US3725722A

US3725722A - Display centering system

Info

Publication number: US3725722A
Application number: US00134961A
Authority: US
Inventors: G Mann
Original assignee: United Aircraft Corp
Current assignee: Raytheon Technologies Corp
Priority date: 1971-04-19
Filing date: 1971-04-19
Publication date: 1973-04-03
Anticipated expiration: 1990-04-03
Also published as: FR2134059B1; IL39196A0; GB1368539A; FR2134059A1; CA996683A; IL39196A

Abstract

A display centering system comprises a cathode ray tube having a metalized screen which is etched to provide at least a pair of electrically discrete sectors. The split screen acts as a target anode; and is operated at a potential to collect all primary and secondary electrons. The beam current is momentarily increased at predetermined points of the raster independently of the normal video signals; and the differential anode currents between the two halves of the split screen are measured at such points of the raster to determine positioning errors. In order to avoid excitation of the phosphors to a visible level, the points in the raster at which the beam current is momentarily increased is successively varied from field to field and from frame to frame. The metalized layer of the screen is preferably split along a line passing through the center of the display so that the display may be centered with the highest accuracy. Information can be readily displayed at the center of the screen, since at any given point the video signal will only occasionally be affected by the pulse of increased beam current which is used to establish the center of the display.

Description

United States Patent [191 Mann [4 1 Apr. 3, 1973 [54] DISPLAY CENTERING SYSTEM [57] ABSTRACT Inventofi George Mann, Galeshead A display centering system comprises a cathode ray 97YA, England tube having a metalized screen which is etched to pro- [73] Assigneez United Aircraft Corporation, East vide at least a pair of electrically discrete sectors. The Hartford Conn. split screen acts as a target anode; and is operated at a potential to collect all primary and secondary elecl Filedl p 19, 1971 trons. The beam current is momentarily increased at [21] AppL No: 134,961 predetermined points of the raster independently of the normal video signals; and the differential anode currents between the two halves of the split screen are U-S- CL R, R measured at uch points of the aster to determine [51] Int. Cl. ..H0lj 29/70 positioning errors In Order to avoid excitation f the Field of Search "315/21 22, 23, 27 27 TD phosphors to a visible level, the points in the raster at which the beam current is momentarily increased is [56] References cued successively varied from field to field and from frame UNITED STATES PATENTS to frame. The metalized layer of the screen is preferably split along a line passing through the center 3,609,219 1/1970 Diehl ..315/27 TD of the display so that the display may be centered with 2,855,540 HOOVCI' et a]. ..315/21 R the highest a curaxa Information can be readily dis- 2,630,548 3/1953 Muller ..315/21 R Primary ExaminerCarl D. Quarforth Assistant ExaminerJ. M. Potenza Attorney-Melvin Pearson Williams played at the center of the screen, since at any given point the video signal will only occasionally be affected by the pulse of increased beam current which is used to establish the center of the display.

27 Claims, 5 Drawing Figures VERY- sweep PATENTEUAPR3 I975 BKV.

sum 2 OF IN VE A 102 76 F Geo/ye Map HT TORNEYS DISPLAY CENTERING SYSTEM BACKGROUND OF THE INVENTION center of the display adjacent the center of the tube.

Furthermore, the beam current is increased at the same points in the raster so that it is necessary to mask the phosphors in these areas adjacent the edges of the display, thus obscuring the field of view.

SUMMARY OF THE INVENTION One object of my invention isto provide a display centering system employing a split metalized screen as the target anode.

Another object of my invention is to provide a display centering system in which the metalized screen is split along a line passing through the center of the display.

A further object of may invention is to provide a display centering system in which the cathode ray tube beam current is momentarily increased at points in the raster which vary from field to field and from frame to frame and which lie along the line of splitting of the metalized screen.

Other and further objects of my invention will appear from the following description.

BRIEF DESCRIPTION OF THE DRAWINGS In the accompanying drawings which form part of the instant specification and which are to be read in conjunction therewith and in which like reference numerals are used'to indicate like parts in the various views: I

FIG. 1 is a schematic view showing a first embodiment of my invention;

FIG. 2 is a fragmentary schematic view showing a second embodiment of my invention;

FIG. 3 is a fragmentaryschematic view showing a third embodiment of my invention;

FIG. 4 is a fragmentary schematic view showing a fourth embodiment of my invention;

FIG. 4a is a diagrammatic view showing the pattern of beam current modulation provided in FIG. 4.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to FIG. 1 of the drawings, a cathode ray tube 10 is provided with a metalized-screen which may comprise a thin and transparent aluminized layer between the transparent glass face of the tube and the phosphor layer. The metalized layer is etched along a horizontal line to provide upper quadrants 12 and and

lower quadrants

14 and 18. The metalized layer is also etched along a substantially vertical line to provide

left quadrants

12 and 14 and

right quadrants

15 and 18.

In FIG. 1, quadrants l2 and 15 determine the lateral position of an upper portion of the display; quadrants l4 and 18 determine the lateral position of a lower portion of the display; and quadrants l5 and 18 determine the vertical position of a central portion of the display. Quadrants l2 and 14 are not used to determine'the vertical position of the display, since they abut along a line which is remote the center thereof.

The embodiment of FIG. 1 is adapted for horizontal and vertical centering of the display as well as correcting for rotational errors.

A 10 kilovolt source of anode potential 26 is connected to center taps of the primary windings of

transformers

20, 22, and 24. The terminals of the primary winding of transformer 20 are connected to

respective quadrants

12 and 15. The terminals of the primary winding of transformer 22 are connected to

respective quadrants

15 and 18. The terminals of the primary winding of transformer 24 are connected to

respective quadrants

14 and 18. Anode source 26 is connected to the positive terminal of a 25 volt battery 26a, the negative terminal of which is connected to the internal aquadag coating on the side walls of the cathode ray tube. One terminal of the secondary winding of each of

transformers

20, 22, and 24 is grounded. The other terminal of the secondary winding of transformer 20 is coupled through a gate 30 to a low-pass R-C filter 36 having a time-constant of 8 microseconds. The other terminal of the secondary winding of transformer 22 is coupled through a gate 34 to a low-pass R-C filter 40 having a time-constant of l as. The other terminal of the secondary winding of transformer 24 is coupled through a gate 31 to a low-pass R-C filter 38 having a time-constant of 8 us. The outputs of

filters

36 and 38 are applied to a differential amplifier 46 and to a summing amplifier 42. The output of differential amplifier 46 is coupled to a 0 input of cathode ray tube 10 which controls rotation of the display. The display may be rotated by mechanically rotating the deflection yoke. Preferably, the rotation input 0 comprises a winding downstream of the deflection yoke which provides an axial field component as shown in the co-pending application of David E. Wadlow for Dynamic Rotation of Cathode Ray Tube Display, Ser. No. 105,918, filed Jan. 12, 1971. The output of summing amplifier 42 is coupled through a resistor 44 to the horizontal deflection input H of cathode ray tube 10. The output of filter 40 is applied to an amplifier 48, the output of which is coupled through a resistor 50 to the vertical deflection input V of cathode ray tube 10. A source of video signals 93 is coupled through a resistor 94 to the intensity input I of tube 10.

A 30,720 Hz oscillator 52 is coupled to a divide-bytwo flip-flop 53. A plus output of flip-flop 53 is applied to a differentiating circuit 54. The output pulses of differentiating circuit 54 are coupled forwardly through a diode 55 to the indexing input of a recycling eight-bit binary counter 58 and to the retrace input of a horizontal sweep generator 56. Sweep generator 56 provides a triangular wave form which rises from zero to volts and then retraces to zero again. The retrace period for sweep generator 56 is 1 us. Sweep generator 56 provides a blanking output (not shown) which is applied to inhibit the intensity input of tube 10 and thus prevent retrace lines from appearing in the display. The repetition frequency of the triangular wave form provided by sweep generator 56 is 15,360 Hz. Counter 58 provides outputs proceeding 0, 1, 2, 126, 127, 128, 129, 254, 255, and 0 again. The output of counter 58 is coupled to a circuit 60 which detects a count of 0 and to a circuit 59 which detects a count of 128. Counter 58 provides bits having respective weights of l, 2, 4, 8, 16, 32, 64, and 128. The output of circuit 60 is applied to a differentiating circuit 61, the output of which is coupled forwardly through a diode 62 to a divide-by-two flipflop 92 and to the retrace input of a 60 Hz vertical sweep generator 63. Sweep generator 63 provides a sawtooth wave form having a linearly rising ramp and a rapid retrace.

The outputs of sweep generator 63 and divide-bytwo flip-flop 92 are coupled through

respective summing resistors

70 and 69 to one terminal of a capacitor 71, the other terminal of which is coupled to the vertical deflection input V of tube 10. The output of divide-by-two flip-flop 92 is coupled forwardly through a diode 65 to index a recycling six-bit binary counter 66. Counter 66 provides counts proceeding 1, 62, 63, and 0 again. Counter 66 provides output bits having respective weights of l, 2, 4, 8, l6, and 32. The six output bits of counter 66 are coupled to one input of a digital comparator 78. The six least significant bits of counter 58 are coupled to the other input of comparator 78.

The output of sweep generator 56 is coupled through a capacitor 57 to the horizontal deflection input H of tube and is further connected to a plus input of a differential amplifier 72. The output bits of counter 66 are applied to a digital-to-analog converter 67 which is supplied with a volt input. As the output of counter 66 varies from 0 to 63", the output of converter 67 correspondingly varies from zero volts to +20 volts. The output of converter 67 is applied to one input of a summing amplifier 68, the other input of'which is supplied with a constant +30 volts. The output of summing amplifier 68 is applied to a minus" input of differential amplifier 72. The output of circuit 59 is coupled to one input of AND circuit 73. The output of divide-by-two flip-flop 92 is coupled to the other input of AND circuit 73. The outputs of

circuits

72 and 73 are applied to respective inputs of AND circuit 74, the output of which is coupled to a differentiating circuit 75. The output of differentiating circuit 75 is coupled forwardly through a diode 76 to trigger a monostable multivibrator 77 which provides an output pulse of 0.25 us duration. The output of multivibrator 77 enables gate 34 and is applied to one input of OR circuit 91. The output of OR circuit 91 is coupled to the intensity input 1 of tube 10.

The 32 bit output of counter 58 is coupled to an enabling input of AND circuit 80 and to an inhibiting input of AND circuit 79. The 64 bit output of counter 58 is applied to an inhibiting input of AND circuit 80 and to an enabling input of AND circuit 79. The outputs of AND circuits 79 and 80 are coupled through an OR circuit 81 to one input of AND circuit 82. The output of comparator 78 is applied to a second input of AND circuit 82; and the minus output of divide-by-two flip-flop 53 is coupled to a third input of AND circuit 82. The output of AND circuit 82 is coupled to inputs of AND

circuits

83 and 84. The 128 bit output of counter 58 is applied to the other input of AND circuit 84 and to an inhibiting input of AND circuit 83. The outputs of AND

circuits

83 and 84 are coupled to respective differentiating circuits 85 and 86. The output of differentiating circuit 85 is coupled forwardly through a diode 87 to trigger a monostable mu1tivibrator 89 which provides an output pulse of 1 us duration. The output of differentiating circuit 86 is coupled forwardly through a diode 88 to trigger a monostable multivibrator 90 which provides an output pulse of 1 us duration. The outputs of

multivibrators

89 and 90 are coupled to further inputs of OR circuit 91. Multivibrator 89 enables gate 30; and multivibrator 90 enables gate 31.

In operation of the circuit of FIG. 1, the display provides 60 fields per second, each of 256 lines, and 30 frames per second, each of 512 lines. Interlaced scanning is provided by flip-flop 92 which couples small positive and negative currents through resistor 69. This varies the vertical position of the horizontal scan lines for alternate fields.

Battery 26a preferably provides a potential of at least 10 to 15 volts so that metallized

quadrants

12, 14, 15, and 18 of the screen will collect substantially all of the beam current. Secondary electrons emitted from the screen by virtue of the high voltage through which the beam of electrons is accelerated will be attracted back to the screen and will not be collected by the aquadag coating on the inner side walls of the tube.

Once each frame multivibrator 77 provides a bright-up pulse which is coupled through OR circuit 91 to the intensity input of tube 10. This bright-up pulse is provided when the output of horizontal line counter 58 is 128 and when the output of divide-bytwo flip-flop 92 is positive. Thus during the 128th line of the first field of each frame, AND circuit 73 provides an output. This horizontal sweep line passes substantially through the center of the display. The position along this horizontal sweep line at which multivibrator 77 provides a bright-up pulse is controlled by differential amplifier 72. During the course of 64 frames, the bright-up pulse provided by multivibrator 77 varies from position .1 to position K. Position .1 is located 37.5 percent of display width from the left-hand margin; and position K is located 62.5 percent of display width from the left-hand margin. For one frame the output of frame counter 66 will be 0; and the output of converter 67 will be zero volts. The output of summing amplifier 68 is thus 30 volts. Since sweep generator 56 provides a maximum output of 80 volts, the output of differential amplifier 72 will become positive at a point during the horizontal sweep which is 30/80 0.375 of display width from the left-hand margin. When the output of differential amplifier 72 becomes positive, AND circuit 74 provides an output which is coupled through differentiating circuit and diode 76 to trigger multivibrator 77 and thus provide bright-up pulse .1. For the subsequent frame, the output of counter 66 will be 1"; and the output of converter 67 will be 20/64 0.3125 volt. Summing amplifier 68 will provide an output of 30.3125 volts. The output of differential amplifier 72 becomes positive at a slightly later point during the horizontal sweep. With a l output from counter 66, the bright-up pulse provided by multivibrator 77 will be shifted to the right by 0.3125/ 0.39 percent of display width, which corresponds to a position of 37.89 percent of display width from the left-hand margin. Since the horizontal sweep frequency is 15,360 Hz, the period between horizontal sweeps is 65 us. With a period of 1 us for horizontal retrace, the effective time for each horizontal sweep is 64 us. The spacing between points .1 and K is 25 percent of display width or 64/4 16 us. The pulse duration of multivibrator 77 has been limited to 0.25 us, so that during the course of 64 frames a corresponding number of bright-up pulses may be provided along the line between points J and K without any overlaps. This is shown in FIG. 4a for the horizontal scan line indicated generally by the reference numeral 128+. With an output from counter 66 of 63, the bright-up pulse provided by multivibrator 77 during the 128+ horizontal scan line will be initiated at 62.11 percent of width from the left-hand margin and will be terminated at 62.5 percent of display width from the left-hand margin corresponding to point K.

Each pulse provided by multivibrator 77 is not only applied through OR circuit 91 to the intensity input of tube but also enables gate 34 to couple the output of the secondary winding of transformer 22 to low-pass filter 40. If the display is above center, then the beam current will be collected by quadrant and a negative output will appear across the secondary winding of transformer 22. On the other hand, if the display is below center, then the beam current will be collected by quadrant 18 and the output across the secondary winding of transformer 22 will be positive. The beam diameter will appreciably exceed the width of the horizontal etched line separating quadrants l5 and 18. When the display is precisely centered, equal portions of the beam current will be collected by

quadrants

15 and 18; and the output voltage across the secondary winding of transformer 22 will be zero. The time-constant of low-pass filter is four times that of the pulses provided by multivibrator 77. Accordingly, filter 40 stores the average error over four successive frames. This provides sufficient integration or smoothing to minimize the effects of noise. The output of filter 40 is applied through amplifier 48 and resistor 50 to the vertical deflection input to adjust the vertical position of the display so that the 128th scan line of the first field of each frame coincides with the etched horizontal line separating quadrants l5 and 18.

In operation of the horizontal centering system of FIG. I, assume frame counter 66 has just been indexed to 0 so that it will retain this count for two successive fields. Comparator 78 will provide an output for those horizontal scan lines where the six least significant bit outputs of counter 58 are all zeros. This will occur when counter 58 provides counts of0, 64, 128, and 192. AND circuit 79 provides an output in the presence of a 64 bit output from counter 58 and in the absence of a 32 bit output. Accordingly, AND circuit 79 provides an output when counter 58 provides counts ranging between 64 and 95 and also between 192 and 223". AND circuit 80 provides an output in the presence of a 32 bit output from counter 58 and in the absence of a 64 bit output. Accordingly, AND circuit 80 provides an output when counter 58 provides counts ranging between 32 and 63 and also between 160 and 191. The plus output of divide-by-two flip-flop 53 becomes positive at the beginning of each horizontal scan line; and the minus output thereof becomes positive at the midpoint of each horizontal scan line. When the count of counter 66 is 0, AND circuit 79, comparator 78, and the minus output of flip-flop 53 cause AND circuit 82 to provide an output at the midpoint of each of scan lines 64 and 192. During horizontal scan line 64, the 128 bit output of counter 58 is zero; and the output of AND circuit 82 is coupled through AND circuit 83 and thence through differentiating circuit 85 and diode 87 to trigger multivibrator 89. The 1 us pulse output of multivibrator 89 is coupled through OR circuit 91 to the intensity input of tube 10 which produces the short horizontal bright-up line B. During horizontal scan line 192, the 128 bit output of counter 58 is one; and the output of AND circuit 82 is coupled through AND cir cuit 84 and thence through differentiating circuit 86 and diode 88 to trigger multivibrator 90. The l as output pulse of multivibrator 90 is coupled through OR circuit 91 to the intensity input of display tube 10 which produces on the screen the short horizontal bright-up line E.

Duringthe second field of the same frame, the count of counter 66 will remain 0. Multivibrator 89 will again provide a bright-up line at the midpoint of the 64th scan line; and multivibrator 90 will again provide a bright-up line at the midpoint of the l92nd scan line. However, because of the interlacing signal from divide by-two flip-flop 92 which is coupled through resistor 69, the scan lines for the second field of each frame are displaced below those of the first frame by half the distance between adjacent scan lines thereof. Accordingly, the bright-up pulse of multivibrator 89 dur ing the second field is slightly below line B; and the bright-up pulse of multivibrator 90 is slightly below line E During each field, there are provided two short horizontal bright-up lines which are separated by 50 per cent of raster height.

For both fields of the next frame, the count of counter 66 will be l Comparator 78 will provide an output when counter 58 provides counts of

l

65, 129", and 193. At the midpoint of the 65th scan line multivibrator 89 will provide a bright-up line which is further below line B; and at the midpoint of the l93rd scan line, multivibrator 90 will provide a bright-up line further below line B.

When the output of frame counter 66 is 31, comparator 78 will provide an output when the count of counter 58 is 31", 95, 159, and 223. AND circuit 79 causes AND circuit 82 to provide an output at the midpoint of the 95th and 223rd scan lines. During the second field of this frame where the output of counter 66 is 31, multivibrator 89 provides the bright-up line C; and multivibrator 90 provides the bright-up line F.

During the next frame the output of counter 66 is 32. Comparator 78 will provide an output when counter 58 provides counts of 32, 96, 160 and g 224. AND circuit 80 now causes AND circuit 82 to provide an output at the midpoint of horizontal scan lines 32 and 160. During the first field of this frame where the output of counter 66 is 32, multivibrator 89 provides the bright-up line A; and multivibrator 90 provides the bright-up line D. During the second field of this frame, multivibrator 89 provides a bright-up line slightly below line A; and multivibrator 90 provides a bright-up line slightly below line D.

When the output of frame counter 66 is 63, comparator 78 will provide an output when the count of counter 58 is 63, 127, 191, and 255. AND circuit 80 causes AND circuit 82 to provide an output at the midpoint of the 63rd and 191st horizontal scan lines. During the second field of this frame where the output of counter 66 is 63", multivibrator 89 provides a bright-up line slightly above line B; and multivibrator 90 provides a bright-up line slightly above line E.

During the next frame, counter 66 is recycled to the count of During the first field of this frame, mul tivibrator 89 again provides the bright-up line B; and multivibrator 90 again provides the bright-up line E. During the course of 64 frames, short bright-up lines will be produced for every raster line between those containing bright-up lines A and C and also between those containing bright-up lines D and F. However, none of these bright-up lines will be visible in the display since any given line appears only once during 64 frames.

Each bright-up pulse provided by multivibrator 89 is not only coupled through OR circuit 91 to the intensity input of display tube 10, but also enables gate 30 to couple the output of the secondary winding of transformer 20 to low-pass filter 36. Transformer 20 responds to those bright-up lines between line A and C. If the display is laterally centered, then the beam current will be initially collected by quadrant 12 for a period of 0.5 1.1.8, producing a negative output across the secondary winding of transformer 20. Subsequently, the beam current will be collected by quadrant 15 for an equal period of 0.5 as, producing a positive output across the secondary winding of trans former 20. When the display is laterally centered, the

v durations of collection of beam current by quadrants l2 and 15 are equal; and the output across the secondary winding of transformer comprises a full cycle of a symmetrical square wave having equal negative and positive durations. Since the average value of a symmetrical square wave is zero, the output from low-pass filter 36 will likewise be zero when the display is laterally centered. The time-constant of filter 36 is eight times that of the pulses provided by multivibrator 89. Since one horizontal bright-up line is provided in the upper portion of the display for each field, filter 36 stores the average error over eight successive fields corresponding to four successive frames. This provides sufficient integration or smoothing to extract the average value of the full cycle square wave output across the secondary winding of transformer 20. If the display is to the left of center, then the beam current will be collected by quadrant 12 for a longer duration than by quadrant 15. Accordingly the output across the secondary winding of transformer 20 will comprise a square wave pulse wherein the duration of the negative half cycle is longer than that of the positive half cycle. The average value of this asymmetrical square wave will be negative; and low-pass filter 36 will correspondingly provide a negative output. On the other hand, if the display is to the right of center, then the beam current will be collected by quadrant 12 for a shorter period than quadrant 15. The output across the secondary winding of transformer 20 will comprise a square wave having a negative half cycle which is of shorter duration than the positive half cycle. The

average value of this asymmetrical square wave is positive; and low-pass filter 36 will correspondingly provide a positive output.

Each bright-up line provided by multivibrator is not only coupled through OR circuit 91 to the intensity input of display tube 10, but also enables gate 31 to couple the output of the secondary winding of transformer 24 to low-pass filter 38. Transformer 24 responds to horizontal bright-up lines between D and F in the lower portion of the display. When the display is laterally centered the beam current is initially collected by quadrant 14 for a period of 0.5 as and is subsequently collected by quadrant 18 for a period of 0.5 pts. The output across the secondary winding of transformer 24 is a symmetrical square wave comprising a negative pulse immediately followed by a positive pulse of equal duration. The average output of this symmetrical square wave is zero; and low-pass filter 38 provides a corresponding output. If the display is to the left of center, the output of filter 38 will be negative; and if the display is to the right of center the output of filter 38 will be positive.

The output of filter 36 is proportional to the lateral deviation of the upper portion of the display, while the output of filter 38 represents the lateral deviation of the lower portion of the display. The outputs of

filters

36 and 38 are combined in summing amplifier 42 to provide an output in accordance with the mean lateral deviation of the display. The output of summing amplifier 42 is applied through resistor 44 to the horizontal deflection input of the display tube to adjust the lateral position of the display.

If the mean lateral position of the display is correct but the display is rotated somewhat clockwise from its proper orientation, the output from filter 36 will be positive while the output from filter 38 will be negative. If the mean lateral position of the display is correct, but the display is rotated somewhat counterclockwise from its proper orientation, the output from filter 36 will be negative while the output from filter 38 will be positive. If the display has the proper angular orientation and is rotated neither clockwise nor counterclockwise, then the outputs of

filters

36 and 38 will be equal irrespective of the lateral position of the display. Differential amplifier 46 responds to any difference in the outputs of

filters

36 and 38 to rotate the display to its proper angular orientation.

It will be noted that since the periods of

multivibrators

89 and 90 are 1 ,us, the outputs of

filters

36 and 38 will be proportional to lateral deviations of the upper and lower portions of the display up to i 0.5 us. Since the effective time for a horizontal sweep is 64 us, the outputs of

filters

36 and 38 will be proportional to lateral deviations up to 0.5/64 i 0.78 percent of display width. The range of proportional response of the horizontal centering system may be increased by increasing the pulse duration of

multivibrators

89 and 90 and correspondingly increasing the time-constants of

filters

36 and 38. For example, the range of proportional response of the horizontal centering system may be doubled by increasing the pulse duration of

multivibrators

89 and 90 to 2 us, and by correspondingly increasing the time-constants of

filters

36 and 38 to 16 us. However, the range of proportional response of the vertical centering system is ordinarily restricted to appreciably smaller values. With 512 horizontal scan lines per frame, the diameter of the beam will be somewhat less than 0.2 percent of raster height. Accordingly the range of proportional response of the vertical centering system will be somewhat less that $0.1 percent of raster height. Because of the appreciably larger range of proportional response of the horizontal centering system as compared with the vertical centering system, horizontal centering errors at two vertically spaced points have been used to correct for rotational errors in the display. The arrangement shown is to be preferred over a configuration where vertical errors measured at two spaced horizontal points are used to correct for rotational errors.

For high accuracy of horizontal centering it is desired that the bright-up lines be placed adjacent the center of the display. However, for'high accuracy in measurement of rotational errors it is desired that the horizontal bright-up lines be placed adjacent the upper and lower margins of the display to maximize the distance between each pair of bright-up lines. If it is desired to reduce rotational errors at the expense of slightly increased horizontal centering errors, then the upper bright-up lines could be displaced upwardly to occupy the region from to percent of display height from the upper margin of the display; and the lower bright-up lines could be displaced downwardly to occupy the region from 75 to 100 percent of display height from the upper margin of the display. If it is desired to reduce horizontal centering errors at the expense of slightly increased rotational errors, then the upper bright-up lines could be moved downwardly to occupy the region from 25 to 50 percent of display height from the upper margin of the display; and the lower bright-up lines could be moved upwardly to occupy the region from 50 to 75 percent of display height from the upper margin of the display.

It will be appreciated that horizontal centering errors will be minimized when a group of bright-up lines occupies a central region extending for example from 37.5 to 62.5 percent of display height from the upper margin of the display. Where rotational errors are also to be corrected, then a second and peripheral group of bright-up lines may extend either from O to 25 percent or from 75 to 100 percent of display height from the upper margin of the display. With such arrangement, horizontal centering errors are determined only from the central group of bright-up lines; and summing amplifier 42 would be replaced by a single input amplifier. To correct for rotational errors, amplifier 46 would respond to the difference in horizontal positions of the central and peripheral groups of bright-up lines. If desired there may be provided three groups of brightup lines comprising an upper group, a central group, and a lower group extending respectively from 0 to 25 percent, from 37.5 to 62.5 percent, and from 75 to 100 percent of display height from the upper margin of the display. Again only the central group would be used to control the horizontal position of the display. Rotational errors are corrected by causing amplifier 46 to respond to the difference in horizontal positions for the upper and lower groups of bright-up lines. Such arrangement minimizes both horizontal centering errors and rotational errors. 4

The period between successive positive pulses coupled through diode 55 is 65 us. The minus output of divide-by-two flip-flop 53 becomes positive 32.5 ts subsequent to each pulse through diode 55. Since the retrace period for horizontal sweep generator 56 is 1 #8, its output begins rising from zero volts 1 #8 subsequent to each retrace pulse through diode 55. Accordingly, the minus output of divide-by-two flipflop 53 becomes positive 31.5 as after the output of horizontal sweep generator 56 begins rising from zero volts. Thus the 1 [LS pulses of

multivibrators

89 and 90 extend from 31.5 [1.8 to 32.5 ,us after the output of horizontal sweep generator 56 begins rising from zero volts. Since the effective period for a horizontal sweep is 64 us, the horizontal center of the raster will occur 32 us after the output of horizontal sweep generator 56 begins rising from zero volts. Hence the output pulses of

multivibrators

89 and 90 extend from 0.5 as to the left of center of the raster to 0.5 as to the right of the center of the raster; and the midpoint of each bright-up line will be horizontally centered in the raster. From the foregoing, it will be appreciated that the midpoint of each bright-up line will be centered in the raster, if the retrace period of horizontal sweep generator 56 is made equal to the duration of the pulses provided by

multivibrators

89 and 90. For example, if the range of proportional response of the horizontal centering system is doubled by increasing the pulse duration of .multivibrators 89 and 90 to 2 [L8, then the retrace period for horizontal sweep generator 56 should correspondingly be increased to 2 us. Since this will decrease the effective period for a horizontal sweep to 63 ps, the pulse duration of multivibrator 77 may be slightly decreased to 0.25(63/64) 0.246 [L8 to avoid overlap of the bright-up pulses for the vertical centering system.

From FIG. 4a it will be noted that scan line 127-, which represents the l27th scan line of the second field of each frame, is slightly above the center of the raster while the 128+ scan line, which corresponds to the 128th scan line of the first field of each frame, is slightly below the center of the raster. Since vertical centering is controlled by the bright-up pulses provided on scan line 128+, the raster will not be precisely centered vertically if the horizontal

line separating quadrants

15 and 18 passes through the center of the display. While the error is small and perhaps negligible, the raster will be displaced upwardly from center by half the distance between adjacent frame scan lines. With 512 horizontal scan lines per frame, the raster will thus be displaced upwardly from center by 0.1 percent of raster height. If desired, this small error may be corrected by displacing the horizontal

line separating quadrants

15 and 18 downwardly from the center of the screen by 0.1 percent of raster height.

Referring now to FIG. 2 there is shown an embodiment of my invention in which the metalized screen is etched to provide only three sectors. Sectors l5 and 18 are identical to

quadrants

15 and 18 of FIG. 1. Sector 13 occupies nearly half of the display area, since the horizontal etched

line separating sectors

15 and 18 is terminated at sector 13. Anode potential source 26 is connected to one terminal of the primary winding of each of

transformers

21, 23, and 25, the other terminals of which are connected respectively to

sectors

13, 15, and 18. One terminal of each of the secondary windings of

transformers

21, 23, and 25 is grounded. The other terminal of the secondary winding of transformer 21 is coupled to the plus input of each of differential amplifiers 95 and 96. The other terminal of the secondary winding of transformer 23 is connected to the minus input of each of differential amplifiers 95 and 97. The other terminal of the secondary winding of transformer 25 is connected to the minus input of differential amplifier 96 and to the plus input of differential amplifier 97. The outputs of

differential amplifiers

95, 96, and 97 are coupled through

gates

30, 31, and 34 to low-

pass filters

36, 38, and 40 in the manner shown in FIG. 1.

In operation of the embodiment of FIG. 2, sector 13 encompasses the entire area of

sectors

12 and 14 of FIG. 1 and cooperates with sector to determine horizontal centering errors in the upper portion of the display and cooperates with sector 18 to determine horizontal centering errors in the lower portion of the display. Again as in FIG. 1,

sectors

15 and 18 cooperate to indicate vertical centering errors. In FIG. 1, the provision of four electrically discrete sectors permits the use of center-tapped primary windings to measure the differential currents between adjacent sectors. In FIG. 2, however, the use of center-tapped primary windings would result in the effective short-circuiting of all sectors since there are an odd number of only three sectors. Accordingly, in FIG. 2, the currents collected by the sectors are sensed by isolated transformers; and the differential currents are measured by

differential amplifiers

95, 96, and 97.

Referring now to FIG. 3, there is shown an embodiment wherein the discrete sectors for measuring centering errors occupy a relatively small area and wherein there are provided further electrically distinct sectors maintained at a constant potential which collect electron beam current but take no part in the measurement of centering errors. The embodiment of FIG. 3 provides a higher frequency response, since the stray capacitances of the electrically discrete sectors for measuring centering errors are considerably reduced.

Sectors

15a and 18a abut along a horizontal line and have a relatively small vertical extent. These sectors are coupled to the terminals of the center-tapped primary winding of transformer 22.

Sectors

12 and 15 abut along a vertical line in an upper region of the display and have a relatively small horizontal extent. These sectors are connected to the terminals of the centertapped primary winding of transformer 20.

Sectors

14 and 18 abut in the lower region of the display along an extension of the vertical line of abutment of

sectors

12 and 15.

Sectors

14 and 18 have a relatively small horizontal extent and are connected to the terminals of the center-tapped primary winding of transformer 24. An electrically distinct sector 17 occupies the area in the upper right-hand portion of the display. An electrically distinct sector 19 occupies the area in the lower right-hand portion of the display. An electrically distinct sector 13 occupies the area in the left-hand portion of the display.

Sectors

17, 19, and 13 are connected to the anode source 26; and the areas of these sectors are large compared with that of any of the discrete measuring sectors coupled to the primary windings of the transformers. The embodiment of FIG.

3 operates in the same manner as FIG. 1, but provides an appreciably higher frequency response because of the reduction in area of the sectors for measuring positioning errors which results in a corresponding reduction of stray capacitances. The areas of the sectors for measuring positioning errors may be made fairly small, since the positioning errors will usually comprise relatively small percentages of display width or display height.

Referring now to FIG. 4, there is shown an embodiment of my invention in which horizontal centering er rors are minimized by measurement of such errors in a central region symmetrically disposed above and below the center of the display. Electrically

discrete sectors

12, 14, 15, and 18 abut along horizontal and vertical lines intersecting adjacent the center of the display.

Sectors

12 and 14 cooperate to measure vertical centering errors in a region to the left of the center of the display. Sectors l5 and 18 cooperate to provide vertical centering errors in a region to the right of the center of the display.

Sectors

12 and 15 cooperate to indicate horizontal centering errors in a region above the center of the display; and

sectors

14 and 18 cooperate to provide horizontal centering errors in a region below the center of the display.

Again as in FIG. 3, electrically

distinct sectors

13, 17, and 19 are each connected to the anode source 26. These three sectors are maintained at a fixed potential and play no part in the measurement of centering errors.

Sectors

12, 14, 15, and 18 each occupy a relatively small portion of thetotal display area to provide low stray capacitance and hence high frequency response. In FIG. 4, horizontal centering errors are measured in only one region; and consequently, rotational errors cannot be sensed. If it were desired to sense rotational errors, then as previously indicated horizontal positioning errors could also be measured in a region adjacent either the upper or the lower margin of the display. Alternatively, horizontal positioning errors might be measured adjacent both the upper and lower margins of the display. Source 26 is connected to one terminal of the primary winding of each of transformers 21 23, 25 and 27. The other terminal of the primary winding of transformer 21 is connected to sector 12; the other terminal of the primary winding of transformer 23 is connected to sector 15; the other terminal of the primary winding of transformer 25 is connected to sector 18; and the other terminal of the primary winding of transformer 27 is connected to sector 14. One terminal of the secondary winding of each of

transformers

21, 23, 25, and 27 is grounded. The other terminal of the secondary winding of transformer 21 is connected to a plus" input of each of amplifiers a and 97a. The other terminal of the secondary winding of transformer 23 is connected to a minus input of amplifier 95a and to a plus input of amplifier 97a. The other terminal of the secondary winding of transformer 25 is connected to minus inputs of

amplifier

95a and 97a. The other terminal of the secondary winding of transformer 27 is connected to a plus input of amplifier 95a and a minus input of amplifier 97a.

Amplifiers

95a and 97a 3

filters

36 and 40 as shown in FIG. 1. Since rotational errors are not corrected in the embodiment of FIG. 4, the output of low-pass filter 36 may be applied to a single input amplifier 42a (not shown) of a construction similar to amplifier 48; and the output of amplifier 42a is coupled through resistor 44 to the horizontal deflection input I-I. Differential amplifier 46 and summing amplifier 42 of FIG. 1 are not required components and may be omitted. In FIG. 4, tube is provided with post deflection acceleration to achieve high deflection sensitivity and high visual sensitivity. The conductive aquadag coating on the inner side wall of the tube is maintained at a potential of 3 Kv by source 26b. The aquadag conductive coating comprises the accelerating electrode; and the metallized screen, which isat the 10 Kv potential of source 26, comprises the intensifying electrode. As is well known in the art of post deflection acceleration tubes, the intensifying electrode may be operated at a potential of between approximately two and four times that of the accelerating electrode. From FIGS. 1 and 4 it will be seen that the potential of the metallized screen may exceed that of the aquadag coat ing by an amount ranging from 10 or volts up to many kilovolts.

In FIG. 4 as in FIG. 1, the outputs of AND circuit 73 and differential amplifier 72 are applied to the inputs of AND circuit 74. The output of AND circuit'74 is coupled through differentiating circuit 75 and forwardly through diode 76 to multivibrator 77. The output of multivibrator 77 actuates gate 34 and is coupled to one input of OR circuit 91a. The 32 and 64 bit outputs of counter 58 are applied to enabling inputs of AND circuit 79a and to inhibiting inputs of AND circuit 80a. The 128 bit output of counter 58 is applied to an inhibiting input of AND circuit 79a and to an enabling input of AND circuit 80a. The outputs of AND

circuit

79a and 80a are applied to the inputs of OR circuit 81, the output of which is applied to one input of AND circuit 82. AND circuit 82 receives further inputs from comparator 78 and from the minus output of divideby-two flip-flop 53 as shown in FIG. 1. The output of AND circuit 82 is coupled to one input of an AND circuit 83a, which receives an inhibiting input from the output of AND circuit 73. The output of AND circuit 83a is coupled through differentiating circuit 85 and forwardly through diode 87 to multivibrator 89. The output of multivibrator 89 enables gate 30 and is coupled to a second input of OR circuit 91a, the output of which is applied to the intensity input of tube 10.

In operation of the vertical centering system of FIG. 4, multivibrator 77 provides a bright-up pulse once each frame which is coupled through OR circuit 91a to the intensity input of tube 10. This bright-up pulse is provided during the 128+ horizontal scan line. The bright-up pulse provided by multivibrator 77 also enables gate 34 to respond to the output of amplifier 970. When the output of frame counter 66 is 0 the brightup pulse is provided at position J. When the output of frame counter 66 is 63 the bright-up pulse will be provided at position K. For outputs of frame counter 66 ranging between 0 and 31 the bright-up pulses of multivibrator 77 will be to the left of the center of the display; and vertical centering errors are measured by the difference in currents drawn by

sectors

12 and 14. The secondary windings of

transformers

21 and 27 are coupled to respective plus and minus inputs of amplifier 970. When the display is vertically centered, equal currents are drawn by

sectors

12 and 14; and equal voltages appear across the secondary windings of

transformers

21 and 27. Thus, when the display is centered vertically, amplifier 97a provides no output. If the display is above center, then more beam current will be collected by sector 12 than by sector 14. The negative output across the secondary winding of transformer 21 will exceed the negative output across the secondary winding of transformer 27; and the output of amplifier 97a will be negative. On the other hand if the display is below center then more beam current will be collected by sector 14 than by sector 12. The negative output across the secondary winding of transformer 27 will exceed the negative output across the secondary winding of transformer 21; and the output of amplifier 97a will be positive.

For outputs of frame counter 66 ranging between 32 and 63 the bright-up pulses of multivibrator 77 will be-provided to the right of the center of the display.

Vertical centering errors are now measured by the difference in currents drawn by

sectors

15 and 18. It will be noted that the secondary windings of transformers 23 and 25'are connected to respective plus" and minus inputs of amplifier 97a. If the display is vertically centered equal currents are drawn by

sectors

15 and 18; and equal negative voltages appear across the secondary windings of

transformers

23 and 25. Thus the output of amplifier 97a will be zero. If the display is above center, then the output of amplifier 97a will be negative; and if the display is below center than the output of amplifier 97a will be positive. In the foregoing description it has been assumed that the display is horizontally centered. However, the vertical centering system still operates properly even if the display is not horizontally centered. If the display is slightly to the left of center, then

sectors

12 and 14 will measure vertical centering errors for outputs of frame counter 66 ranging between 0 and 32 while

sectors

15 and 18 will measure vertical centering errors for outputs of frame counter 66 ranging between 33 and 63. If the display is slightly to the right of center, then

sectors

12 and 14 will measure vertical centering errors for outputs of frame counter 66 ranging between 0 and 30, while

sectors

15 and 18 will measure vertical centering errors for outputs of frame counter 66 ranging between 31 and 63.

In operation of the horizontal centering system of FIG. 4, assume frame counter 66 has just been indexed to 0. As in FIG. 1, comparator 78 will provide an output when counter 58 provides counts of 0, 64, 128, and 192. AND circuit 79a provides an output when the output of counter 58 ranges between 96 and 127; and AND circuit 800 provides an output when the output of counter 58 ranges between 128 and 159. Again the minus output of divide-by-two flip-flop 53 becomes positive at the mid-point of each horizontal scan line. During the first field of the frame where the output of frame counter 66 is 0, AND circuit a, comparator 78, and the minus output of flip-flop 53 cause AND circuit 82 to provide an output at the mid-point of scan line 128+. However, during the 128+ horizontal scan line, AND circuit 73 provides an output which inhibits AND circuit 83a from coupling the output of AND circuit 82 to multivibrator 89. Thus,

no horizontal bright-up line may be provided during the 128+ scan line. As may be seen by reference to FIG. 4a, scan line 128+ is reserved for the bright-up pulses of the vertical centering system.

During the second field of this frame where the output of frame counter 66 is AND circuit 82 will provide an output at the mid-point of scan line 128. Since AND circuit 73 does not provide an output, AND circuit 83a is enabled; and the output of AND circuit 82 is coupled through AND circuit 83a, differentiating circuit 85, and diode 87 to trigger multivibrator 89. The 1 us pulse output of multivibrator 89 is coupled through OR circuit 91a to the intensity input of tube which produces the horizontal bright-up line 128- as shown in FIG. 4a.

For both fields of the next frame, the output of frame counter 66 will be 1". Comparator 78 willprovide an output when counter 58 provides outputs of l 65 I29, and 193. For the first field of this frame, AND circuit 82 will trigger multivibrator 89 to provide the bright-up line 129+ of FIG. 4a; and during the second field of this frame, AND circuit 82 will trigger multivibrator 89 to provide the bright-up line 129 of FIG. 4a.

When the output of frame counter 66 is 31, comparator 78 will provide an output when the count of counter 58 is 31, 95, I59, and 223. During the first field of this frame, AND circuit 80a causes multivibrator 89 to provide a bright-up pulse at the mid-point of the 159+ scan line which lies just above line C of FIG. 4. During the second field of this frame, multivibrator 89 provides the bright-up line C durin the 159 scan line.

During the next frame the output of counter 66 is 32. Comparator 78 will provide an output when counter 58 provides counts of 32, 96, 160, and 224. AND circuit 79a now causes AND circuit 82 to provide an output at the mid-point of horizontal scan line 96. During the first field of this frame, multivibrator provides the bright-up line A during the 96+ scan line. During the second field of this frame, multivibrator 89 provides a bright-up line slightly below line A.

When the output of frame counter 66 is 63, comparator 78 will provide an output when the count of counter 58 is 63, l27, 191, and 255. AND circuit 79a causes AND circuit 82 to provide an output at the mid-point of horizontal scan line 127. During the first field of this frame multivibrator 89 provides the bright-up line 127+ shown in FIG. 4a; and during the second field of this frame multivibrator 89 provides the horizontal bright-up line 127- as shown in FIG. 4a.

During the next frame counter 66 is recycled to the count of 0. During the first field of this frame, AND circuit 73 inhibits AND circuit 83a so that multivibrator 89 provides no horizontal bright-up line. The first field of this frame is reserved for the short bright-up pulses of the vertical centering system which appear on horizontal scan line 128+. During the second field of this frame multivibrator 89 provides the horizontal bright-up line 128- as shown in FIG. 4a. During the course of 64 frames, short horizontal bright-up lines will be produced for every raster line between those containing bright-up lines A and C except scan line 128+.

Each bright-up pulse provided by multivibrator 89 is not only coupled through OR circuit 91a to the intensity input of display tube 10 but also enables gate 30 to couple the output of amplifier 95a to low-pass filter 36. For those bright-up lines above the center of the display, the beam current will be initially collected by quadrant 12, producing a negative output across the secondary winding of transformer 21. Since this output is coupled to a plus" input of amplifier 950, a negative output will be initially produced therefrom. Subsequently the beam current will be collected by quadrant l5, producing a negative output across the secondary winding of transformer 23. Since this output is coupled to a minus input of amplifier 950, the output therefrom will be positive. If the display is laterally centered the duration of collection of beam current by

quadrants

12 and 15 will be equal; and the output of amplifier 95a will comprise a full cycle of a symmetrical square wave having equal negative and positive durations. Accordingly the average value of the output of amplifier 95a will be zero. If the display is to the left of center, then the average value of the output of amplifier 95a will be negative, while if the display is to the right of center then the average value of the output of amplifier 95a will be positive.

For those bright-up lines below the center of the display, the beam current will be initially collected by quadrant 14 producing a negative output across the secondary winding of transformer 27. Since this output is coupled to a plus input of amplifier 95a the output thereof will initially be negative. Subsequently the beam current will be collected by quadrant 18, producing a negative output across the secondary winding of transformer 25. Since this output is coupled to a minus input of amplifier 95a, the output thereof will become positive. When the display is laterally centered, the durations of collection of beam currents by

quadrants

14 and 18 are equal; and the output of amplifier 95a comprises a full cycle of a symmetrical square wave having equal negative and positive durations. The average value of this symmetrical square wave is zero. If the display is to the left of center then the average value of the output of amplifier 95a will be negative, while if the display is to the right of center then the average value of the output of amplifier 95a will be positive. Filter 36 extracts the average value of the output of amplifier 95a and applies it to the single input amplifier 42a (previously described) and thence through register 44 to the horizontal deflection input of display tube 10.

Since in FIG. 4 rotational errors are not sensed, it is not necessary to provide a wide range of proportional response for the horizontal centering system. Accordingly if desired, the range of proportional response may be reduced by reducing the pulse duration of multivibrator 89 and correspondingly reducing the timeconstant of filter 36. For example the range of proportional response of the horizontal centering system may be halved by reducing the pulse duration of multivibrator 89 to 0.5 us and by correspondingly reducing the time-constant of filter 36 to 4 ,lLS. In such event when the display is laterally centered,

sectors

12 and 15 and

sectors

14 and 18 will each collect beam current for a period of 0.25 us, which is one-half the pulse period of multivibrator 89. Since the horizontal and vertical centering systems are both subject to the same frequency restrictions, brighttup lines for the horizontal centering system should be not less than twice as long as the bright-up pluses for the vertical centering system. Accordingly the pulse duration of multivibrator 89 should be at least twice that of multivibrator 77.

It will be seen that the objects of my invention have been accomplished. My display centering system employs a metallized screen split along lines passing adjacent the center of the display. Bright-up pulses for the vertical centering system and bright-up lines for both the horizontal centering system and the rotational correction system are provided at positions in the raster which vary from field to field and from frame to frame so as not to be visible in the display. High linearity of the deflection system is not required to maintain the center of the display adjacent the'center of the tube; and the entire display area is available for presenting information.

It will be understood that certain features and subcombinations are of utility and may be employed without reference to other features and subcombinations. It is further obvious that various changes may be made in details without departing from the spirit of my invention. It is therefore to be understood that my invention is not to be limited to the specific details shown and described.

Having thus described my invention, what I claim is:

1. In a display centering system, the combinationincluding a cathode ray tube having horizontal and vertical deflection inputs and an intensity input, means for applying sweep signals to said deflection inputs to provide a raster having a certain frame repetition frequency, and first means for pulsing the intensity input at a predetermined point in the raster during each frame and for successively shifting the position of said point from frame to frame.

2. A system as in claim 1 wherein the first means is so constructed that said points lie along a horizontal line adjacent the center of the raster.

3. A system as in claim 1 wherein the first means is so constructed that said points lie along a vertical line adjacent the center of the raster.

4. A system as in claim 1 wherein the first means is so constructed that said points lie remote the center of the raster along a vertical line extending adjacent the center thereof.

5. A system as in claim 1 wherein the first means is so constructed that said points lie along a horizontal line and wherein the first means provides pulses of relatively short duration.

6. A system as in claim 1 wherein the first means is so constructed that said points lie along a vertical line and wherein the first means provides pulses of relatively long duration.

7. A system as in claim 1 further including second means for pulsing the intensity input at a certain point in the raster during each frame and for successively shifting the position of said certain point from frame to frame.

8. A system as in claim 7 wherein the first means is so constructed that said predetermined points lie along a horizontal line and wherein the second means is so constructed that said certain points lie along a vertical line.

9. A system as in claim 8 wherein the second means provides pulses of appreciably longer duration than those provided by the first means.

10. A system as in claim 7 wherein the first means is so constructed that said predetermined points lie in a first region along a vertical line and wherein the second means is so constructed that said certain points lie in a second and different region along said vertical line.

11. A system as in claim 10 wherein the first and second means provide pulses of substantially equal durations.

12. A system as in claim 10 wherein the first region is entirely above the center of the raster and the second region is entirely below the center of the raster.

13. A system as in claim 10 wherein the first region has substantially equal extents above and below the center of the raster and the second region is adjacent one of the upper and lower margins of the raster.

14. A system as in claim 10 wherein the regions are of substantially equal vertical extents.

15. A system as in claim 10 wherein the regions have vertical extents which do not exceed one-third of raster height.

16. A display centering system including in combination a cathode ray tube having horizontal and vertical beam deflection inputs and a display rotation input and a metallized screen divided into at least two electrically discrete sectors, means for collecting beam current from each sector, means responsive to the currents collected from the sectors for providing a control signal, and means coupling the control signal to the rotation input.

17. A display centering system including in combination a cathode ray tube having horizontal and vertical beam deflection inputs and a metallized screen divided into at least two electrically discrete sectors, means for collecting beam current from each sector, means including a gate and responsive to the currents collected from the sectors for providing a control signal, and means for momentarily enabling the gate at predetermined intervals of time.

18. A display centering system including in combination a cathode ray tube having horizontal and vertical beam deflection inputs and a metallized screen divided into at least two electrically discrete sectors, means for collecting beam current from each sector, means responsive to the currents collected from the sectors for providing a control signal, an algebraic combining device having a plurality of inputs and means including a gate and a low-pass filter coupling the control signal to one input of said plurality.

19. A display centering system including in combination a cathode ray tube having horizontal and vertical beam deflection inputs and a metallized screen divided into at least two electrically discrete sectors, means for collecting beam current from each sector, means responsive to the currents collected from the sectors for providing a control signal, an algebraic combining device having a plurality of inputs, means coupling the control signal to one input of said plurality, a low-pass filter and means comprising a gate for coupling the device to the filter.

20. A system as in claim 16 including means coupling the control signal to the horizontal deflection input.

21. A system as in claim 17 wherein the tube is provided with an intensity input, the system further including means responsive to the enabling means for exciting the intensity input.

22. A system as in claim 17 including a low-pass filter and means coupling the gate to the filter.

23. In a display centering system, the combination including a cathode ray tube having horizontal and vertical deflection inputs and an intensity input, means for applying horizontal and vertical sweep signals to the respective deflection inputs to provide a raster having a certain frame repetition frequency, means including a recycling counting device responsive to, the sweep signal means for providing a digital indication which changes from frame to frame, and means responsive to the counting device for controlling the intensity input.

24. A system as in claim 23 wherein the control means includes a digital-to-analog converter responsive 1

Claims

1. In a display centering system, the combination including a cathode ray tube having horizontal and vertical deflection inputs and an intensity input, means for applying sweep signals to said deflection inputs to provide a raster having a certain frame repetition frequency, and first means for pulsing the intensity input at a predetermined point in the raster during each frame and for successively shifting the position of said point from frame to frame.

23. In a display centering system, the combination including a cathode ray tube having horizontal and vertical deflection inputs and an intensity input, means for applying horizontal and vertical sweep signals to the respective deflection inputs to provide a raster having a certain frame repetition frequency, means including a recycling counting device responsive to the sweep signal means for providing a digital indication which changes from frame to frame, and means responsive to the counting device for controlling the intensity input.

24. A system as in claim 23 wherein the control means includes a digital-to-analog converter responsive to the counting device.

25. A system as in claim 24 wherein the control means includes an analog comparator responsive to the converter and to the horizontal sweep signal.

26. A system as in claim 23 wherein the sweep signal means includes a recycling counter providing a digital indication which changes from line to line of each frame.

27. A system as in claim 26 wherein the control means includes a digital comparator responsive to the counting device and to the counter.