US3427497A - Means for controlling distortion in a cathode ray tube - Google Patents

Description

Feb. H, 19% E. GOSTYN 3,427,497

MEANS FOR CONTROLLING DISTORTION IN A CATHODE RAY TUBE Filed July 7, 1965 Sheet of 2 mvzmon ERA/6 57' 66575 ATTORNEY Feh H, 1969 E. GOSTYN 3,427,497 v MEANS FOR CONTROLLING DISTORTION IN A CATHODE RAY TUBE Filed July 7, 1965 Shet 2 of 2 C I; 35 F L; +24 I627 C 3 KL INVENTOR 519M625 7' 60: r/A/ ATTORNEY United States Patent 3,427,497 MEANS FOR CONTRGLLING DISTORTION IN A CATll-IODE RAY TUBE Ernest Gostyn, Longmeadow, Mass, assignor to General Instrument orporation, a corporation of Delaware Filed .lt'uly 7, 1965, Set. No. 470,024 US. Cl. 31527 17 Claims Int. Ci. Htllj 29/70; Htllf 21/00 ABSTRACT OF THE DISCLGSURE Magnetic apparatus for controlling distortion in a cathode ray tube and capable of having the magnitude and distribution of the correcting effect baried, a permanent magnet being rotatable relative to the fixed magnetic structure to vary the magnitude of the correcting effect and being shiftable along the magnetic structure for varying the distribution of the correcting effect.

The present invention relates to means for controlling or modifying distortion in cathode ray tube displays, particularly distortion of the pincushion type. It is particularly well adapted for use in conjunction with color display tubes.

The so-called pincushion type distortion of cathode ray tube displays has long been recognized. In black-andwhite displays this type of distortion is to considerable extent corrected through the use of permanent magnets which are so shaped and so fixed in position relative to the cathode ray tube as to produce an appropriate magnetic biasing effect on the cathode ray beam. However, in the case of color display tubes which are based on use of the shadow mask or similar principles such fixed correcting magnets cannot be used.

One approach which has been adopted in connection with pincushion distortion correction in color displays involves the modulation or variation of one of the sweep currents in such a fashion as to produce the desired re sults. It has further been suggested in the prior art that this modulation be accomplished in an electromagnetic manner, using a combination of magnetic and electrical circuitry which works on the principle of magnetic saturability. In general, adequate nominal correction is produced by this means, but only for one particular design of display system, and it is difficult if not impossible for a given device to be readily adjustable or modifiable so as to compensate for the individual perculiarities of a series of display tubes as they come from a production line, or to make correction adjustments from time to time in order to compensate for changes in the operating characteristics of the cathode ray tube which occur with use. Thus it has been difficult, on a production line, to produce a large number of color displays all of which have the same degree of excellence insofar as absence of pincushion distortion is concerned, and it has been correspondingly difficult to prevent an increase in pincushion distortion in a color display tube as it is used.

The prime object of the present invention is to a highly effective modulation of the sweep current of a display tube in order to minimize or eliminate pincushion distortion, the magnitude of the compensating correction being readily variable, and the relative magnitudes of the corrections at the opposite extremes of the display likewise being variable. Thus a single system design can be used with display tubes of different characteristics, or with display tubes nominally having the same characteristics but in which individual such tubes depart somewhat from their nominal characteristics.

In accordance with the present invention the electromagnetic modulation of the sweep current is accomplished in electromagnetic fashion, as in the prior art, but the 3,427,497 Patented Feb. 11, 1969 ice system is so constructed and arranged as to provide for adjustable variation in the modulating effect, thereby to produce the desired variations in magnitude or location of the distortion-correcting effects. More specifically, the modulating structure comprises a magnetic circuit having a plurality of legs with which coils are associated, the legs being connected by a magnetizable arm in such a fashion that a current through one winding will effect or modulate the current in another winding. A permanent magnet is associated with the magnetic circuit in order to provide a desired magnetic bias, the location and magnitude of the bias determining, at least in part, the modulation effect produced. The permanent biasing magnet is formed separately from the main magnetic circuit, and is mounted so as to be movable relative to a portion of the core with which it is magnetically associated, thereby to vary its biasing effect. In order to vary the maximum amplitude of the modulating effect produced, the biasing magnet is rotatably mounted relative to the main magnetic circuit so that the direction of magnetization of the permanent magnet may be shifted between positions parallel to and perpendicular to that arm of the magnetic circuit with which the biasing magnet is associated. In addition, the biasing magnet is mounted for translation along the arm of the magnetic circuit with which it is associated, so that the magnitude of its biasing effect can be varied as between different portions of the magnetic circuit. In this way the pincushion correction can be made greater at one end of the display than at the other end thereof, and the absolute magnitude of the correction produced can be independently adjusted and controlled.

It is preferred that the biasing magnet be mounted on a carriage which is itself movably mounted on the basic magnetic circuit, the carriage being shiftable in position in order to produce the desired distribution of magnetic biasing effect in the different portions thereof, [and means are provided for reliably retaining the carriage in its adjusted position. The biasing magnet may itself be rotatably mounted on the carriage, thereby to permit variation of the absolute magnitude of the magnetic biasing force produced independently of the distribution thereof among the various parts of the basic magnetic circuit. Additional structural arrangements may be utilized to enhance and intensify the effect of the biasing magnet on the basic magnetic circuit and thus produce a desired degree of adjustment sensitivity.

To the accomplishment of the above, and to such other objects as may hereinafter appear, the present invention relates to a device for controlling distortion in a cathode ray tube, as defined in the following claims and as described in this specification, taken together with the accompanying drawings, in which:

FIG. 1 is atop plan view of one structural embodiment of the present invention;

FIG. 2 is a front elevational view thereof;

FIG. 3 is a side elevational view thereof;

FIG. 4 is a schematic representation illustrating the geometric requirements for correction of pincushion effect;

FIG. 5 is a graphical representation of a typical sweep current cycle without correction in accordance with the present invention;

FIG. 6 is a view similar to FIG. 5 but showing the sweep current with the correction applied; and

FIG. 7 is an electromagnetic circuit diagram showing the use of the device of FIGS. 1-3 for producing sweep current corrections of the type shown in FIG. 6.

Pincus-hion distortion is a type of distortion in which the outlines of a nominally rectangular display depart from rectangularity in a concave fashion, the resultant outline having the shape of a conventional pincushion in which the corners are spaced outwardly from the midpoint of the sides. FIG. 4 discloses, in more or less schematic form, such a distortion. For simplicity of explanation, the entire display is represented by five horizontal lines, a-e, which represent horizontal sweeps which are repeated in order cyclically for each vertical scan or sweep. The central line is straight, but is shorter than the other lines. The lines I) and d are curved, and the lines a and e are curved still more, the length of the lines b and d being intermediate between that of the line 0 and those of the lines a and e. It will be seen from FIG. 4 that deviation from the rectangular pattern occurs at the top and bottom and at the left and right sides. Either type of deviation or distortion can be corrected in accordance with the present invention. In the description to follow correction will be 7 applied to the top and bottom type of pincushion distortion.

From an examination of FIG. 4 it will be seen that the top and bottom type of pincushion distortion can be counteracted if the vertical sweep current is varied in accordance with the horizontal sweep, with that variation in vertical sweep current changing as the horizontal sweep moves from line a to line b and on to line 2. Line a will be converted to a straight line (thus eliminating the top and bottom pincushion effect) if the vertical sweep current is reduced as the horizontal beam deflection approaches the left and right sides. The same is true of line b, except that the amount of reduction in the vertical sweep current will be less than that of line a. For line 0 the vertical sweep current should remain constant. The correction for line d is similar to that for line b, but in the opposite direction, and the correction for line e is similar to that for line a, but in the opposite direction.

A typical vertical deflection current wave, in idealized form, is illustrated graphically in FIG. 5. The graph portion 2 represents the change in vertical deflection current during a single display cycle, while the graph portion 4 indicates the more rapid change of the vertical sweep current as it resets to its initial condition, ready for the next cycle. It will be understood that during the time that the graph portion 2 goes from maximum to minimum the horizontal sweep occurs a multiplicity of times, thereby to form the horizontal sweep lines a-e (plus as many other horizontal sweep lines as are necessary to produce a display of desired quality. In a typical TV display the vertical sweep occurs at the rate of 60 c. p.s., while the horizontal rate occurs at 15,750 c.p.s.).

Typical points On the curve 2 corresponding to the horizontal sweeps a-e are identified in FIGS. 5 and 6 with corresponding letters. FIG. 6 represents the idealized graph of FIG. 5 with pincushion distortion correcting modulations applied thereto. As shown in FIG. 6, the correcting modulation applied at point a is of the same magnitude as, but in the opposite direction from, the modulation correction applied at point e, the modulation corrections at points b and d are similarly related to one another but have a magnitude less than that applied at points a and e respectively, and no modulation correction at all is applied at point 0.

To produce the modulations shown in FIG. 6, the electromagnetic circuit shown in FIG. 7 is employed. It comprises a ferromagnetic core generally designated 6 and having legs 8, 10 and 12 connected at their lower ends by arm 14 and at their upper ends by arm 16. The cathode ray tube vertical deflection windings 18 and 20 are energized by a vertical sweep current source 22 having an output of the type shown. The ends of the deflection windings 18 and 20 are connected by wires 24 in series with a winding 26 on the intermediate leg 10 of the magnetic core 6. The horizontal deflection windings 28 and 30 of the cathode ray tube are connected in series with a horizontal sweep current source 32, having an output of the character shown, and they are further connected by wires 34 in series with one another and with windings 36 and 38 on the outer legs 8 and 12 respectively of the magnetic core 6. A permanent magnet 40, magnetically polarized as indicated, is magnetically operatively associated with the upper arm 16 of the magnetic core 6. The biasing magnetic field produced by the magnet 40 is represented by the broken line arrows 42. Those arrows, it will be noted, extend in opposite directions through the legs 8 and 12, and with the magnet 40 centrally positioned along the arm, none of that biasing field passes through the leg 10. The magnetic field produced in the leg 10 by the current in the winding 26 is represented by the dot-dash arrows 44. These arrows 44, it will be noted, extend in the same direction through the legs 8 and 12. During the time that current is flowing in the winding 26 in a given direction the current through the windings 36 and 38 will alternate, because the horizontal scan is accomplished many times for each vertical scanl The magnetic field produced in the core 6 by reason of the horizontal deflection current in the windings 36 and 38 is represented by the solid line arrows 46, which are double-headed to indicate that their direction alternates. The windings 26 correspond to the gate winding of a magnetic amplifier, while the windings 36 and 38 contitute the control windings. The horizontal deflection current variations produce variations in the magnetic fields indicated by the solid line arrows 46. These, in conjunction with the biasing magnetic field 42 and the excitation status of the magnetic fields in the legs 8 and 12, control the permeability of the leg 10 and thus affect the reactance of the Winding 26. That reactance will vary in step with the horizontal sweep current, and as the reactance varies, the magnitude of the current flowing therethrough, and hence the magnitude of the current flowing through the vertical deflection windings 18 and 2.0, will also vary.

The modulation effect on the Winding 26 will be determined not only by that portion of the field 46 which passes through the leg 10, but also by the instantaneous current in the winding 26 itself. Thus with the vertical sweep current declining from its maximum value toward zero (represented by the point c on graph 2) there will be a constantly declining inductive drop in the winding 26, and the modulation thereof by the current in the windings 36 and 38 will therefore also decline. When the vertical deflection beam is half-way down (when the current in winding 26 is zero) there will be no modulation .eflect. As the vertical sweep current rnOVes down below zero the current through the winding 26 will increase in the opposite direction, and hence the modulation effect thereon will increase, but in the opposite direction. Thus modulations of the types schematically indicated in FIG. 6 are produced.

Further analysis of the operation of the magnetic system of FIG. 7 reveals that the amplitude of the correcting modulation produced for any given value of vertical deflection current (other than zero) will vary in accordance with the magnitude of the biasing magnetic field represented by the arrows 42. Analysis also reveals that with the vertical sweep current in one direction one leg 8 or 12 of the core 6 predominates over the other in effecting modulation control, and when the vertical deflection current is in the other direction it is the other of the legs 8 or 12 which predominates in modulation control. This may be demonstrated by assuming a direction of vertical deflection current which produces the magnetic field indicated by the arrows 44. The biasing field 42 and the vertical-deflection-current-producing field 44 add in the leg 8 and subtract in the leg 12. The addition of the fields 42 and 44 in the leg 8 tend to saturate that leg and thus disable its control function. The resultant magnetic field in the leg 12, however, is capable of producing a magnetic operating level which permits modulation. When the vertical deflection current is in the opposite direction, the arrows 44 will be reversed in direction, and then the arrow 42 and 44 will add in the leg 12 and subtract in the leg 8, thus shifting control from the former to the latter. The degree of modulation control will, of course, be determined primarily by the resultant magnetic field in the leg or legs which are exercising control at any given moment, and that value of magnetic operating level will, for a given magnitude of vertical deflection current, be determined by the effective magnetic field produced by the biasing magnet 40.

From this analysis it appears that the magnitude of modulation, and hence the magnitude of the resultant pincushion correction, can be achieved by varying the intensity of the magnetic biasing field 42. Further, the pincushion correction at the top and bottom portions of the display (corresponding in FIGS 56 to positive and negative vertical deflection current respectively) can be varied, so that more correction is provided at the top than at the bottom or vice versa, by varying the distribution of the biasing field 42 between the legs 8 and 12. This can be done by shifting the magnet 40 toward the leg 8 or the leg 12, depending upon the variation desired, the biasing magnetic field then partially passing through the control leg 10, so that the biasing fields in the legs 8 and 12 are no longer equal. This causes one leg 8 or 12 to produce a greater modulation control than the other. Since, as we have seen, one leg predominates for one direction of vertical deflection current and the other leg dominates for the other direction of that current, the resultant will be a variation in the amount of correction produced at the top half of the display as compared with that produced at the bottom half of the display.

FIGS. 1-3 illustrate a preferred structural embodiment of an electromagnetic modulator corresponding to that schematically shown in FIG. 7 and capable of effecting the adjustments in modulation effect described above. The magnetic core 6, with the windings thereon, is received within a frame generally designated 48 and formed of some suitable non-magnetic material. It comprises a bottom wall 50 having outwardly extending ears 52 with apertures 54 therethrough, through which apertures screws may pass to secure the structure to any desired support. Lips 56 extend up from the side edges of the wall 50, defining a channel within which the magnetic core assembly 6 is relatively snugly received and is constrained against lateral movement. Appropriately secured to the bottom wall 50 in any desired manner, and extending up therefrom between the lips 56, are end walls 58 which substantially engage and confine the magnetic core assembly 6 against longitudinal movement, the walls 58 at each end of the core assembly 6 having straps 60 which extend toward one another and are adapted to be secured together in any appropriate fashion, as by having the tab 62 on the right hand strap 60 pass through an aperture in the ear 64 on the left hand strap 60 and then be bent up so as to prevent disengagement of the straps 60. The magnetic core assembly 6 is constructed as shown in FIG. 7, but the individual legs and arms, and the coils carried thereby, are encapsulated or otherwise surrounded by insulating material, the portions 8', and 12 shown in FIG. 2 representing the core legs 8, 10 and 12 of the schematic embodiment of FIG. 7 with windings 36, 26 and 38 respectively secured thereto and surrounded by encapsulating material.

The end walls 58 of the frame are provided with upward extensions 58a which project well above the top of the magnetic core 6, and which are provided with wings 66 which are bent therefrom so as to extend toward the center line of the core assembly 6. The bottom edges 68 of the wings 66 are spaced a short distance above the upper edges 70 of the straps 60, the edges 70 more or less corresponding to the upper edges of the core assembly 6. A carriage generally designated 72 is provided, that carriage being generally of H-shape and having a crosspiece 80 from which right and left hand pairs of arms 82 extend, the crosspiece 80 resting on the edges 70 and being received between the frame extensions 58a with sufficient clearance in the direction from one extension 58a to the other so that the crosspiece may be shifted in that direction. The arms 82 lie outside the frame extensions 58a and are received under the wings 66. In this way the wings 66 keep the carriage 72 in vertical position substantially against the upper edge 70 of the frame straps 60, and consequently substantially against the upper arm 16 of the core frame 6, While permitting the carriage 72 to be shifted from right to left as viewed in FIGS. 1 and 2. In order to retain the carriage 72 in a given adjusted position, a wire spring element 84 is carried by one of the frame extensions 58a, and is adapted to resiliently engage a serrated portion 86 on the upper surface of the carriage arms 82.

The carriage 72 carries, in any appropriate fashion, the biasing permanent magnet 40. As here specifically disclosed the magnet 40 is cylindrical in shape and is magnetically polarized as indicated by the letters N and S in FIG. 1. It is mounted on the carriage 72 so as to move with the carriage as the latter is moved from right to left in FIGS. 1 and 2, and it is also preferably mounted in the carriage 72 so as to be pivotal about a vertical axis, thereby to move its axis of magnetization between positions parallel to and at right angles to the lengthwise dimension of the core arm 16.

As here specifically disclosed, the magnet 40 is received within an aperture 88 in the crosspiece portion of the carriage 72. Partially surrounding the magnet 40 are a pair of magnetizable elements generally designated 90, each located between the magnet 40 and one of the frame extensions 58a. They are held in position by means of the C-spring 92, have upstanding arcuate shaped portions 94 which partially surround the periphery of the magnet 40, have top bars 96 which overlie the magnet 40 and thereby retain it in position, and have outwardly extending pole piece portions 98 oriented toward the respective frame extensions 58a, the portions 98 being bent down around the left and right hand edges 100 of the crosspiece portion 80 of the carriage 72. These elements assist in retaining the magnet 40 in position on the carriage 72, help to constitute a bearing for the magnet 40, permitting it to be adjustably rotated in order to vary the effective magnetic biasing force, and by reason of the action of the spring washer 92 they frictionally grasp the magnet 40 and retain it in its rotatably adjusted position. Moreover, the magnetizable portions 98 make a more efficient magnetic circuit for the biasing magnetic flux, thus intensifying the degree to which the biasing magnet 40 can be effective in giving rise to the current modulations previously described. If desired, special magnetizable pole piece extensions 102 can be provided which extend up vertically inside the frame extensions 58a from the core legs 8 and 12 respectively to points substantially on a level with the pole piece portions 98, thus further improving the efficiency of the magnetic circuit for the biasing flux.

From the above it will be seen that by rotating the magnet 40 relative to the carriage 72 the magnitude of the effective biasing magnetic flux reaching the legs 8 and 12 will be varied, and by shifting the carriage 82 to the right or the left, the magnet 40 moving along therewith, the relative distribution of the biasing magnetic force in the legs 8 and 12 will be varied, thus providing respectively the adjustment in the magnitude of the sweep current modulation and in the relative magnitudes of that modulation at the upper and lower portions of the scan respectively.

The structure involved is simple, inexpensive, and need not be made to any high degree of precision, yet through its use a device is produced which Will not only give rise to such modulation of the sweep current as will minimize the pincushion effect in a given direction, but will also permit the degree of correction to be adjusted and modified at will. Consequently, a device of a single design can be used with many different cathode ray tube systems and, when used with tubes of a given system, can be adjusted to provide corrections appropriate for the departure of individual tubes from nominal geometric or electrical characteristics. It is, in addition, possible to employ the relative magnitude correction in applications in which viewing angle problems make an unequal pincushion correction of opposing sides advantageous.

While but a single embodiment of the present invention has been here specifically disclosed, it will be apparent that many variations may be made therein, all within the scope of the instant invention as defined in the following claims.

I claim;

1. A device for controlling distortion in a cathode ray tube comprising a magnetic core having first and second legs connected by an arm, a biasing magnet, and means operatively connected to said magnet for mounting it adjacent said arm for movement in the direction of the length of said arm between said legs, and windings on said legs, said mounting means comprising a frame on which said core is mounted and a carriage mounted on said frame adjacent said core arm and movable in the direction of the length of said arm, said magnet being mounted on said carriage between magnetizable pole pieces which extend from said magnet toward said core legs and are movable with said carriage and magnet.

2. The device of claim 1, in which said core legs have magnetizable pole piece extensions operatively extending therefrom, said magnet and said magnetizable pole pieces being received between said pole extensions.

3. In the device of claim 2, resilient means engaging said magnetizable pole pieces and urging them into engagement with said magnet.

4. In the device of claim 1, resilient means engaging said magnetizable pole pieces and urging them into engagement with said magnet.

5. A device for controlling distortion in a cathode ray tube comprising a magnetic core having first and second outer legs and an intermediate leg all connected by an arm, a biasing magnet, means operatively connected to said magnet for mounting it adjacent said arm generally in registration with said intermediate leg, said magnet being magnetically polarized substantially in a plane parallel to said arm and being rotatably mounted in said mounting means to vary the effective magnetic strength in the direction of the length of said arm, windings on said outer legs and a winding on said intermediate leg, said windings on said outer legs constituting a control winding, said winding on said intermediate leg being connected to deflection means in a cathode ray tube assembly.

6. The device of claim 5, in which said mounting means comprises a frame on which said core is mounted and a carriage mounted on said frame adjacent said core arm, said magnet being rotatably mounted on said carriage.

7. A device for controlling distortion in a cathode ray tube comprising a magnetic core having first and second legs connected by an arm, a biasing magnet, means operatively connected to said magnet for mounting it adjacent said arm for movement in the direction of the length of said arm between said legs, said magnet being magnetically polarized substantially in a plane parallel to said arm and being rotatably mounted in said mounting means to vary the effective magnetic strength in the direction of the length of said arm, and windings on said legs, at least one of said windings being connected to deflection means in a cathode ray tube assembly.

8. The device of claim 7, in which said mounting means comprises a frame on which said core is mounted and a carriage mounted on said frame adjacent said core arm and movable in the direction of the length of said arm, said magnet being mounted on said carriage.

9. The device of claim 8, in which said magnet is mounted on said carriage between magnetizable pole pieces which extend from said magnet toward said outer core legs and are movable with said carriage and magnet.

10. The device of claim 8, in which said magnet is mounted on said carriage between magnetizable pole pieces which extend from said magnet toward said outer core legs and are movable with said carriage and magnet,

said outer core legs having magnetizable pole piece extensions operatively extending therefrom, said magnet and said magnetizable pole pieces being received between said pole extensions.

11. The device of claim 7, in which said mounting means comprises a frame on which said core is mounted and a carriage mounted on said frame adjacent said core arm, said magnet being rotatably mounted on said carriage.

12. The device of claim 7, in which said mounting means comprises a frame on which said core is mounted and a carriage mounted on said frame adjacent said core arm and movable in the direction of the length of said arm, said magnet being rotatably mounted on said carriage.

13. The device of claim 12, in which said magnet is mounted on said carriage between magnetizable pole pieces which extend from said magnet toward said outer core legs and are movable with said carriage and mag net.

14. The device of claim 12, in which said magnet ismounted on said carriage between magnetizable pole pieces which extend from said magnet toward said outer core legs and are movable with said carriage and magnet, said outer core legs having magnetizable pole piece extensions operatively extending therefrom, said magnet and said magnetizable pole pieces being received between said pole extensions.

15. The device of claim 7, in which said mounting means comprises af rame on which said core is mounted and a carriage mounted on said frame adjacent said core arm and movable in the direction of the length of said arm, said magnet being rotatably mounted on said carriage, and means on said frame operatively engaging said carriage to hold it in an adjusted position along said core arm.

16. A magnetic modulator comprising a magnetic core having first and second outer legs and an intermediate leg all connected by an arm, a biasing magnet, means operatively connected to said magnet for mounting it adjacent said arm generally in registration with said intermediate leg for movement in the direction of the length of said arm between said outer legs, said magnet being magnetically polarized substantially in a plane parallel to said arm and being rotatably mounted in said mounting means to vary the effective magnetic strength in the direction of the length of said arm, and windings on said outer legs and a winding on said intermediate leg.

17. The magnetic modulator of claim 16, in which said mounting means comprises a frame on which said core is mounted and a carriage mounted on said frame adjacent said core arm and movable in a direction of the length of said arm, said magnet being rotatably mounted on said carriage, magnetizable pole pieces on said carriage between which said magnet is mounted and from which magnet said pole pieces extend toward said outer core legs, said outer core legs having magnetizable pole piece extensions operatively extending therefrom, said magnet and said magnetizable pole pieces being received between said pole piece extensions, and means on said frame operatively engaging said carriage to hold it in an adjusted position along said core arm.

References Cited UNITED STATES PATENTS 2,702,874 2/1955 Adler 323-92 X 2,802,140 8/1957 Mattingly -336 X 3,283,279 11/1966 Garlotte 336110 3,346,765 10/1967 Barkow 315-27 RODNEY D. BENNETT, Primary Examiner. R. E. BERGER, Assistant Examiner.

US. Cl. X.R. 336-110

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