IMPERIAL CT2026 CHASSIS 711 (TELEFUNKEN) CRT TUBE TELEFUNKEN A66-140X

 

 

CRT TUBE TELEFUNKEN  A66-140X.

DEFLECTION UNIT TELEFUNKEN AS110K213

As is well known to those skilled in this art, the delta-gun, color cathode ray tube (CRT) differs most significantly from conventional CRT's having a single gun in that the delta-gun has three guns, each of which generate a distinct electron beam for exciting its corresponding phosphor dot of the red, green and blue dot triad matrix constituting the CRT display surface.

Each of the delta-arranged gun beams must land at precisely the same opening in the shadow mask at every point on the screen in order to produce a perfect rendition of a desired color and symbol shape. To achieve the desired rendition on the screen, the beams, substantially forward of where they leave the guns, are magnetically deflected, as a set, by excitation of a main deflection yoke comprising an electromagnet arrangement mounted around the neck of the CRT so as to control the angle at which the beams, as a set, approach the desired landing site on the display screen.

Registration of all three beams at the landing site cannot be accomplished through all deflection angles without some individual deflection of each beam in addition to the main deflection. The auxiliary deflection system is termed convergence deflection because of the function it performs in causing each beam to converge to a single point for all deflected angles.

Aside from the geometry errors (pin cushion effect) which necessitate a convergence system, there are errors due primarily to alignment tolerances associated with the deflection yoke and electron gun assemblies. These errors have the effect of distorting the ideal convergence function and consequently, each CRT assembly must be uniquely converged.

In most, if not all delta-gun CRT's, convergence deflection is accomplished by electrical current excitation of a small deflection yoke for each yoke is usually part of the electron gun assembly .

 Each of these yokes produce a magnetic field which is applied to the beam emanating from their respective gun in such a way that deflection of the beam is along a single axis, that is, the axis which extends radially from the center of the CRT, through the center of each of the three guns. These axes are called red radial, green radial and blue radial, for a typical CRT and are then, by the nature of the delta-gun, 120 angular degrees apart.

It is customary to align the blue radial with the vertical deflection of the main deflection yoke. Thus, as facing the front of the CRT, the blue gun converges along a vertical axis, red along an axis 120° left and green along an axis 120° to the right (240° to the left).

The full function of convergence is not possible without a fourth degree of freedom of movement associated with one of the electron beams. This is generally applied to the blue gun along the lateral axis of the CRT at 90° to the blue radial and is consequently termed blue lateral convergence.

As is the case with radial convergence, the blue lateral convergence can be partitioned into two components, a dynamic portion and a static, or constant portion. The cynamic portion of blue lateral correction (unlike that of the dynamic portion of the radial correction) can be, in general, performed by the design of the main deflection yoke. When this is the case, only an adjustable external static magnet is required to position the blue beam laterally to a reference location at the center of the screen. This magnetic adjustment is termed the static blue lateral adjustment.

CRT TUBE TELEFUNKEN  A66-140X  Assembly and adjustments

 The following sequence must be followed in the adjustments.
 
 a)
 Deflection yoke, radial converging assembly, lateral converging device and purifying magnet must be fitted on the tube neck in the prescribed position.
 
 b)
 Apply supply voltages and adjust focusing to Optimum.
 
 c)
 Demagnetize the tube for several seconds. Thus local colour impurity zones will be eliminated, which are due to magnetization of the metal shield and the mask. Take care with magnetized chassis or other iron parts.
 
 d)
 Adjust the static convergence. Using a cross-hatch or dot pattern the electron beams must converge in a dot at screen centre. This adjustment is performed with the aid of radial permanent or electromagnets on the radial converging assembly and the lateral converging device. If the blue beam is blocked, Optimum convergence is obtained once the pattern or dots in the screen centre are yellow. Subsequently the blue beam must be applied and brought horizontally to the side of the yellow dot using the corresponding radial magnet, and then brought to coincidence with it adjusting the lateral converging magnet.
 
 e)
 Adjust colour purity. With a red area raster switched on and the deflection unit placed as for back as possible (12 mm) the field of the colour purifying magnet must be adjusted as to intensity and direction until as uniform as possible c red area appears at screen centre. (Optimum beam centering to red phosphor dots in screen centre.)
 
)
Push the deflection unit forwards until the entire screen is illuminated with o uniform red. Subsequently the colour purity of the green and blue rasters must be checked Over the whole screen and, if necessary, a compromise must be found in the adjustment for all colours. Prior to, and following, each colour purity adjustment the static convergence must be checked. Centre a test pattern by means of DC pre deflection. Check static convergence as well as colour purity and readjust if necessary.

Adjustment of dynamic convergence.
Use a bright cross-hatch or dot pattern for this purpose. By adjusting the alternating currents in the converging coils the three coloured pattern must be brought to coincidence Over the entire screen (without the Corners) in such a manner that white dots or pattern lines are produced. After blocking the blue beam adjust the red a n d green roster to coincidence. During this procedure the static convergence must be readjusted and the colour purity checked several times. By regulating the corner convergence currents in the deflection coils raster coverage is performed in the corners as next Step. In conclusion pincushion distortion must be eliminated. Care must be token that at no time o considerable amount of beam current reaches the walls of the tube or ports of the electron gun.

Adjusting colour purity with o microscope

Observation of register through a microscope (magnification approx. 40) is recommended for  colour purity adjustment. For this purpose a white raster is used. The phosphor dots must be illuminated from the side with a light source (e. g. lamp). In the screen centre the beam position is influenced by means of the purifying magnet in such manner that the centres of the two triangles, which are formed by a phosphor triplet and the excited surfaces, are in coincidence. Afterwards the convergence must be checked and readjusted if necessary, the colour purity likewise being corrected once more. After switching to red raster o uniform red screen surface must be adjusted as described in e) by sliding the deflecting yoke axially.

Purifying magnet
Permanent magnet with field perpendicular to tube axis. The intensity and direction of the field must be adjustable. Lateral converging device Permanent- or electromagnet with field perpendicular to tube axis, whose direction causes a horizontal movement of the blue beam in opposite direction to red and green beams. The field intensity must be adjustable. Radial converging assembly Permanent magnets or electromagnet fed with DC having a field perpendicular to tube axis are used to adjust static convergence. The field intensity must be adjustable. The dynamic convergence is achieved by alternating magnetic fields, which are obtained through superposed AC currents in the coils attached to the permanent- or electromagnets.

Deflection unit
The axes of the deflection unit and tube must coincide. The deflection unit has to be movable along the neck for a distance of min. 12 mm and a slight turning has to be allowed. Beam centering is effected by DC exclusively, pincushion distortion correction exclusively by superposing AC.

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.