US2831145A - Anti-distortion means for cathode ray tube displays - Google Patents

Description

ANTI-DISTORTION MEANS FOR CATHODE RAY TUBE DISPLAYS Filed Dec. 31, 1956 April 15, 1958 M. P. ALBERT ETAL 3 Sheets-Sheet 1 INVENTORS MILTON RALBERT Hm+xww THEIR ATTORNEYS 5159a A F: whww April 15, 1958 ANTI-DISTORTION MEANS FOR CATHODE RAY TUBE DISPLAYS Filed Dec. 31, 1956 P. ALBERT ETAL 5 SheetsSheet 2 sm REM S OBC U TLSBM N. NA R EDI-L O V NN w I W M HQ Y B M B H T April 15, 1958 M. P. ALBERT E" AL ANTIDISTORTION MEANS FOR CATHODE RAY TUBE DISPLAYS 3 Sheets-Sheet '3 Filed Dec. 31. 1 956 INVENTQRS MILTON P. ALBERT GLENN L. SCOTT BY ,B/w-AM 1m 1- gu THEIR ATTORNEYS United States atent ANTI-DISTORTION MEANS FOR CATHODE. RAY TUBE DHSPLAYS Milton P. Albert, Apalachin, N. Y., and Glenn L. Scott,

Orleans, Nehru, assignors to International Business Machines Corporation, New York, N. .Y., a corporation of New York Application December 31, 1956, Serial No. 631,780

16 Claims. (Cl. 315-24) This invention relates generally to apparatus for correcting distortion appearing in displays produced by deflection of an electron beam in a cathode ray tube. More particularly, this invention relates to apparatus for correcting that form of distortion which is known in optics as pincushion or barrel distortion.

While the invention hereof will be described with particular reference to a flat-faced cathode ray tube wherein the electron beam is magnetically deflected, the invention is also of application in connection with other types of cathode ray tubes as, say, tubes wherein the beam is electrostatically deflected.

For a better understanding of theinvention, reference is made to the following description and to the accompanying drawings wherein:

Fig. 1 is a geometric diagram representing in side elevation the electron beam deflection of Fig. 1 as seen from a point directly in front of the face of the tube of Fig. l;

Figs. 3 and 4 are illustrations of the type of distortion encountered with the tube geometry shown in Figs. 1 and 2; v

Fig. 5 illustrates in block diagram an anti-distortion circuit according to the invention;

Fig. 6 illustrates in block diagram the form taken by the circuit of Fig. 1 when the vector deflection of the electron beam is the resultant of two component deflections at right angles to each other; and

Fig. 7 is a detailed schematic diagram of the circuit of Fig. 6. 1 V

Referring now to Figs. 1 and 2, the number 10 generally designates a cathode ray tube having a flat face 11.

The tube generates an electron beam 12 which, when undefiected, coincides with the center axis 13 of the tube. The beam is deflected from this center axis by a magnetic yoke means 14. The yoke means 14 is a simple yoke means in the sense that it provides a uniform magnetic field. It will be assumed for the present that the yoke means 14 comprises a single yoke which is energized by a single yoke current I to deflect the beam by an angle 6 from the axis 1.3. The yoke 14 is assumed for the present to be rotatable about the axis 13 to produce deflection of the beam in various directions from this axis. Rotatable yokes of this sort are used, for example, in radar to produce P. P. I. (plan position indication) displays on the face of the cathode ray tube.

As shown in Figs. 1 and 2, the total deflection D at face 11 of beam 12 from axis 13 can be considered to be the resultant of a first deflection d and a second deflection' e, both having the same direction as D. The deflection d is proportional to sin p and can be shown to be a linear function of I for the relatively small values of which are used. The deflection e is not a linear function of l and. is the cause of the distortion to which this invention relates. If, as shown in Figs. 1 and 2 the deflection e adds to the deflection d, the result will be pincushion distortion. This type of distortion is shown in Fig. 3 wherein the beam 12 is considered to produce on face 11 a square picture 20 which is symmetrical about axis 13, and wherein the presence of an additive non-linear deflection e causes the corners of the square picture to be pulled outward from axis 13. If, on the other hand, the deflection e subtracts from deflection d, the result will be barrel distortion. This latter type of distortion is shown in Fig. 4 wherein the presence of a subtractive non-linear deflection e causes the corners of a like square picture 21 to be pushed inward whereby the square picture is given some roundness.

Relating the total deflection D to the yoke current I, it can be demonstrated, for the tube geometry of Figs. 1 and 2, that D varies with I as indicated by the expression:

D=K I+K I +K I K,-I" (1) wherein the quantities K K K K are all constants and wherein the quantities i l I" are all odd powers of I. values of 5 of less than 12 degrees, the contribution of the fifth power and higher order terms is entirely negligible, and that the third power term, K 1 is always less than approximately 2 percent of the linear term. Hence, for most applications, expression (1) can be simplified to the following:

From (2) it will be seen that the total deflection D of the electron beam has a non-linear relation to the yoke current I because of the presence of the term K 1 in (2). In the instance where no correction is made for: this non-linear relation, the non-linearity will cause pincushion distortion when K has a positive sign and the non-linearity will cause barrel distortion when K has a negative sign.

It has been assumed until now that the deflection of beam 12 is produced by a yoke current I energizing a single yoke, and that the yoke is rotatable to deflect the beam in various directions from axis 13. In practice, however, it is more often the case that the beam is deflected by yoke means comprised of an X-deflecting yoke and a Y-deflecting yoke arranged at right angles to each other. The X-deflecting yoke is energized by a yoke current I to deflect the beam in the X coordinate of an XY coordinate system. The Y-deflecting yoke is 6116i: gized by a current I to deflect the beam in the Y coordinate of the mentioned coordinate system. The total deflection of the electron beam will be the vector sum of the two components of beam deflection which are respectively produced by the X-deflecting yoke and by the Y-deflecting yoke.

Referring to Fig. 2, it will be seen that the total deflec tions D and D of the electron beams in, respectively, the X and Y coordinates are given by the following expressions:

term given by expression (2) for D into expressions (3) and (4), we get:

It can be further demonstrated that, for

. Referring to. (5), (6), (7 and (8), it will be recalled that I was said to represent a single yoke current which by itself produced the entire deflection of the electron beam. In the case first considered wherein the yoke was said to be rotatable, the symbolI does, in fact, represent a real current which produces the entire deflection. In the case at hand, however, wherein the beam deflection is produced by two yokes at right angles toeach other, I does not represent a single real current which produces the entire deflection since there is no such single real current. Instead, the total deflection D is produced by the I and I currents which act in essentially independent circuits rather than being combined with each other to form a resultant current which deflects the beam. Mathematically speaking, however, it is possible to consider, even in the arrangementof two yokes at right angles, that thetotal radial vector deflection of beam 12 from axis 13 is the result of the action of an imaginary current I which is the counterpart of the real Qcurrentl .in the rotatable yoke arrangement, and which will be referred to herein as the total efiectivef current. This total effective current I is in the nature of a vector which, like the radial deflection vector of the electronbeam, has an angle 6 in Fig. 2 with reference to the X 'axis in this figure. The magnitude of the vector I is given by the following expression:

x l- Y (9) from which it is evident that:

I2=IX2+IY2 Also, it will be evident from vector theory that:

- cos (11) sin 0: (12

I cos 0:1,; "(13) 1 sin 0:1,. 14)

Expression and expressions (13), (14) permit operation on expression'(7') and (glas'follows. Substitute for the terms I cos 0 and "1 sin 0f in (7) and (8) the equivalents ltliere of given'in ('13) and (14). -Also substitute for the term Fin (7) and (8) the equivalent thereof given in (10). The result will be the belowlisted expressions:

r=' i y+ z r( x d-' r In each: of expression (15)v and (T6) the presence of the second term renders the deflection in the X or Y coordinat e, as the case may be, a non-linear function of the yoke current'acting in that coordinate to thereby produce distor'tion. 'The second term of expression (15) when multiplied out demonstrates that'in the X coordinate the orthogonal yoke arrangement resembles the rotatable yoke arrangement in that in the X 'coordinatethe distortion accompanying deflection 'of the 'bear'n by the yoke c'u'rrent I is a function of I3 Further, however, the second term of (15) demonstrates that distortion 'in the X coordinate is a function of Y. Similarly, the second term of (16) demonstrates both" that distortion in the Y coordinate is a function of 1Y and that this lastnameddistortion is also a function of I Endeavors have been made in the prior art to remedy the pincushion or. barrel form of 'cubic' distortion which has been described. For example, the expedients have been tried of distributing and shaping the windings of the deflecting yoke, adding permanent magnets near the face ofthe tube, placing 'an opticallensinfront of the tube face to contribute an opposite compensating distortion, and altering the accelerator potential of'the tube as a function of yoke current. Each of these expedients is 4 characteriied by one or more disadvantages as, say, inadaptability to a flat-faced tube, inflexibility in the presence of different operating conditions for the tube, difliculty of reduction to practice, deteriorated focus or production of other undesired side effects.

It is accordingly an object of the invention to provide apparatus adapted to reduce or eliminate distortion in cathode ray tube displays in a manner which is free of the disadvantages listed above.

Another object of the invention is to provide apparatus of the sort described which is specifically adapted to reduce or eliminate cubic distortion in a flat-faced cathode ray tube.

Yet another object of the invention is to provide anti-distortion apparatus of the sort described and adapted for use with a cathode ray tube wherein the electron beam beam is magnetically deflected.

A further object of the invention is to provide antidis-tortion apparatus which will reduce or eliminate cubic distortion in a cathode ray tube display wherein the electron beam is deflected by two magnetic yokcs at right angles to each other. I

These and other objects are realized according to the invention by providing an anti-distortion circuit whereby the deflection undergone by the electron beam, Whether total vector deflection or total component deflection in a given deflection coordinate, is rendered a linear function of the driving signal which initiates the deflection as contrasted with the signal which actually produces the deflection. This anti-distortion circuit comprises output terminal means, source-means-of an electric driving signal having a variable characteristic, signal modifier means, and electric analog computer means. In the circuit the source means is connected to the cathode ray tube "through the output terminal means to deflect the electron beam of the tube as a functionof the variable characteristic of the driving signal. Thus in, say, a cathode raytube characterized by electrostatic deflection, the driving signal deflects the beam as a function of a variable voltage provided 'by the drivingsignal, whereas in, say, a cathode ray tube characterized by magnetic deflection the beam is deflectedas afunction ofa'variable current provided by the driving signal.

To correct for distortion the said signal modifier means is connectedin circuit betweenthesource means of the driving signal and the out-putterminal means. Thesignal modifier means responds to a correction 'signal to modify the variable characteristic of the driving signal before thensignal is'applied tothe tube andin a manner whereby the deflection impartedto the beam'by the modifiedcharacteristicof-thedriving-signal will be a linear function, to a preselected degree, of the unmodified characteristic-of the driving signal despite the-distortion introduced into the deflection bythegeometry ofthe tube.

The correction signal to which the modifier means rc-- sponds is developed by the electric analog computer means. This computer means responds to the modified characteristic of the driving signal to-render the correction signal a'preselected function of the third power of the'modified characteristic. It is this relation between ,thescorrection signal and the modified characteristic whichipermits attainment of a preselected degree of linearity between thedeflection of the-beam and the valued the variable characteristic of the unmodified drivingsignal. While ordinarily it is desirable to'render the deflection asclose as practicable to an exact linear function of the unmodifiedlinear characteristic in order to exactly compensate for the-cubic distortion introduced by the tube-geometry, in some instances it may bepreferable to depart somewhat from -linearityto provide overcompensation-or under-compensation. Such elimination of distortion-at the face of the 'tube to only a partial extent may be 'usefuL'for' example, to balance out an opposite distortion introduced by an optical system through which the tubeis viewed.

As a feature according to the invention, the antidistortion circuit'may be adapted for use with a cathode ray tube wherein the total vector deflection of the electron beam is the resultant of two component deflections of the beam at right angles to each other. In this lastnamed instance, the two component deflections are produced by two driving signals each of which is modified as described by a respective signal modifier means before being applied to the tube. The electric analog computer means responds to both modified driving signals to produce two correction signals which are respectively supplied to one and the other of the two modifier means of the two signals.

'For a further understanding of the invention reference is made to Fig. 5 which illustrates the invention when used to correct pincushion distortion in a cathode ray tube wherein the beam is magnetically deflected and wherein the entire vector deflection of the beam is produced by a single yoke current I. In Fig. 5 a source 30 of driver current I is connected to supply this driver current to a signal modifier :stage 31 which also receives a correction current 'I The stage 31 adds the currents I and I in an algebraic sense to produce the yoke current I. This yoke current I is supplied through the output terminal 32 to the yoke 14 of the cathode ray tube of Fig. 1 when, in the first instance, the antidistortion circuit and the cathode ray tube have been brought into assembled relation and when, at times thereafter, the circuit and the tube are operating.

In order to provide the correction current l a signal S proportional to the yoke current I is supplied as an input to an analog computer circuit 35. The computer circuit responds to signal S to develop the correction current I as a function of I and to supply this correction current to the modifier stage 31.

The requirement to be satisfied by the Fig. 5 circuit is that the circuit render the total deflection D of the electron beam a linear function of thedriver current I A linear relation of this sort is given by the expression D=K I (l7) and the particular relation expressed by *(17) can be realized by the Fig. 5 circuit in the following manner.

It is known from the algebraic adding action of modifier stage 31 that, a

I n+ e Substituting in (17) the equivalent established for I by (19) there is obtained the expression As the next step, the equivalent established for D by (2) can be substituted for D in (20) to yield the expression and, hence K1I+K2I =K1IK1Ic and solving (21) for 1 there is obtained n 3 I c Expression (22) is of significance in that it establishes the relation between I and I which is necessary to validate the equation set forth by expression (17). In other words, if the computer circuit of Fig. 5 responds to the signal S to develop a current I which varies as the function of I which is indicated in (22), the total vector deflection of the electron beam will, in fact, be a linear function of the driver current I in accordance with 17). It is to be noted in this connection that (22) was derived for the case where pincushion distortion is to be corrected. In this case I is, as indicated by the minus sign in (22), opposite in sense to I which is of the same sense as I Hence, I acts to correct pincushion distortion by rendering I less in magnitude than I If, on the other hand, the distortion to be corrected for were patter dist'ortion,the current 1 would be of the same 6 sense as I and I to render I of greater magnitude than I As indicated, the Fig. 5 circuit is adapted to correct distortion in the operation of a cathode ray tube wherein the beam is deflected by a single yoke. When distortion is to be corrected in a cathode ray tube having two orthogonal yokes, the Fig. 5 circuit is modified to the extent shown in Fig. 6. Referring to this last-named figure, the anti-distortion circuit of Fig. 6 comprises a pair of sources 46, 40 respectively supplying the driver currents I and I These two currents respectively control the deflection of the electron beam in the X coordinate and in the Y coordinate. The current 1 is fed to a modifier stage 41 adapted to algebraically add I and a correction current I to produce the yoke curent I Similarly, the current I is fed to a modifier stage 41 adapted to algebraically add I and a correction current Icy to produce the yoke current I The currents I and T are respectively supplied through the output terminals 42, 42 to the yokes 43, 43' of a cathode ray tube 44 when, in the first instance, the Fig. 6

circuit and the tube have been brought into assembled relation, and when, at times thereafter, the Fig. 6 circuit and the cathode ray tube are operating. As is evident, the passage of current I through yoke 43 will deflect the beam of tube 44 in the X coordinate, and the passage of current I through yoke 43' will deflect the said beam in the Y coordinate.

The correction currents I and Icy are developed by the analog computer circuit 45. This circuit responds to a signal 8;; proportional to I and the circuit also responds to a signal S proportional to I The computer circuit 45 operates in accordance with the information provided by the signals 8;; and 5,; to render I a preselected function of both I and T and to also render I another preselected function of both I and I As will soon be described, these preselected functions are of a character to produce a linear relation both between the driver current I and D (the total amount .of electron beam deflection in the X-coordinate) and between the driver current I and Dy (the total amount of electron beam deflection in the Y-coordinate).

The relation of each of correction currents I and Icy to the currents I l and I I of the Fig. 6 circuit can be derived in the same manner as the relation between I I and I was derived for the Fig. 5 circuit. Thus, by analogy to expressions (17), 19) and (20) for the Fig. 5 circuit, there are obtained the following expressions for the Fig. 6 circuit:

If, now, the equivalent established for D by (15) is substituted for D in (27) and if, likewise, the equivalent established for D by (16) is substituted for D in (28) the results are the following:

l X+ 2 X( X Y 1 X 1 CX i y-iz yux -iy )=K1 Y 1 CY The expression (29) and (30) can be manipulated to solve for I in (29) and to solve for I in (30). Such manipulation provides as an outcome the relations: IcX=- IX(IX +IY Expressions (31) and (32) are of significance in that they establish the manner in which I must be a 7 function of I and L and the manner in which Icy must he a function of 1;; and I in order to respectively w le validate the expressions (23) and (24). To put it another way, ifv in the Fig. 6 circuit l is rendered a function of I and 1-; in accordance with ('31), and if in the Fig. 6 circuit Icy is rendered a function of I and I in accordance with (32), then D (the amount of total deflection of the electron beam in the X coordinate) will, in fact, be rendered a linear function of the driver current I in accordance with (23), and D (the amount of total deflection of the electron beam in the Y coordinate) will, in fact, be rendered a linear function of the driver current I in accordance with (24).

In connection with the above, it will be noted, as in the case of .the Fig. circuit, that, if the Fig. 6 circuit is to be used to correct pincushion distortion, the correction currents I Icy are indicated 'by (31') and (32') as currents of opposite .sense to, respectively, I and 1,, whereby fI .and Icy will render I and L; of lesser magnitudes than, respectively, the currents 1 and I It is also to be noted, in connection with (31) that I is a function of I as well as I .and, in connection with (32) that Icy is a .function of I as well as I In other words, in order to *-fully correct for distortion in the X coordinate, it is necessary to also consider what is happening .in the Y coordinate. Conversely, in order to fully correct for distortion in the Y coordinate, itis necessary to also consider .what is happening in the X coordinate.

A specific embodiment of the Fig. '6 .circuit is shown inFig. 7. Referring to thislast-named figure, the block (shownin dotted outline) represents apush-pull circuit for deflecting the electronbeam of the cathode ray tube in the X coordinate. This deflection is accomplished by anX- coil 51 and by an X+ yoke coil 52 disposed in the deflection circuit 5.0. The driver current 'I for coil .51 passes through a current path extending from a +270 volt supply, through a junction 53, asensing resistor 54, the coil 51, the plate and cathode of a triode 55, a junction 56, the .plate and cathode of a triode 57 whose grid is connected to '-50 volts, a resistor58, and to a 270 volt supply. Similarly-the driver current I for coil -'52 flows through a current path extending from the +270 volt supply through junction 53, a sensing resistor 59, the coil52, the plate and cathode of a triode 60, the junction 56, and then back tothe 270 volt supply via the same path from.-junction 56 as that taken, by the driver current for coil 51. A resistor 61 and 'a resistor 62 are respectively connected in parallel with the series combinations of elements 54,51 and the series combinationof elements 59, 52.

The separate driver currents for the coils 51 and 52 are respectively produced by applying a pair of sawtooth signals of equal amplitude but of opposite polarity to'the terminals 63, 64, and from thence to the grids of the triodes and 60. ,Th@ opposite polarity variations of the sawtooths .causecpposite sense changes in the currents threading coils 51 and 52. In the view, however, that circuit 50 is a balanced push-pull circuit, the opposite sense variations of therespective currents in coils 5t and 52 will cancel each other out at junctions 53 and 56. Accordingly, an essentially constant current will flow from the '270 volt supply to junction 53 and an essentially constant current will-flow tronrjunction 56 towards triode 57.

It will be assumed in connection with circuit .50, that the coils 51 and 52 are arranged in relation .to-the cathode ray tube so that a leftward deflection of the electron beam is produced by an increase in current through coil 51 and by a corresponding decrease in current through coil 52. It follows that a rightward deflection of the beam will be poduced by a-decrease in current through coil 51 and by a corresponding increase in current througlreoil -52. As-willbe made clear hereafter, the currents which produce these beam deflections are not exactly the same as the-driver currents L;,;; and

I J but are instead the respective yoke currents which actually flow through the coils 51.and-52. These yoke currents will be designated as.I and I In circuit 5.0 the voltage drop across sensing resistor 5-4 will be proportional to the current I;; flowing through coil 51. Similarly, the voltage drop across sensing resistor 59 will be proportional to the current I flowing through coil 52. The two mentioned voltage drops are related to each other in that if one voltage drop changes, the other voltage drop will change by the same amount but in an opposite sense.

The voltage drops across resistors 54 and 59 are utilized to produce corresponding sensing signal inputs for. a squaring circuit generally designated by the block 70. These sensing signal inputs are developed in the following manner. A resistor 71, an adjustable gain potentiomcter 72 and a resistor 72a extend in series from the junction 73 of resistor 54 and coil 51 to a lower junction 74 which is connected through a resistor 75 to the -270 volt supply. This serial connection of resistor 71, pctentiometer 72 and resistor 72a forms a voltage dividing network which bleeds down the voltage drop across resistor 54 to produce a sensing signal of lowered but proportional voltage value on the tap 76 of the potentiometer 72. A similar voltage dividing network is formed by a resistor 77, an adjustable gain potentiometer 78 and a resistor 78a connected in series from the junction 79 of resistor 59 and coil 52 to the junction 74. This latter network bleeds down the voltage drop across resistor 59 to produce a sensing signal of lowered but proportional voltage value on the tap 80 of potentiometer 78. 'The sensing signals appearing on taps 76 and 80 will be representative, respectively, of the yoke current I and 1 f These two sensing signals will, of course, vary in opposite directions in the same way as their corresponding yoke currents vary inopposite directions.

The tap 76 of potentiometer 72 is connected to supply the sensing signal appearing thereon to the control grid of a pentode In like manner, the tap 38 0 of 'potentiometer 78 is connected to supply the sensing signal appearing on this last-named tap to the control grid of a pentode 86 which is similar to pentode 85. The pentodes 85 and 86 have commoned plates in the .sense that both pentodes draw plate current from the 270 volt supply through a common plate resistor 87 and aeommon junction 88.

When a pair of pentodes are connected in this manner to have commoned plates and to be respectively driven by two input signals which vary in opposite directions 'but whose .variations are equal .in magnitude, the pentodes will act to provide a squaring operation in the-sense that the combined current drawn by the pentodes will vary as thesquare of .the absolute magnitude of the these changes in -.value, although opposite indirectign,

are equal in magnitude, the pentodes 85 and 86 will together draw through resistor ;87 a combined plate current which is representative of the quantity 1 Such current produces .acrossrresistor .87'a lvoltage drop which isrepresentativev of the same quantity.

-.It iS'IO be-noted'in this connection that the variation occurring-in the ,voltagedrop across resistor .87 will be avariation in the same direction both whenthe variation is'induced by a negative-going signal on tap "76. and a.positive-going signal on tapfitl, andwhen the .variation is induced by .a-positive-going signal on tap 76 and a negative-going signalon tapr8tb. This is so, since his a matter of indiiference insofar as the observable voltageydrop aerossresistor--87 is concerned, as to which of pentodes -85,-86 produces thelarger fraction and which;the lesser fraction of the voltage drop by drawing the larger fraction and the lesser'fraction of the combined current. At the same time, it 'iS to be noted, however,

that the variation from zero value of the voltage across resistor 87 is a negative-going variation. Accordingly, the voltage signal produced by pentodes 85, 88 at junction 88 must be considered to represent 1,2 in the negative sense, or, in other words, be representative of -I Considering now the right-hand side of Fig. 7, the block 50' (shown in dotted outline) represents the circuit which is utilized to produce Y-coordinate deflections of the electron beam of the cathode ray tube. This Y-deflection circuit is analogousin all respects to the X- defiection circuit which has been hitherto described. As a further resemblance, the Y-deflection circuit 58 supplies to a squaring circuit 70' a pair of sensing signal inputs analogous to the hitherto described inputs which are supplied to the squaring circuit 78. As will be seen from Fig. 7, the squaring circuit 719 is a full counterpart of the squaring circuit '70. Moreover, the circuit 71) is connected like the circuit '70 in the sense that the pentodes 85, 86 have cornmoned plates which are connected to the junction 88 to draw a combined plate current through the resistor 87.

Reasoning by analogy from the heretofore described action of pentodes 85, 86 in circuit 70, it will be clear that the combined current drawn by the pentodes 85', 86 in circuit 70' will produce .across resistor 87 a voltage drop representative of the quantity I This lastnamed voltage drop will be in superposed relation with the voltage drop produced by pentodes 85, 86 and representing the quantity I Accordingly, the total voltage drop appearing at junction 88 will be representative of the quantity (I +I As stated, the voltage drop at junction 88 has a negative-going direction of variation with respect to its refer-- ence value. For purposes, however, of utilizing the voltage signal on junction 88 farther on in the Fig. 7 circuit, the polarity of the signal should be reversed so that the signal has a positive-going variation with respect to refer ence voltage. Such polarity reversal is accomplished as follows. A resistor 86 an adjustable gain potentiometer 91, and a resistor 92 are connected in series from the junction 88 to the 270 volt supply. The mentioned three resistance elements form a voltage divider network which bleeds down the voltage signal of junction 88 to produce a signal of lowered but proportional voltage value on the tap 93 of potentiometer 91. The signal on this tap is applied to the control grid of a triode 94 which, with another triode 95, forms an inverting stage 96 which is represented in Fig. 7 as a block having a dotted outline.

In the stage 96 the plates of triodes 94, 95 are connected to the +270 volt supply through similar plate resistors 97, 98. Also, the cathodes of these triodes are respectively connected through the similar cathode resistors 99, 101 to a junction 101 which is in turn connected through a resistor 102 to the 270 volt supply. The grid of triode 95 is connected to the tap 103 of an adjustable gain potentiometer 1% extending between ground and 50 volts.

A resistor 105 and another resistor 106 are connected in series to extend between the plate of the triode 95 and -270 volts. In like manner, a resistor 167, an ad justable gain potentiometer 108, and a resistor 109 are connected between the plate of triode 94 and -270 volts. The output of the inverting stage appears on the tap 110 of the adjustable gain potentiometer 108.

The inverting stage just described provides an output representing to a selected scale relation (by adjustment of taps 103, 110) and with reversed polarity the magnitude of the input signal to the stage. Since, as described, the input to stage 96 represents the quantity (IX2+IY2), the output at tap 110 of stage 96 will represent the quantity +(I +I This output is supplied both to a multiplying circuit 115 for the X-coordinate and to a multiplying circuit 115' for the Y-coordinate. Since the circuits 115 and 115 are counterparts, it will be neces- 1O sary to consider the output (I +Iy of stage 96 only insofar as this output affects the multiplying circuit 115.

The circuit comprises a pair of pentagrid tubes 120, 121 having grounded cathodes. The third grid of each of these tubes receives from tap 110 the signal representing (If-H 7 The first grid of tube is connected to the tap 122 of an adjustable gain potentiometer 123 in parallel with the potentiometer 72 in squaring circuit 70. Hence, in accordance with the prior description, the first grid of tube 120 will receive a signal representing the quantity of I;; In like manner, the first grid of tube 121 is connected to the tap 124 of an adjustable gain potentiometer 125 shunting potentiometer 78 whereby the last-named grid receives a signal representing the quantity 1 The plate of tube 120 is connected to the junction of the coil 51 and the triode 55. The plate of tube 121 is connected to the junction of the coil 52 and the triode 60.

When connected in the manner described, the pentagrid tubes 120 and 121 are adapted to act as multipliers. Hence, the plate current drawn by tube 120 will be representative of the quantity I (I +I and the plate current drawn by tube 121 will be representative of the quantity I X+ (I H By properly adjusting various adjustable gain potentiometers in the Fig. 7 circuit, these plate currents drawn by the tubes 120 and 121 can be made representative, respectively, of the quantities These last-named quantities correspond in form to the right-hand side of expression (31). Hence, from the foregoing description, it will be seen that the plate currents drawn by the pentagrid tubes 12%, 121 will not only act as respective correction currents I I for the driver currents energizing yoke coils 51, 52, but that, further, the said correction currents can be adjusted in value to render the deflection in the X-coordinate of the electron beam a linear function of the driving currents I and I J In exactly the same manner, the deflection of the electron beam in the Y-coordinate may be made a linear function of the driving currents which produce this last-named deflection.

The over-all operation of the Fig. 7 circuit may be understood by considering what happens when the electron beam of the cathode ray tube is given a leftward deflection in the X-coordinate. As stated, this leftward deflection is produced by varying the current in coil 51 in the direction which increases the current and by varying the current in coil 52 in the direction which decreases the last-named current. Theses current variations respectively produce an increase in the voltage drop across resistor 54 and a decrease in the voltage drop across resistor 59. As described, the changes in the voltage drops across resistors 54 and 59 will, in consequence of the squaring action of circuit '70, the contribution made to the squaring action by circuit '76 and the polarity reversing action of inverter stage 96, be translated into a positive-going signal representing (I +I The mentioned positive-going signal is supplied in equal measure to the third grid of pentagrid tube 120 and to the third grid of pentagrid tube 121. in the case of tube 120, the signal received by this tube on the first grid is a signal which is negative-going in consequence of the increasing voltage drop across resistor 54. This negative-going signal causes the plate current I drawn by tube 128 through coil 51 to decrease to thereby oppose the driver current 1 which is increasing. In the case of tube 121, the signal received by this tube on the first grid is a signal which is positive-going in consequence of the decreasing variation in the voltage appearing across sensing resistor 59. This last-named positive-going sig nal causes the plate current I drawn by tube 121 through coil 52 to increase to thereby oppose the driver current I which is decreasing.

To summarize, a leftward deflection of the electron beam is assumed to be produced by a positive-going variaqnp d ver ur e hroug c il .1 an by..a.,n s v going variation of driver currentthroughcoil 52. The positive-going driver current variationin coil :51 isopposed-by anegative-going variation of, correction current developed,in coil 51 by tube 120. The negative-going driver current variation in coil 52 is oppqsedinrthis coil bya positive-going variation of correction current de veloped in this coil by tube 121. The respective variations of the correction currents in coils 51 and 52 will act in push-pulLrelation to tendto decrease the leftward de flection of the electron beam, or, in other words, to tend to pull the electronbeam to the right. ,In theconverse situation where the beam is deflected to the right, the sense of the variationsof all currentswould be reversed, but the corrcction currents would still oppose their .re spectivelyassociateddriver currents to tend to decrease the deflection ofthe beam. Hence, by appropriately relating the values which are developed for the correction urrentsto the values which are developed .for the driver currents,.either a leftward or rightward .deflectionof the beam in the X-coordinate can be made substantiallya linear function of the magnitude of the driver currents producing the X-coordinate deflection. Inlike manner, either a leftward or rightward deflection ofthe beam in the Y -coordinate can be made substantially a linear function of the magnitude of the driver currents producing the .Y-cc-ordinate deflection. Thus, it is possible to eliminate most or allof the pincushion distortion which would otherwise be present in the X and Y coordinates. Barrel distortion can also be eliminated to the extent desired by modifying the Fig. 7 circuit slightly to have the correction currents aid rather than oppose the driver currents.

Suitable resistance values and other identifying characteristics of the elements of theFig. 7 circuit are as follows.

Element(s) 2 Characteristic 51 and 52 -100 ma. current variation. 54 and 59 100 ohms. 55, 57 and 60 Type 6550. 58 Q 1570 ohms. 61 and 62 K. ohms. 71 and 77 263K ohms. 72 and 78 K ohms. 72 and 72a 235K ohms. 5K ohms. and 86 Type 6SK7 87 5K ohms. 90 250K ohms. 91 c 25K ohms. 92 245K ohms. 94 and 95 Type 5965 97 and 98 5K ohms.

99 and 100 910 ohms. 102 43K ohms. 105 and 107 250K ohms.

106 270K ohms. 108 25K ohms. 1 09 245K ohms. and 121 Type 6356. 124 and .125 50K ohms.

Among the numerous advantages of the present invention, there are several which should be particularly mentioned. First, the practice of the invention does not eq ire the alteration of conventional cathode ray tube display equipment. The sole possible exception to the statement just made is that it is often desirable, as shown in Fig. 7, to insert the current sensing resistors at the center taps of the push-pull deflection yokes. Second, the

distortion correction provided by the invention is indcpendent, within the working frequencyrange, of thetime element of the display and of the sequence of positions that the beam is caused to traverse. The correction which is provided is a point-bypoint correction. Third, the

correction currents introduced into each of the half axes of each deflection yokeare adjustable independently of each other. Thus, it .is possible to correct eachedge of the display picture separately to compensate for a nonsymmetric distortion caused by misalignement of the cathode ray tube axis and ofthe axis of one or both of the yokes.

Fourth, in accordance with the invention,.the sense or sign of the correction currents may be reversed toremove barrel distortion that might be caused byan .overcompensated yoke. This would be done by reversing the connections to the sensing resistors. Fifth, a controlled amount of pincushion or barrel distortion may be introduced by the invention into a cathoderay tube display in order to compensate for a distortion of .an opposite sense existing in an optical system used in conjunction with the cathode ray tube display. Sixth, very exact correction of distortion may be obtained with the :invention due to the flexibility of adjustment permitted by the-invention, and due to the firm mathematical basis onwhich the invention is grounded.

The above-described embodiments being exemplary only, it will be understood that the invention comprehends embodiments differing in form and/ or detail from the above-described embodiments. For example, while the invention in some applications may include the means which deflects the electron beam as an integral part of the anti-distortion circuit, in oth r applications of the invention the beam deflecting means .may be a means which is primarily associated with the cathode ray tube but which acts as a component of the anti-distortion circuit when the circuit and the tube are operating. As another example, while the one or more correction signals vhave been described as being applied to the beam deflecting means to be directly combined therein with the one or more driving signals which operate the deflecting means, it is evident that the one or more correction signals may be applied to the deflecting means in such manner that the correcting signals are not directly combined with the driving signals but are, at the same time, effective to reduce distortion. Thus, .in-the instance where the beam is magnetically deflected, .the yoke means may be comprised of principal yoke means which responds to one or more drivercurrents .to develop a prin-' cipal magnetic .flux and of auxiliary yoke .means which responds to one or more correction currents to develop an auxiliary magnetic flux, the principal flux and the auxiliary flux being spatially related whereby the conjoint deflecting action of both fluxes on the beam produces a distortionless deflection thereof. Moreover, while, as exemplified by the Fig. 7 circuit, the invention may be utilized in conjunction with cathode ray tubes which employ double-ended deflection yokes, the invention may also be utilized in conjunction with tubes of this sort which employ only single-ended yokes. As another consideration, while, as exemplified by the Fig. 7 circuit, vacuum tubes may be used to carry out the various electrical operations by means of which the invention obtains its results, it is consistent with the invention to. use diodes or other non-linear devices to accomplish these operations. In fact, it may in some instances be found preferable to use such diodes or other non-linear elements in order to simplify the circuitry of the invention, and in order to obtain improvement in the factors of speed and reliability. Accordingly, the invention is not to be considered as limited, save as is consonant with the scope of the following claims.

We claim:

1. A circuit to correct cubic distortion in a display produced by an electron beam in a cathode ray tube having means to deflect said beam, said circuit compris ing, source means of a variable amplitude driver signal adapted to provide a variable amplitude input to said deflecting means to thereby cause said deflection of said beam, electric analog computer means responsive .to the amplitude variation of said input to develop a correction signal as a function of the third power of the variation of said input, and means to apply said correction signal to said deflecting means in an amount appropriate to render the observed deflection of said beam a linear function of the variation of said driver signal.

2. A circuit to correct cubic distortion in a display produced by magnetic deflection of an electron beam in a cathode ray tube, said circuit comprising, yoke means to magnetically deflect said beam, source means of variable amplitude driver current providing variable amplitude yoke current for said yoke means, electric analog computer means responsive to the variation of said yoke current to develop a correction current as a function of the third power of said variation, and means to apply said correction current to said yoke means in an amount appropriate to render the observed deflection of said beam a linear function of the variation of said driver current.

3. A circuit to correct cubic distortion in a display produced by magnetic deflection of an electron beam in a cathode ray tube, said circuit comprising, yoke means to magnetically deflect said beam, source means of variable amplitude driver current providing variable amplitude yoke current for said yoke means, means responsive to the variation of said yoke current to develop a voltage signal as a function of said variation, electric analog computer means responsive to said voltage signal to develop a correction current as a function of the third power of said variation, and means to apply said correction current to said yoke means in an amount appropriate to render the observed deflection of said beam a linear function of the variation of said driver current.

4. A circuit as in claim 3 wherein said means which develops said voltage signal is in the form of resistor means in series with said yoke means.

5. A circuit as in claim 3 wherein said computer means comprises, a squaring stage responsive to said voltage signal to develop an intermediate signal as a function of the square of said voltage signal, and a multiplying stage responsive to both said voltage signal and to said intermediate signal to develop said correction current as a function of the product of said last-named signals.

6. A circuit to correct cubic distortion in a display produced by magnetic deflection of an electron beam in a cathode ray tube, said circuit comprising, yoke means to magnetically deflect said beam, source means or" variable amplitude driver current providing variable amplitude yoke current for said yoke means, electric analog computer means having an output and responsive to the variation of said yoke current to develop at said output a correction current as a function of the third power of said variation, and means connecting said output in circuit with said yoke means to render said yoke current the resultant of said driver current and of an amount of said correction current appropriate to render the observed deflection of said beam 2. linear function of the variation of said driver current.

7. A circuit as in claim 6 wherein said yoke current is the resultant of a driver current and a correction current of opposite sense whereby said circuit corrects for pincushion distortion.

8. A circuit as in claim 6 wherein said yoke current is the resultant of a driver current and a correction current of the same sense whereby said circuit corrects for barrel distortion.

9. A circuit to correct cubic distortion in a display produced by magnetic deflection of an electron beam in a cathode ray tube, said circuit comprising, yoke means to magnetically deflect said beam, source means of variable amplitude driver current providing variable amplitude yoke current for said yoke means, means responsive to the variation of said yoke current to develop a voltage signal as a function of said variation, electric analog com puter means having an output and responsive to said voltage signal to develop at said output a correction current as a function of the third power of said variation, and means connecting said output in circuit with said yoke means to render said yoke current the resultant of said driver current and of an amount of said correction current appropriate to render the observed deflection of said beam a linear function of the variation of said driver current.

10. A circuit to correct cubic distortion in a display produced by an electron beam in a cathode ray tube having first and second means to respectively deflect said beam in first and second coordinates at right angles, said circuit comprising, source means of first and second variable amplitude driver signals adapted to provide first and second variable amplitude inputs to, respectively, said first and second deflecting means to thereby cause said deflections of said beam in said first and second coordinates, electric analog computer means responsive to the ampli tude variation of each of said first and second inputs to develop a first correction signal as a function of the variation of said first input as multiplied by a factor representing the sum of the squares of the variations of said first and second inputs, and to develop a second correction signal as a function of the variation of said second input as multiplied by said factor, and means to apply said first and second correction signals to said first and second deflecting means in amounts appropriate to render the observed deflections of said beam in said first and second coordinates related in a linear manner to, respectively, the variations of said first and second driver signals.

ll. A circuit to correct cubic distortion in a display produced by magnetic deflection of an electron beam in a cathode ray tube, said circuit comprising, first and second yoke means to magnetically deflect said beam in first and second coordinates at right angles, source means of first and second variable amplitude driver currents providing first and second variable amplitude yoke currents for, respectively, said first and second yoke means to thereby cause said respective deflections of said beam in said first and second coordinates, electric analog computer means responsive to the amplitude variation of each of said first and second yoke currents to develop a first correction current as a function of the variation of said first yoke current as multiplied by a factor representing the sum of the squares of the variations of said first and second yoke currents, and to develop a second correction current as a function of the variation of said second yoke current as multiplied by said factor, and means to apply said first and second correction currents to, respectively, said first and second yoke means in amounts appropriate to render the observed deflections of said beam in said first and second coordinates related in a linear manner to, respectively, the variations of said first and second yoke currents.

12. A circuit to correct cubic distortion in a display produced by magnetic deflection of an electron beam in a cathode ray tube, said circuit comprising, first and second yoke means to magnetically deflect said beam in first and second coordinates at right angles, source means of first and second variable amplitude driver currents pro viding first and second variable amplitude yoke currents for, respectively, said first and second yoke means to thereby cause said respective deflections of said beam in said first and second coordinates, electric analog computer means having first and second outputs and responsive to the amplitude variation of each of said first and second yoke currents to develop at, respectively, said first and second outputs a. first correction signal as a function of the variation of said first yoke current as multiplied by a factor representing the sum of the squares of the variations of said first and second yoke currents, and to develop a second correction current as a. function of the variation of said second yoke current as multiplied by said factor, and means respectively connecting said outputs in circuit with said first and second yoke means to'render said first and second yoke currents the resultants, respectively, of said first driver current and said first correction current and of said second driver current and said second correction current, said first and second correction currents being present as components of said first and second yoke currents in amounts appropriate to render the observed deflections of saidbeam in said first and second coordinates related in a linear manner to, respectively, the variations of said first and second yoke currents.

13. A circuit to correct cubic distortion in a display produced by magnetic deflection of an electron beam in a cathode ray tube, said circuit comprising, first and second yoke means to magnetically deflect said beam in first and second coordinates at right angles, source means of first-and second variable amplitude driver currents providing first and secondvariable-amplitude yoke currents for, respectively, said-first and second yoke means to thcrebycause said respective deflections of saidbeam in said first and second coordinates, first and second sensing means responsive .to the respective variations of said first and second yoke currents to develop first and second voltage signals as respective functions of said variations, and analogicomputer means comprising first squaring stage responsive to-said first voltage signal to develop a first intermediate signal as a function of thesquarc of said first voltage signal, a second squaring stage responsive to said second voltage signal to develop asecond intermediate signal as a functionof the square of said second voltage signal, a stage wherein said first and'seconcl intermediate signals combined to provide a combincdsignal'reprcsenting the sum of the squares of said first and second voltage signals, a firstmultiplying stage responsive to both said first voltage signal and said combined signal to develop a first correction curr nt as a fun tion of the variation of said first yoke current as multipliedby a factor representing the sum of the squares of the variations of said first and second yoke currents, and a second mu't ing stage responsive to both said second voltage signal and said combined signal ;to develop a second correction current as a function of the variation of said second yoke current as multiplied by said factor, said circuit further comprising means to apply said firstand second correcton currents to, respectively, said first and second yoke means in amounts appropriate to render the observed deflections of said beam in said first and second coordinates related in a linear manner to, respectively, the variations of said first and second yoke currents.

14. A circuit to correct cubic distortion in a display produced by the deflection of an electron beam in a cathode ray tube wherein said deflection is produced by pushpull means formed of first and second deflecting rneans.v said circuit comprising, source means of first and second driving signals which undergo opposite sense variations of equal magnitude to provide first and. second input signals which undergo corresponding opposite sense variations of equal magnitude and which are applied to, respectively, said first and second deflecting cans to thereby cause said deflection of said beam, electric analog computer means responsive to both the variation of s -.d first input signal and the variation of said second input signal to produce first and second corre ion signals of equal nitude which vary in an opposite sense and for each of which the magnitude is a function of the third power of the variation in magnitude of said input signals, and means to apply said first and second correction signals to, respectively, said first and second deflecting means in amounts appropriate to render the observed defic ic of said beam a linear function of the variation in ma tude of said driver signals.

15. A circuit to correct cubic distortion in a display produced by magnetic deflection of an electron beam in a cathode ray tube, said circuit comprising, push-pull yoke means formed of first and scond winding means disposed to producerespective magnetic fluxes in a common direction and'in aiding relation when said winding means are respectively energized by first and second input currents whichundergo opposite sense variations of equal magnitude, said winding means being further disposed to be adapted to deflect said beam by the magnetic field set up by said fluxes, source means of first and second driver currents which undergo'opposite sense variations of equal magnitude'to provide said first and second input currents, electric analog computer means responsive to both the variation of said first input current and the variation of said second input current to produce first and second correction currents which undergo opposite sense variations of equal magnitude and for each of which the magnitude isa function of the third power of the variation in magnitude of said input currents, and means to apply said first and second correction currents-to, respectively, said first and second Winding means in amounts appropriateto render the observed deflection of said beam a linear function of the variation in magnitude of said driver currents.

16. A circuit to correct cubic distortion in a display produced by magnetic deflection of an electron beam in a cathode ray tube, said circuit comprising, first pushpull yoke means formed of first and second winding means disposed to produce respective magnetic fluxes in a common direction and in aiding relation when said winding means are respectively energized by first and second input currents which undergo opposite sense variations of equal magnitude, said Winding means being further disposed to be adapted to deflect said beam in a first coordinate by the magnetic field set up by 'said fluxes, source means of first and second driver currents which undergo opposite sense variations of equal magnitude to provide said first and second input currents, second push-pull yoke means formed of third and fourth winding means disposed to produce respective magnetic fluxes in a common direction and in aiding relation when said last-named Winding means are respectively energized by third and fourth input currents which undergo opposite sense variations of equal magnitude, said third and fourth Winding means being further disposed to be adapted to deflect said beam by the magnetic 'field set up by the fluxes of said third and fourth means in a second coordinate at right angles to said first coordinate, source means of third and fourth driver currents which undergo opposite sense variations of equal magnitude to provide said third and fourth input currents, electric analog computer means responsive to the variations of all said input currents to develop first and second correction currents which undergo opposite sense variations of equal magnitude and for each of which the magnitude is a function of the variation in magnitude of said first and second input currents as multiplied by a factor representing the sum of the square of the variation in magnitude of said last-named currents and the square of the variation in magnitude of said third and fourth input currents, said computer means further developing third and fourth correction currents which undergo opposite sense variations of equal magnitude and for each of which the magnitude is a function of the variation in magnitude of said third and fourth input currents as multiplied by said factor, means to apply said first and second correction cur rents to, respectively, said first and second winding means in amounts appropriate to render the observed deflection of said beam in said first coordinate a linear function of the variation in magnitude of said first and second driver currents, and means to apply said third and fourth correction currents to, respectively, said third and fourth Winding means in amounts appropriate to render the observed deflection of said beam in said second coordinate a linear function of the variation in magnitude of said third and fourth driver currents.

No references I cited.

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