment of grid No. 2. Here, the widening 6 is formed by a step, and its depth is designated c. In this embodiment, too, the grid can have the shape of a cup whose bottom 7 contains the aperture 4. The bottom 7 then bears the plate S, whose aperture is aligned with the aperture 4 and has the diameter d1 (FIG. 9). This diameter is greater than the diameter dO of the aperture in the bottom 7, so that the step is obtained Here, the condition d1=d0+2ctanα should be satisfied, where α≥10°. FIG. 9 shows the detail X of FIG. 8. In this embodiment, too, the bottom 7 may contain a rectangular aperture which acts as an astigmatic beam hole.
ITT NOKIA DIGIVISION 7170 VT CHASSIS DIGI B-E ITT DIGIT2000 CATHODE RAY TUBE (Kinescope) driver with kinescope current sensing circuit:
A television receiver includes a kinescope and a current sensing transistor for conveying amplified video signals to the kinescope, and for providing at a sensing output terminal an output signal related to the magnitude of kinescope current conducted during given sensing intervals. A clamping circuit clamps the sensing output terminal during normal image intervals, and unclamps the sensing output terminal during the sensing intervals. The clamping circuit facilitates interfacing the sensing transistor with utilization circuits which process the sensed output signal, and assists to maintain a proper operating condition for the sensing transistor.
1. In a video signal processing system including an image reproducing device for displaying video information in response to a video signal applied thereto, apparatus comprising:
a video output driver stage with a video signal input and a video signal output for providing an amplified video signal;
means for conveying said amplified video signal to said image reproducing display device, said conveying means having a sensing output for providing thereat a sensed signal representative of the current conducted by said image reproducing display device;
utilization means responsive to said sensed signal; and
cl
amping means for selectively clamping said sensing output during normal image intervals, and for unclamping said sensing output during intervals when said sensed signal representative of current conducted by said image reproducing display device is subject to processing by said utilization means; wherein said clamping means comprises clamping transistor means with an output first electrode coupled to said sensing output, a second electrode coupled to an operating potential, and an input third electrode coupled to said sensing output, the conduction of said clamping transistor means being controlled in accordance with the magnitude of said sensed signal as received by said third electrode; and
said clamping transistor means is self-keyed to exhibit clamping and non-clamping states in response to said sensed representative signal.
2. Apparatus according to claim 1, wherein:
said video output stage comprises a video amplifier with a video signal input and a video signal output for providing said amplified video signal; and
said conveying means comprises an active current conducting device with an input first terminal for receiving said amplified video signal, an output second terminal for conveying said amplified video signal to said image reproducing display device, and a third terminal for providing said sensed signal.
3. Apparatus according to claim 2, wherein
said active current conducting device is a transistor with a base input for receiving said amplified video signal, an emitter output for providing said amplified video signal to said image reproducing display device, and a collector output for providing said sensed signal.
4. Apparatus according to claim 1, wherein
said first and second electrodes define a main current conduction path of said clamping transistor means.
5. Apparatus according to claim 4, wherein
said clamping means includes resistive means coupled to said sensing output for providing a voltage in accordance with the magnitude of said sensed signal; and
said third electrode of said clamping transistor me
ans is coupled to said resistive means.6. Apparatus according to claim 1, and further comprising
filter means for bypassing high frequency signal components at said sensing output.
7. In a video signal processing system including an image reproducing device for displaying video information in response to a video signal applied thereto, apparatus comprising:
a video output driver stage coupled to said image reproducing display device for providing an amplified video signal thereto, and having a sensing output for providing thereat a sensed signal representative of the current conducted by said image reproducing display device;
control means responsive to said sensed signal for developing a control signal;
means for coupling said control signal to said image reproducing display device to maintain a desired conduction characteristic of said image reproducing display device; and
clamping means for selectively clamping said sensing output during normal image intervals, and for unclamping said sensing output during intervals when said control means operates to monitor said sensed signal; wherein
said clamping means comprises clamping transistor means with an output first electrode coupled to said sensing output, a second electrode coupled to an operating potential, and an input third electrode coupled to said sensing output, the conduction of said clamping transistor means being controlled in accordance with the magnitude of said sensed signal as received by said third electrode; and
said clamping transistor means is self-keyed to exhibit clamping and non-clamping states in response to said sensed signal.
8. Apparatus according to claim 7, wherein
said control means includes digital signal processing circuits; and
said control means includes an input analog-to-digital signal converter network.
9. In a video signal processing
system including an image reproducing device for displaying video information in response to a video signal applied thereto, apparatus comprising: a video amplifier with a video signal input for receiving video signals, and a video signal output for providing an amplified video signal;
a signal coupling transistor with an input first electrode for receiving said amplified video signal from said video amplifier, an output second electrode for providing a further amplified video signal to said image reproducing display device, and a third electrode for providing a sensed signal representative of the magnitude of the current conducted by said image reproducing display device;
utilization means responsive to said sensed signal; and
clamping means for selectively clamping said third electrode of said coupling transistor during normal image intervals, and for unclamping said third electrode during interval when said sensed representative signal is subject to processing by said utilization means, said clamping means comprising clamping transistor means with an output first electrode coupled to said third electrode of said signal coupling transistor, a second electrode coupled to an operating potential, and an input third electrode coupled to said third electrode of said signal coupling transistor, the conduction of said clamping transistor means being controlled in accordance with the magnitude of said sensed signal as received by said input third electrode of said clamping transistor means.
10. Apparatus according to claim 9, wherein
said coupling transistor is an emitter follower transistor with a base input electrode, an emitter output electrode, and a collector output electrode corresponding to said third electrode.
Description:
This invention concerns a video output display driver amplifier for supplying high level video output signals to an image display device such as a kinescope in a television receiver. In particular, this invention concerns a display driver stage associated with a sensing circuit for providing a signal representative of the magnitude of current conducted by the kinescope during prescribed intervals.
Video signal processing and display systems such as television receivers commonly include a video output display driver stage for supplying a high level video signal to an intensity control electrode, e.g., a cathode electrode, of an image display device such as a kinescope. Television receivers sometimes employ an automatic black current (bias) control system or an automatic white current (drive) control system for maintaining desired kinescope operating current levels. Such control systems typically operate during image blanking intervals, at which time the kinescope is caused to conduct a black image or a white image representative current. Such current is sensed by the control system, which generates a correction signal representing the difference between the magnitude of the sensed representative current and a desired current level. The correction signal is applied to video signal processing circuits for reducing the difference.
Various techniques are known for sensing the magnitude of the black or white kinescope current. One often used approach employs a PNP emitter follower current sensing transistor connected to the kinescope cathode signal coupling path. Such sensing transistor couples video signals to the kinescope via its base-to-emitter junction, and provides at a collector electrode a sensed current representative of the magnitude of the kinescope cathode current. The representative current from the collector electrode of the sensing transistor is conveyed to the control system and processed to develop a suitable correction signal.
In accordance with the principles of the present invention, there is disclosed a kinescope current sensing arrangement wherein a current sensing device is coupled to a kinescope for providing at an output terminal a signal representative of the magnitude of the kinescope current. A clamping circuit clamps the output terminal to a given voltage during normal image trace intervals. During prescribed kinescope current sensing intervals, however, the clamping circuit is inoperative and the sensed signal representative of the kinescope current is developed at the output terminal. The clamping circuit advantageously facilitates interfacing the current sensing device with control circuits for processing the sensed signal, and assists to maintain a proper operating condition for the current sensing device which, in a disclosed embodiment, also conveys video signals to the display device. In accordance with a feature of the invention, the clamping circuit is self-keyed between clamping and non-clamping states in response to the representative signal at the output terminal.
In the drawing:
FIG. 1 shows a circuit diagram of a kinescope driver stage with associated kinescope current sensing and clamping apparatus in accordance with the present invention; and
FIG. 2 depicts, in block diagram form, a portion of a color television receiver incorporating the current sensing and clamping apparatus of FIG. 1.
In FIG. 1, low level color image representative video signals r, g, b are provided by a source 10. The r,g and b color signals are coupled to similar kinescope driver stages. Only the red (r) color signal video driver stage is shown in schematic circuit diagram form.
Red kinescope driver stage 15 comprises a driver amplifier including an input common emitter amplifier transistor 20 arranged in a cascode amplifier configuration with a common base amplifier transistor 21. Red color signal r is coupled to the base input of transistor 20 via a current determining resistor 22. Base bias for transistor 20 is provided by a resistor 24 in association with a source of negative DC voltage (-V). Base bias for transistor 21 is provided from a source of positive DC voltage (+V) through a resistor 25. Resistor 25 in the base circuit of transistor 21 assists to stabilize transistor 21 against oscillation.
The output circuit of driver stage 15 includes a load resistor 27 in the collector output circuit of transistor 21 and across which a high level amplified video signal is developed, and opposite conductivity type emitter follower transistors 30 and 31 with base inputs coupled to the collector of transistor 21. A high level amplified video signal R is developed at the emitter output of follower transistor 30 and is coupled to a cathode electrode of an image reproducing kinescope via a kinescope arc current limiting resistor 33. A resistor 34 in the collector circuit of transistor 31 also serves as a kinescope arc current limiting resistor. Degenerative feedback for driver stage 15 is provided by series resistors 36 and 38, coupled from the emitter of transistor 31 to the base of transistor 20.
A diode 39 connected between the emitters of transistors 30 and 31 as shown is normally reverse biased and therefore nonconductive by the voltage difference across it equalling the sum of the two base-emitter voltage drops of transistors 30 and 31, but is forward biased and therefore rendered conductive under certain conditions in response to positive-going transients at the emitter of transistor 30, corresponding to the output terminal of driver stage 15. The arrangement of transistor 31 prevents the amplifier feedback loop including transistors 20, 21 and 31 and resistors 36 and 38 from being disrupted, thereby preventing feedback transients and signal ringing from occurring. Additional details of the arrangement including transistors 30 and 31 and diode 39 are found in my copending U.S. patent application Ser. No. 758,954 titled "FEEDBACK DISPLAY DRIVER STAGE".
The emitter voltage of transistor 30 follows the voltage developed across load resistor 27, and transistor 30 conducts the kinescope cathode current. Substantially all of the kinescope cathode current flows as collector current of transistor 30, through a kinescope arc current limiting protection resistor 37a, to a clamping network 40. Transistor 30 acts as a current sensing device in conjunction with network 40 as will be explained. Clamping network 40 in this example is self-keyed to exhibit clamping and non-clamping states in response to the magnitude of the current conducted by transistor 30.
Clamping network 40 is common to all three driver stages of the receiver, as will be seen subsequently in connection with FIG. 2, and is coupled to the green and blue signal driver stages via protection resistors 37b and 37c. Network 40 includes clamping transistors 41 and 42 arranged in a Darlington configuration, and series voltage divider resistors 43 and 44 which bias clamp transistors 41 and 42. A high frequency bypass capacitor 46 filters signals in the collector circuit of transistor 30 in a manner to be described below. The series combination of a mode control switch 49 and a scaling resistor 48 is coupled across resistors 43 and 44. A voltage related to the magnitude of kinescope current is developed at a terminal A and, as will be explained with reference to FIG. 2, the voltage at terminal A can be used in conjunction with a feedback control loop to maintain a desired kinescope operating current condition which is otherwise subject to deterioration due to kinescope aging and temperature effects, for example.
Assuming switch 49, the function of which will be explained below, is open, the kinescope cathode current flowing in the collector of transistor 30 is conducted to ground via resistors 43 and 44. When this current causes a voltage drop across resistor 44 to sufficiently forward bias the base-emitter junctions of transistors 41 and 42, transistor 42 will conduct in a linear region, and will clamp terminal A to a voltage VA according to the following expression, where V BE41 and V BE42 are the base-emitter junction voltage drops of transistors 41 and 42: VA=(V BE41 +V BE42 ) (R43+R44)/R44
During normal image intervals typically there are greater than approximately 25 microamperes of current conducted by transistor 30, which is sufficient to render transistors 41 and 42 conductive for developing clamping voltage VA at terminal A. At other times, as will be discussed, transistors 41 and 42 are rendered nonconductive whereby clamping action is inhibited and a (variable) voltage is developed at node A as a function of the magnitude of the kinescope cathode current, for processing by succeeding control circuits.
Illustratively, the arrangement of FIG. 1 can be used in connection with digital signal processing and control circuits in a color television receiver employing digital signal processing techniques, as will be seen in FIG. 2. Such control circuits include an input analog-to-digital converter (ADC) for converting analog voltages developed at terminal A to digital form for processing.
When the control circuits are to operate in an automatic kinescope black current (bias) control mode, wherein during image blanking intervals the kinescope conducts very small cathode currents on the order of a few microamperes, approximating a kinescope black image condition, clamp transistors 41 and 42 are rendered nonconductive because such small currents flowing through resistors 43 and 44 from the collector of transistor 30 are unable to produce a large enough voltage drop across resistor 44 to forward bias transistors 41 and 42. Consequently terminal A exhibits voltage variations, as developed across resistors 43 and 44, related to the magnitude of kinescope black current. The voltage variations are processed by the control circuits coupled to terminal A to develop a correction signal, if necessary, to maintain a desired level of kinescope black current conduction by feedback action. In this operating mode switch 49, e.g., a controlled electronic switch, is maintained in an open position as shown in response to a timing signal VT developed by the control circuits.
When the control circuits are to operate in an automatic kinescope white current (drive) control mode wherein during image blanking intervals the kinescope conducts much larger currents representing a white image condition, switch 49 closes in response to timing signal VT, thereby shunting resistor 48 across resistors 43 and 44. The value of resistor 48 is chosen relative to the combined values of resistors 43 and 44 so that the larger current conducted via the collector of transistor 30 divides between series resistors 43, 44 and resistor 48 such that the magnitude of current conducted by resistors 43 and 44 is insufficient to produce a large enough voltage drop across resistor 44 to render clamping transistors 43 and 44 conductive. Unclamped terminal A therefore exhibits voltage variations related to the magnitude of kinescope white current, which voltage variations are processed by the control circuits to develop a correction signal as required. As used herein, the expression "white current" refers to a high level of individual red, green or blue color image current, or to combined high level red, green and blue currents associated with a white image.
With the illustrated configuration of transistors 41 and 42 clamping voltage VA is relatively low, approximately +2.0 volts. The clamping voltage could be provided by a Zener diode rather than the disclosed arrangement of Darlington-connected transistors 41 and 42, but the disclosed clamping arrangement is preferred because Zener diodes with a voltage rating less than about 4 volts usually do not exhibit a predictable Zener threshold voltage characteristic, i.e., the "knee" transition region of the Zener voltage-vs-current characteristic is usually not very well defined. In addition, the disclosed transistor clamp operates with better linearity than a Zener diode clamp and radiates less radio frequency interference (RFI).
The relatively low clamping voltage is compatible with the analog input voltage requirements of the analog-to-digital converter (ADC) at the input of the control circuits which receive the sensed voltage at terminal A as will be explained in greater detail with respect to FIG. 2. In this example the ADC is intended to process analog voltages of from 0 volts to approximately +2.5 volts, and the clamping voltage assures that excessively high analog voltages are not presented to the ADC during normal video signal intervals.
The relatively low clamping voltage also assists to prevent transistor 30 from saturating, which is necessary since transistor 30 is intended to operate in a linear region. To achieve this result and to maximize the cathode current conduction capability of transistor 30, the clamping voltage should be as low as possible to maintain a suitably low bias voltage at the collector of transistor 30. On the other hand, the value of arc current limiting resistor 37a should be large enough to provide adequate arc protection without compromising the objective of maintaining the collector bias voltage of transistor 30 as low as possible. Operation of transistor 30 in a saturated state renders transistor 30 ineffective for its intended purpose of properly conveying video drive signals to the kinescope cathode, and for conveying accurate representations of cathode current to clamping network 40 particularly in the white current control mode when relatively high cathode current levels are sensed. In addition, undesirable radio frequency interference (RFI) can be generated by transistor 30 switching into and out of saturation. Also, when saturation occurs transistor base storage effects can result in video image streaking due to the time required for a transistor to come out of a saturated state.
Thus clamping network 40 advantageously limits the voltage at terminal A to a level tolerable by the analog-to-digital converter at the input of the control circuits coupled to terminal A, and protects the analog-to-digital converter input from damage due to signal overdrive. Network 40 also provides a collector reference bias for transistor 30 to prevent transistor 30 from saturating on large negative-going signal amplitude transitions at its emitter electrode. The clamping voltage level is readily adjusted simply by tailoring the values of resistors 43 and 44.
Capacitor 46 bypasses high frequency video signals to ground to prevent transistor 30 from saturating in response to such signals. Capacitor 46 also serves to smooth out undesirable high frequency variations at terminal A to prevent potentially troublesome signal components such as noise from interfering with the signal processing function of the input analog-to-digital converter of the control circuits, e.g., by smoothing the current sensed during the settling time of the analog-to-digital converter.
The latter noise reducing effect is particularly desirable, for example, when the input ADC of the control circuits coupled to terminal A is of the relatively inexpensive and uncomplicated "iterative approximation" type ADC, compared to a "flash" type ADC. The operation of an iterative ADC, wherein successive approximations are made from the most significant bit to the least significant bit, requires a relatively constant or slowly varying analog signal to be sampled during sampling intervals, uncontaminated by noise and similar effects.
The value of capacitor 46 should not be excessively large because a certain rate of current variation should be permitted at terminal A with respect to kinescope cathode currents being sensed. If the value of capacitor 46 is too small, excessive voltage variations, particularly high frequency video signal variations, will appear at terminal A, increasing the likelihood of transistor 30 saturating. The speed of operation of the clamp circuit itself is restricted by an RC low pass filter effect produced by the base capacitance of transistor 41 and the equivalent resistance of resistors 43 and 44.
FIG. 2 shows a portion of a color television receiver system employing digital video signal processing techniques. The FIG. 2 system utilizes kinescope driver amplifiers and a clamping network as disclosed in FIG. 1, wherein similar elements are identified by the same reference number. By way of example, the system of FIG. 2 includes a MAA 2100 VCU (Video Codec Unit) corresponding to video signal source 10 of FIG. 1, a MAA 2200 VPU (Video Processor Unit) 50, and a MAAA 2000 CCU (Central Control Unit) 60. The latter three units are associated with a digital television signal processing system offered by ITT Corporation as described in a technical bulletin titled "DIGIT 2000 VLSI DIGITAL TV SYSTEM" published by the Intermetall Semiconductors subsidiary of ITT Corporation.
In unit 10, a luminance signal and color difference signals in digital form are respectively converted to analog form by means of digital-to-analog converters (DACs) 70 and 71. The analog luminance signal (Y) and analog color difference signals r-y and b-y are combined in a matrix amplifier 73 to produce r, g and b color image representative signals which are processed by preamplifiers 75, 76 and 77, respectively, before being coupled to kinescope driver stages 15, 16 and 17 of the type shown in FIG. 1. A network 78 in unit 10 includes circuits associated with the automatic white current and black current control functions.
The high level R, G and B color signals from driver stages 15, 16 and 17 are coupled via respective current limiting resistors (i.e., resistor 33) to cathode intensity control electrodes of a color kinescope 80. Currents conducted by the red, green and blue kinescope cathodes are conveyed to network 40 via resistors 37a-37c, for producing at terminal A a voltage representative of kinescope cathode current conducted during measuring intervals, as discussed previously.
VPU unit 50 includes input terminals 15 and 16 coupled to terminal A. Through terminal 15 the VPU receives the analog signal from terminal A and, via an internal multiplex switching network 51, the analog signal is supplied to an analog-to-digital-converter (ADC) 52. Terminal 16 is connected to an internal switching device (corresponding to switch 49 in FIG. 1) which, in conjunction with scaling resistor 48, controls the impedance and therefore the sensitivity at input terminal 15. High sensitivity for black current measurement is obtained with resistor 48 ungrounded by internal switch 49, and low sensitivity for white current measurement is obtained with resistor 48 grounded by internal switch 49.
The digital signal from ADC 52 is coupled to an IM BUS INTERFACE unit 53 which coacts with CCU unit 60and provides signals to an output data multiplex (MPX) unit 55. Multiplexed output signal data from unit 55 is conveyed to VCU unit 10, and particularly to control network 78. Control network 78 provides output signals for controlling the signal gain of preamplifiers 75, 76 and 77 to achieve a correct white current condition, and also provides output signals for controlling the DC bias of the preamplifiers to achieve a correct black current condition.
More specifically, during vertical image blanking intervals the three (red, green, blue) kinescope black currents subject to measurement and the three white currents subject to measurement are developed sequentially, sensed, and coupled to VPU 50 via terminal 15. The sensed values are sequenced, digitized and coupled to IM Bus Interface 53 which organizes the data communication with CCU 60. After being processed by CCU 60, control signals are routed back to interface 53 and from there to data multiplexer 55 which forwards the control signals to VCU 10.
Video signal processing and display systems such as television receivers commonly include a video output display driver stage for supplying a high level video signal to an intensity control electrode, e.g., a cathode electrode, of an image display device such as a kinescope. Television receivers sometimes employ an automatic black current (bias) control system or an automatic white current (drive) control system for maintaining desired kinescope operating current levels. Such control systems typically operate during image blanking intervals, at which time the kinescope is caused to conduct a black image or a white image representative current. Such current is sensed by the control system, which generates a correction signal representing the difference between the magnitude of the sensed representative current and a desired current level. The correction signal is applied to video signal processing circuits for reducing the difference.
Various techniques are known for sensing the magnitude of the black or white kinescope current. One often used approach employs a PNP emitter follower current sensing transistor connected to the kinescope cathode signal coupling path. Such sensing transistor couples video signals to the kinescope via its base-to-emitter junction, and provides at a collector electrode a sensed current representative of the magnitude of the kinescope cathode current. The representative current from the collector electrode of the sensing transistor is conveyed to the control system and processed to develop a suitable correction signal.
In accordance with the principles of the present invention, there is disclosed a kinescope current sensing arrangement wherein a current sensing device is coupled to a kinescope for providing at an output terminal a signal representative of the magnitude of the kinescope current. A clamping circuit clamps the output terminal to a given voltage during normal image trace intervals. During prescribed kinescope current sensing intervals, however, the clamping circuit is inoperative and the sensed signal representative of the kinescope current is developed at the output terminal. The clamping circuit advantageously facilitates interfacing the current sensing device with control circuits for processing the sensed signal, and assists to maintain a proper operating condition for the current sensing device which, in a disclosed embodiment, also conveys video signals to the display device. In accordance with a feature of the invention, the clamping circuit is self-keyed between clamping and non-clamping states in response to the representative signal at the output terminal.
In the drawing:
FIG. 1 shows a circuit diagram of a kinescope driver stage with associated kinescope current sensing and clamping apparatus in accordance with the present invention; and
FIG. 2 depicts, in block diagram form, a portion of a color television receiver incorporating the current sensing and clamping apparatus of FIG. 1.
In FIG. 1, low level color image representative video signals r, g, b are provided by a source 10. The r,
g and b color signals are coupled to similar kinescope driver stages. Only the red (r) color signal video driver stage is shown in schematic circuit diagram form. Red kinescope driver stage 15 comprises a driver amplifier including an input common emitter amplifier transistor 20 arranged in a cascode amplifier configuration with a common base amplifier transistor 21. Red color signal r is coupled to the base input of transistor 20 via a current determining resistor 22. Base bias for transistor 20 is provided by a resistor 24 in association with a source of negative DC voltage (-V). Base bias for transistor 21 is provided from a source of positive DC voltage (+V) through a resistor 25. Resistor 25 in the base circuit of transistor 21 assists to stabilize transistor 21 against oscillation.
The output circuit of driver stage 15 includes a load resistor 27 in the collector output circuit of transistor 21 and across which a high level amplified video signal is developed, and opposite conductivity type emitter follower transistors 30 and 31 with base inputs coupled to the collector of transistor 21. A high level amplified video signal R is developed at the emitter output of follower transistor 30 and is coupled to a cathode electrode of an image reproducing kinescope via a kinescope arc current limiting resistor 33. A resistor 34 in the collector circuit of transistor 31 also serves as a kinescope arc current limiting resistor. Degenerative feedback for driver stage 15 is provided by series resistors 36 and 38, coupled from the emitter of transistor 31 to the base of transistor 20.
A diode 39 connected between the emitters of transistors 30 and 31 as shown is normally reverse biased and therefore nonconductive by the voltage difference across it equalling the sum of the two base-emitter voltage drops of transistors 30 and 31, but is forward biased and therefore rendered conductive under certain conditions in response to positive-going transients at the emitter of transistor 30, corresponding to the output terminal of driver stage 15. The arrangement of transistor 31 prevents the amplifier feedback loop including transistors 20, 21 and 31 and resistors 36 and 38 from being disrupted, thereby preventing feedback transients and signal ringing from occurring. Additional details of the arrangement including transistors 30 and 31 and diode 39 are found in my copending U.S. patent application Ser. No. 758,954 titled "FEEDBACK DISPLAY DRIVER STAGE".
The emitter voltage of transistor 30 follows the voltage developed across load resistor 27, and transistor 30 conducts the kinescope cathode cur
rent. Substantially all of the kinescope cathode current flows as collector current of transistor 30, through a kinescope arc current limiting protection resistor 37a, to a clamping network 40. Transistor 30 acts as a current sensing device in conjunction with network 40 as will be explained. Clamping network 40 in this example is self-keyed to exhibit clamping and non-clamping states in response to the magnitude of the current conducted by transistor 30. Clamping network 40 is common to all three driver stages of the receiver, as will be seen subsequently in connection with FIG. 2, and is coupled to the green and blue signal driver stages via protection resistors 37b and 37c. Network 40 includes clamping transistors 41 and 42 arranged in a Darlington configuration, and series voltage divider resistors 43 and 44 which bias clamp transistors 41 and 42. A high frequency bypass capacitor 46 filters signals in the collector circuit of transistor 30 in a manner to be described below. The series combination of a mode control switch 49 and a scaling resistor 48 is coupled across resistors 43 and 44. A voltage related to the magnitude of kinescope current is developed at a terminal A and, as will be explained with reference to FIG. 2, the voltage at terminal A can be used in conjunction with a feedback control loop to maintain a desired kinescope operating current condition which is otherwise subject to deterioration due to kinescope aging and temperature effects, for example.
Assuming switch 49, the function of which will be explained below, is open, the kinescope cathode current flowing in the collector of transistor 30 is conducted to ground via resistors 43 and 44. When this current causes a voltage drop across resistor 44 to sufficiently forward bias the base-emitter junctions of transistors 41 and 42, transistor 42 will conduct in a linear region, and will clamp terminal A to a voltage VA according to the following expression, where V BE41 and V BE42 are the base-emitter junction voltage drops of transistors 41 and 42: VA=(V BE41 +V BE42 ) (R43+R44)/R44
During normal image intervals typically there are greater than approximately 25 microamperes of current conducted by transistor 30, which is sufficient to render transistors 41 and 42 conductive for developing clamping voltage VA at terminal A. At other times, as will be discussed, transistors 41 and 42 are rendered nonconductive whereby clamping action is inhibited and a (variable) voltage is developed at node A as a function of the magnitude of the kinescope cathode current, for processing by succeeding control circuits.
Illustratively, the arrangement of FIG. 1 can be used in connection with digital signal processing and control circuits in a color television receiver employing digital signal processing techniques, as will be seen in FIG. 2. Such control circuits include an input analog-to-digital converter (ADC) for converting analog voltages developed at terminal A to digital form for processing.
When the control circuits are to operate in an automatic kinescope black current (bias) control mode, wherein during i
mage blanking intervals the kinescope conducts very small cathode currents on the order of a few microamperes, approximating a kinescope black image condition, clamp transistors 41 and 42 are rendered nonconductive because such small currents flowing through resistors 43 and 44 from the collector of transistor 30 are unable to produce a large enough voltage drop across resistor 44 to forward bias transistors 41 and 42. Consequently terminal A exhibits voltage variations, as developed across resistors 43 and 44, related to the magnitude of kinescope black current. The voltage variations are processed by the control circuits coupled to terminal A to develop a correction signal, if necessary, to maintain a desired level of kinescope black current conduction by feedback action. In this operating mode switch 49, e.g., a controlled electronic switch, is maintained in an open position as shown in response to a timing signal VT developed by the control circuits. When the control circuits are to operate in an automatic kinescope white current (drive) control mode wherein during image blanking intervals the kinescope conducts much larger currents representing a white image condition, switch 49 closes in response to timing signal VT, thereby shunting resistor 48 across resistors 43 and 44. The value of resistor 48 is chosen relative to the combined values of resistors 43 and 44 so that the larger current conducted via the collector of transistor 30 divides between series resistors 43, 44 and resistor 48 such that the magnitude of current conducted by resistors 43 and 44 is insufficient to produce a large enough voltage drop across resistor 44 to render clamping transistors 43 and 44 conductive. Unclamped terminal A therefore exhibits voltage variations related to the magnitude of kinescope white curre
nt, which voltage variations are processed by the control circuits to develop a correction signal as required. As used herein, the expression "white current" refers to a high level of individual red, green or blue color image current, or to combined high level red, green and blue currents associated with a white image. With the illustrated configuration of transistors 41 and 42 clamping voltage VA is relatively low, approximately +2.0 volts. The clamping voltage could be provided by a Zener diode rather than the disclosed arrangement of Darlington-connected transistors 41 and 42, but the disclosed clamping arrangement is preferred because Zener diodes with a voltage rating less than about 4 volts usually do not exhibit a predictable Zener threshold voltage characteristic, i.e., the "knee" transition region of the Zener voltage-vs-current characteristic is usually not very well defined. In addition, the disclosed transistor clamp operates with better linearity than a Zener diode clamp and radiates less radio frequency interference (RFI).
The relatively low clamping voltage is compatible with the analog input voltage requirements of the analog-to-digital converter (ADC) at the input of the control circuits which receive the sensed voltage at terminal A as will be explained in greater detail with respect to FIG. 2. In this example the ADC is intended to process analog voltages of from 0 volts to approximately +2.5 volts, and the clamping voltage assures that excessively high analog voltages are not presented to the ADC during normal video signal intervals.
The relatively l
ow clamping voltage also assists to prevent transistor 30 from saturating, which is necessary since transistor 30 is intended to operate in a linear region. To achieve this result and to maximize the cathode current conduction capability of transistor 30, the clamping voltage should be as low as possible to maintain a suitably low bias voltage at the collector of transistor 30. On the other hand, the value of arc current limiting resistor 37a should be large enough to provide adequate arc protection without compromising the objective of maintaining the collector bias voltage of transistor 30 as low as possible. Operation of transistor 30 in a saturated state renders transistor 30 ineffective for its intended purpose of properly conveying video drive signals to the kinescope cathode, and for conveying accurate representations of cathode current to clamping network 40 particularly in the white current control mode when relatively high cathode current levels are sensed. In addition, undesirable radio frequency interference (RFI) can be generated by transistor 30 switching into and out of saturation. Also, when saturation occurs transistor base storage effects can result in video image streaking due to the time required for a transistor to come out of a saturated state. Thus clamping network 40 advantageously limits the voltage at terminal A to a level tolerable by the analog-to-digital converter at the input of the control circuits coupled to terminal A, and protects the analog-to-digital converter input from damage due to signal overdrive. Network 40 also provides a collector reference bias for transistor 30 to prevent transistor 30 from saturating on large negative-going signal amplitude transitions at its emitter electrode. The clamping voltage level is readily adjusted simply by tailoring the values of resistors 43 and 44.
Capacitor 46 bypasses high frequency video signals to ground to prevent transistor 30 from saturating in response to such signals. Capacitor 46 also serves to smooth out undesirable high frequency variations at terminal A to prevent potentially troublesome signal components such as noise from interfering with the signal processing function of the input analog-to-digital converter of the control circuits, e.g., by smoothing the current sensed during the settling time of the analog-to-digital converter.
The latter noise reducing effect is particularly desirable, for example, when the input ADC of the control circuits coupled to terminal A is of the relatively inexpensive and uncomplicated "iterative approximation" type ADC, compared to a "flash" type ADC. The operation of an iterative ADC, wherein successive approximations are made from the most significant bit to the least significant bit, requires a relatively constant or slowly varying analog signal to be sampled during sampling intervals, uncontaminated by noise and similar effects.
The value of capacitor 46 should not be excessively large because a certain rate of current variation should be permitted at terminal A with respect to kinescope cathode currents being sensed. If the value of capacitor 46 is too small, excessive voltage variations, particularly high frequency video signal variations, will appear at terminal A, increasing the likelihood of transistor 30 saturating. The speed of operation of the clamp circuit itself is restricted by an RC low pass filter effect produced by the base capacitance of transistor 41 and the equivalent resistance of resistors 43 and 44.
F
IG. 2 shows a portion of a color television receiver system employing digital video signal processing techniques. The FIG. 2 system utilizes kinescope driver amplifiers and a clamping network as disclosed in FIG. 1, wherein similar elements are identified by the same reference number. By way of example, the system of FIG. 2 includes a MAA 2100 VCU (Video Codec Unit) corresponding to video signal source 10 of FIG. 1, a MAA 2200 VPU (Video Processor Unit) 50, and a MAAA 2000 CCU (Central Control Unit) 60. The latter three units are associated with a digital television signal processing system offered by ITT Corporation as described in a technical bulletin titled "DIGIT 2000 VLSI DIGITAL TV SYSTEM" published by the Intermetall Semiconductors subsidiary of ITT Corporation. In unit 10, a luminance signal and color difference signals in digital form are respectively converted to analog form by means of digital-to-analog converters (DACs) 70 and 71. The analog luminance signal (Y) and analog color difference signals r-y and b-y are combined in a matrix amplifier 73 to produce r, g and b color image representative signals w
hich are processed by preamplifiers 75, 76 and 77, respectively, before being coupled to kinescope driver stages 15, 16 and 17 of the type shown in FIG. 1. A network 78 in unit 10 includes circuits associated with the automatic white current and black current control functions. The high level R, G and B color signals from driver stages 15, 16 and 17 are coupled via respective current limiting resistors (i.e., resistor 33) to cathode intensity control electrodes of a color kinescope 80. Currents conducted by the red, green and blue kinescope cathodes are conveyed to network 40 via resistors 37a-37c, for producing at terminal A a voltage representative of kinescope cathode current conducted during measuring intervals, as discussed previously.
VPU unit 50 includes input terminals 15 and 16 coupled to terminal A. Through terminal 15 the VPU receives the analog signal from terminal A and, via an internal multiplex switching network 51, the analog signal is supplied to an analog-to-digital-converter (ADC) 52. Terminal 16 is connected to an internal switching device (corresponding to switch 49 in FIG. 1) which, in conjunction with scaling resistor 48, controls the impedance and therefore the sensitivity at input terminal 15. High sensitivity for black current measurement is obtained with resistor 48 ungrounded by internal switch 49, and low sensitivity for white current measurement is obtained with resistor 48 grounded by internal switch 49.
The digital signal from ADC 52 is coupled to an IM BUS INTERFACE unit 53 which coacts with CCU unit 60
and provides signals to an output data multiplex (MPX) unit 55. Multiplexed output signal data from unit 55 is conveyed to VCU unit 10, and particularly to control network 78. Control network 78 provides output signals for controlling the signal gain of preamplifiers 75, 76 and 77 to achieve a correct white current condition, and also provides output signals for controlling the DC bias of the preamplifiers to achieve a correct black current condition. More specifically, during vertical image blanking intervals the three (red, green, blue) kinescope black currents subject to measurement and the three white currents subject to measurement are developed sequentially, sensed, and coupled to VPU 50 via terminal 15. The sensed values are sequenced, digitized and coupled to IM Bus Interface 53 which organizes the data communication with CCU 60. After being processed by CCU 60, control signals are routed back to interface 53 and from there to data multiplexer 55 which forwards the control signals to VCU 10.
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