US3854005A

US3854005A - Film stabilizing system for electron beam recorder

Info

Publication number: US3854005A
Application number: US00346723A
Authority: US
Inventors: R Castrignano
Original assignee: Columbia Broadcasting System Inc
Current assignee: CBS Broadcasting Inc
Priority date: 1973-04-02
Filing date: 1973-04-02
Publication date: 1974-12-10
Anticipated expiration: 1991-12-10
Also published as: JPS49131339A; JPS5225695B2

Abstract

The invention is applicable to an electron beam recording system which includes a film transport for moving a film past a scanning window, the system including an electron sensitive plate positioned adjacent to the window, and means for scanning the modulated electron beam in a repetitive scanline pattern over a reference position which includes the plate and the window. In the system, the rate of scanning is synchronized with the motion of the film and the plate has an output which is a function of the intensity of the beam that is incident on an active area thereof. The invention comprises an improved subsystem for stabilizing the position of the beam with respect to the film. In accordance with the invention, there are provided interrupt means located at a prescribed position on the plate for causing an interruption signal on the output of the plate means when the electron beam is scanned over the prescribed position. Means are provided for comparing the interrupt signal to the timing of individual scanlines and for generating a correction signal which reflects the comparison. Finally, means are provided for varying the reference position of the scanline pattern in accordance with the correction signal. In a preferred embodiment of the invention, the interrupt means comprises a vertical slot in the plate. In this embodiment, the interrupt signal is used to sample a ramp voltage that is, in turn, synchronized with horizontal synchronizing pulses. The sampled voltage is applied to a sample and hold circuit which generates the correction signal for application to the horizontal centering circuitry of the electron beam scanner.

Description

Waited States Eatent n91 Castrignano Dec. 10, 1974 1 1 .FILM STABILIZING SYSTEM FOR ELECTRON BEAM RECORDER Inventor: Robert A. Castrignano, Stamford,

Conn.

[73] Assignee: Columbia Broadcasting Systems,

Inc, New York, NY.

[22] Filed: Apr. 2, 1973 [21] Appl. No.: 346,723

[52] US. Cl 178/6.7 A, l78/DIG. 28, l78/7.7 [51] Int. Cl. H04n 5/84 [58] Field of Search 178/6.7 A, 5.4 M, 5.2 D, 178/DIG. 28, 7.7; 358/6, 7

[56] References Cited UNITED STATES PATENTS 3,221,337 11/1965 Quinn 178/6.7 A 3,225,137 12/1965 Johnson l78/6.7 A 3,267,212 8/1966 Goldmark..... l78/6.7 A 3,290,437 12/1966 Goldmark 178/6.7 A 3,755,622 8/1973 Horowitz l78/D1G. 28 3,780,222 12/1973 Pickstock l78/DIG. 28

Primary Examiner-Bernard Konick Assistant ExaminerAlan Faber Attorney, Agent, or FirmSpencer E. Olson, Esq.; Martin Novack, Esq.

[57] ABSTRACT 'The invention is applicable to an electron beam recording system which includes a film transport for moving a film past a scanning window, the system including an electron sensitive plate positioned adjacent to the window, and means for scanning the modulated electron beam in a repetitive scanline pattern over a reference position which includes the plate and the window. In the system, the rate of scanning is synchronized with the motion of the film and the plate has an output which is a function of the intensity of the beam that is incident on an active area thereof. The invention comprises an improved subsystem for stabilizing the position of the beam with respect to the film. In accordance with the invention, there are provided interrupt means located at a prescribed position on the plate for causing an interruption signal on the output of the plate means when the electron beam is scanned over the prescribed position. Means are provided for comparing the interrupt signal to the timing of individual scanlines and. for generating a correction signal which reflects the comparison. Finally, means are provided for varying the reference position of the scanline pattern in accordance with the correction signal. In a preferred embodiment of the invention, the interrupt means comprises a vertical slot in the plate. In this embodiment, the interrupt signal is used to sample a ramp voltage that is, in turn, synchronized with horizontal synchronizing pulses. The sampled voltage is applied to a sample and hold circuit which generates the correction signal for application tothe horizontal centering circuitry of the electron beam scanner.

9 Claims, 4 Drawing Figures 7'O AGO C/RCU/TRY L//VE V COUNTER ONE SHOT H ONE SHOT HOLD - CORR. S/G/VAL V/DEO SAMPLE E RAMP GENERATOR PAIENTEB DEB I 01974 SHE! 1 0F 3 HH' llll l l l lm ll lfl illl l l ll lll "1" II I mu I I IE]. PRIOR ART FILM STABILIZING SYSTEM FOR ELECTRON BEAM RECORDER BACKGROUND OF THE INVENTION This invention relates to the recording of data on a film and more particularly, to an improved system for recording an image on photographic film using an electron beam.

It is well known that information recorded in a succession of frames on a photographic film can be scanned electronically such as by a flying spot scanner to provide an electrical output signal representative of scanned information and can be reproduced on a display such as a television receiver. One system for recording picture information on photographic film is known as electronic video recording (EVR), wherein picture information is recorded in successive frames and a television picture reproduced from this film by means of electronic scanning and processing of resulting video signals. Both monochrome and color pictures can be recorded and reproduced by electronic video recording techniques.

In the case of monochrome pictures, the film contains a picture track comprising a succession of photographic frames with a sound track disposed along the film. For color recording, two picture tracks are provided along the film, one track being a luminance track comprising a succession of monochrome frames, the other track being a color track containing frames of encoded chroma information. In both monochrome and color recording, a synchronization track is provided along the film and generally includes an aperture in alignment with each frame from which synchronization signals are derived. To reproduce or play the recorded picture information, the recorded frames are each scanned in a rasterpattern compatible with a conventional television receiver and a video signal generated to cause display of the scanned picture on a television receiver.

In one version of the EVR player system, the film is scanned while moving by a flying spot that follows the direction of the film motion but at twice the film velocity. The vertical scan starts at the top of a frame and at the end of one-sixtieth of a second reaches the bottom of the frame. In this time the film moves one frame and the scan moves a vertical distance equivalent to about the height of two frames. During vertical blanking, the spot returns to its original position to start the process over again on the next film frame. This technique, as well as an overall description of the EVR system, can be found in an article entitled Color EVR" which appeared in the September 1970 issue of IEEE Spectrum.

Electron beam recording is found to be a particularly advantageous technique for producing quality films in the EVR format. One present production method involves a multimaster film which is 40mm wide and includes four parallel masters, each master comprising a frame track pair. After it is made, the multimaster film is copied using a high speed printing technique and each copy thereof is slit into four individual EVR films.

In recording a color EVR master, an NTSC color signal from a video tape recorder is processed in two parallel channels, one for the color signal and one for the luminance. After appropriate processing, the EVR color and luminance signals are utilized to respectively modulate the intensity of electron beams from a pair of electron guns in an electron beam recorder. Generally, the electron beam recorder consists of three chambers: one for the film magazine, one for the film drive, and the third .for the electron guns and beams. To insure satisfactory beam focus and cathode life, the electron gun chamber is maintained at a pressure of approximately 10 atmosphere. The film tends to bemoist and give ofi vapor, so the vacuum in the film chamber is poorer; e.g., 10* atmosphere. The film is exposed to the beams through a small window or aperture and is kept as small as possible to facilitate maintenance of the gun chamber at the substantially lower pressure. Typically, the window is slightly larger than the width of two side-by-side frames and has a height which corresponds to the height of two frames. The two electron guns simultaneously scan the film moving past the window. The horizontal scan rate is the standard TV horizontal rate of 15,750 Hz. The film moves vertically through the chamber at about six inches per second, or 60 frames per second; the beams vertical scan rate being 12 inches per second. The recording scan is similar to the above-described scan used in playing developed film. Each beam begins scanning at the top of..the frame and by the time that frame has moved the distance above one frame height, about 0.1 inch, the beam has reached the bottom of the frame. A short blanking period occurs and the beams fly back to the topof the next pair of side-by-side frames.

As above stated, the film in the electron beam recorder is 40mm wide and ultimately contains four separate and parallel two-track masters. The gun chamber sits on a pair of trunnions and can be indexed to five distinct horizontal positions; i.e., one position known as the monoscope position, and the four positions corresponding to the four frame track pairs. In practice the chamber is first indexed at the monoscope position where a specially designed monoscope target allows adjustment of the beam parameters such as focus and scan dimensions. The monoscope is an electron beamsensitive semiconductor target which is provided with a precise grid pattern that allows an accurate optical display of the pattern which the electron beam is scanning and can thus be used to achieve the type of adjustments just listed. After appropriate adjustments at the monoscope position, the gun chamber is indexed to a horizontal position on the film that is to be occupied by the first of the two-track masters; i.e., the window is moved horizontally to the desired position on the film. The master is then recorded by advancing the film and activating the two electron guns which simultaneously record the side by side luminance and chrominance frames of the first master. The film is then rewound and the gun chamber is moved over to the second indexing position so that the window is now at a position on the film where the second two-track master is to be recorded. The second master is then recorded and the procedure followed two more times to complete a 40mm multimaster film.

During the recording just described a technique is employed for continually monitoring and automatically adjusting the gain of the two electron beams. This technique employs a pair of special plates, known as VIT plates which are located just above the window and over the positions of the two side-by-side frame tracks being recorded. The letters VIT stand for vertical of a material such as molybdenum which acts as an anode, so signals from output conductors coupled to the plates indicate the instantaneous'intensities of the electron beams. These outputs are received by automatic gain control circuits which regulate the gain level of the electron guns.

The use of the described monoscope and VIT correction techniques effectively minimize some of the problems associated with achieving an accurate electron beam scan and film exposure level. However, there remains a problem of maintaining an accurate registration reference as between the raster scan and the film at the different indexing positions of the gun chamber. Stated another way, when the window has been moved to one of the four indexing positions on the film, thereis no way of knowing exactly where the beam will expose the film. For example, a stray magnetic field may shift the raster reference slightly to the left or right of the intended position on the film. Magnetized metal parts in or near the chamber may have this effect and the film copies ultimately made from an improperly registered multimaster will have frames which are off center and which reproduce with the loss of video at one edge.

' Accordingly, it is an object of the present invention to provide a subsystem for stabilizing the position of the electron beam in an apparatus of the type described.

SUMMARY OF THE INVENTION The present invention is applicable to an electron Further features and advantages of the invention will become more readily apparent from the following detailed description when taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a fragmentary view of a typical film format of the type which can be produced using the present invention;

FIG. 2 is a simplified diagram of portions of an electron beam recorder and associated circuitry suitable for producing four-track multimaster films;

FIG. 3 is a diagram, partially in block form, of a portion of an electron beam recorder and a subsystem in accordance with the embodiment of the present invention; and

FIG. 4 is a series of timing graphs which facilitate description of the operation of the circuitry of FIG. 3;

DESCRIPTION OF THE PREFERRED EMBODIMENT Before considering the operation of a system embodying the invention, it is useful to consider the forincluding an electron sensitive plate positioned adjacent the window, and means for scanning the modulated electron beam in a repetitive scanline pattern over a reference position whichincludes the plate and the window. In the system, the rate of scanning is synchronized with the motion of the film and the plate has an output which is a function of the intensity 'of the beam that is incident on an active area thereof. The invention comprises an improved subsystem for stabilizing the position of the beam with respect to the film.

In accordance with the invention, there are provided interrupt means located at a prescribed position on the plate for causing an interruption signal on the output of the plate means when the electron beam is scanned over the prescribed position. Means are provided for comparing the timing of the interrupt signal to the timing of individual scanlines and for generating a correction signal which reflects the comparison. Finally, means are provided for varying the referenceposition of the scanline pattern in accordance with the correcmat of a film which can be produced thereby. A coded monochrome film format for color programming material is depicted in FIG. 1 and includes a luminance track 20 comprised of a succession of frames 22 of black and white pictures, and a color track 24 comprised of a succession of encoded frames 26 each associated with a respective frame 22 and each containing coded chroma information. It can be noted that the beam modulated by the color signal produces rows of dots on the half of the film it scans. The horizontal scan frequency and the color carrier frequency are related by an integral multiple, so the dots occur at about the same spots on each scanline. Therefore, the color encoded frames 26 appear to be made up of thin vertical stripes,. the stripes varying in spacing in accordance with the color information. A sync track 28 is provided on a longitudinal strip intermediate the two successions of frames and includesfa plurality of light transmissive apertures 30 each aligned with the upper edges of re spect'ive frames 22- and 26. One or more sound tracks 32 along one or both edges of the film provide monaural or binaural audio information for reproduction along with the picture information. These audio tracks can be of magnetic form and can be applied to the film independent of the picture information. I

The film depicted in FIG. 1 would typically be one of four individual EVR films which are slit from a printed copy of a four-track multimaster as described above in the background section. Portions of an electron beam recorder and associated circuitry suitable for producing the four-track multimaster are shown, in simplified form, in FIG. 2. The multimaster film 50 is fed from a supply reel 51 to a takeup reel 52 by suitable film drive means (not shown). The film 50 is enclosed in a suitable vacuum chamber (not shown). An electron gun chamber 60 is shown as being transparent for purposes of illustration, the enclosure 60 housing a pair of electron guns and 80. The chamber 60 is evacuated and maintained at a required vacuum level by suitable pump means (not shown). At the end of the chamber 60 opposite the electron gun 70 and in an end plate having a small aperture or window therein. Typically, the window is slightly larger than the width of two side by side film frames and has a height which corresponds to the height of two film frames.

The

electron guns

70 and 80 simultaneously scan the film moving past the window. Each vertical scan starts at the top of the window and reaches the bottom of the window after one-sixtieth of a second has elapsed. During this time the film moves a vertical distance equivalent to the height of about one frame and the scanning beam moves a vertical distance equivalent to about the height of two frames. During this excursion through the window, the two electron beams each continuously record a frame of video information and the beams then fly back to the top of the window (during the vertical blanking interval) to record the next pair of side-byside frames.

The input color video information to be recorded is received by video processing circuitry 81 which generates luminance and encoded chrominance signals that are suitable for modulating the intensity of the

electron gun

70 and 80 during their respective scans. Deflection of the electron beams during recording is controlled by horizontal and vertical deflection signals produced by a sync signal generator 82. The vertical and horizontal sync signals are also utilized to synchronize the video processing circuitry 81. Sync mark processing circuitry within the video processor 82 generates an output signal that can be added to the input signal to either

electron gun

70 or 80 in order to produce the sync marks 30 which ultimately appear on the film (FIG. 1). This is accomplished, in known manner, by unblanking one of the scanning beams during a portion of each of the first few active scanlines of each frame. i

The entire gun chamber 60 is mounted on a slidable mechanism (not shown) so that it can be indexed to five distinct horizontal positions indicated in FIG. 2 by the circled

designations M

1, 2, 3 and 4. The M" position is the monoscope position, referred to above, where certain adjustments of the electron beams can be accomplished. The four reference positions on the film correspond to the positions on the film at which the four two-track masters are to be recorded. Depending on the type of film used, the images recorded on the master film with the electron beam will generally not be visible to the eye until after a'subsequent developing step, but for purposes of illustration the frames which have been exposed by the beam are depicted as being visible in FIG. 2. As is evident in the FIGURE, the gun chamber 60 has already performed its function at index position 1" so that the first twotrack master has been completely recorded on the film. The gun chamber was then movedto index position 2 where it is now shown during the operation of exposing the second two-track master on a multimaster film 50. When this is complete-the multimaster 50 will be rewound and the gun chamber 60 moved to index position 3" and, after appropriate recording, to index position 4.

Located just-above the window 95 and over the positions of the two side-by-side frame tracks being recorded are a pair of

VIT plates

96 and 97. The raster scans of the beams from

electron gun

70 and 80 are preadjusted to impinge upon these plates during the initial horizontal line scans which occur during a portion of the vertical blanking interval between fields of video information. The video processing circuitry 81 inserts appropriate test signals in the vertical blanking interval which may, for example, modulate the electron beams with a prescribed grey scale stairstep signal. The plates are formed of a material such as molybdenum which acts as an anode, so the signals from

output conductors

96A and 97A, coupled to the plates, indicate the instantaneous intensities of the electron beams at various amplitude levels. These outputs are received by automatic

gain control circuits

98 and 99 which regulate the gain level of the electron guns and by applying appropriate control signals over

lines

98A and 99A.

Up to this point, the detailed description of the beam recording apparatus has dealt with a system that is presently known in the art. Referring to FIG. 3, there is shown an embodiment of a system in accordance with the improvement of the present invention. An enlargement of the window is shown in conjunction with the

VIT plates

96 and 97. Only one electron gun 70 and the beam therefrom are shown in FIG. 3 along with circuitry for controlling same, but it will be appreciated that similar circuitry is used to control the other electron gun. i

In the present embodiment of the invention the

VIT plates

96 and 97 are provided with

vertical slots

101 and 102, respectively. These thin slots may be cut into the VIT plates and can have a typical width of about 0.005 inch. The important property of each of the slots is that it gives rise to a discontinuity in the output taken from the VIT plate when an electron beam is scanned over the slot. Thus, the slot acts as a precisely located interrupt means and it will be appreciated that alternate means for accomplishing such an interruption, for example a masking of aportion of the plate, could be employed. The output of the plate 96 is coupled over the output conductor 96A to AGC circuitry, as was shown in FIG.'2, and is also coupled to an input of a gate 110. The gate is enabled by acircuit which consists of a pair of monostable or one-shot" multivibrators and 130, a line counter 140, and an AND gate 150. The one-shot multivibra'tor 120 receives as an input the horizontal sync pulse and generates a posi tive-going output pulse for the duration of its intrinsic unstable state. This on time is typically selected as being about 20 microseconds. The output of one-shot 120 is coupled to the other one-shot multivibrator which is adapted for triggering by the-negative-going voltage excursion which occurs when one-shot 120 goes off; i.e., it is triggered 20 microseconds after the horizontal sync pulse, H. The one-shot 130 is provided vwith a short intrinsic unstable state time of about 5 microseconds, so it generates apositive-going pulse on' output line 130A, the'pulse occurring from about 20 microseconds after the start of a scanline and continuing until about 25 microseconds after the start of the scanline. The line 130A is one of two inputs to AND gate 150, the other input being the output of a line counter 140. The line-counter receives the vertical and horizontal input pulses from the sync signal genera tor 82 (FIG. 2) and produces an output only during the first three lines of each'scanning field/The output of AND gate 150, which enables the gate 110, is thus present for 5 microseconds at about the center of each of the first three horizontal scanlines of each scanning field.

During thepresence of the enable signal from AND gate 150, the signal on conductor 96A is passed by the gate 110 to the sampling input of a smaple and hold circuit 160. The other input to the circuit is the output of a ramp generator 170 which is, in turn, triggered by the horizontal sync pulse H. The output of sample and hold circuit 160 is a correction signal which is applied to the horizontal centering circuitry of the electron gun 70.

A description of the operation of the circuitry of FIG. 3 is facilitated by referring to the graphs of FIG. 4. The graph 4A shows the output derived from VIT plate 96 during two successive scanlines which occur during the vertical interval when the electron beam is scanned the VIT plate and crossing the slot 101. The video information during these scanlines consists of a stairstep test pattern of the type shown. The negative-going pulses P result from the beam crossing the slot 101 and the accordant interruption of the VIT plate output. The graph 48 shows the timing of the horizontal sync pulses, the time base of all graphs in FIG. 4 being the same. Graph 4C illustrates the output of the one-shot multivibrator 120. The one-shot 120 is triggered by the leading edge of the horizontal sync pulse, and the output of the one-shot is on for 20 microseconds whereupon it returns to the off state. FIG. 4D shows the output of the one-shot 130 which is triggered by the trailing edge of the output of one-shot 120 and then remains on for 5 microseconds.

The slot 101 is positioned at a prescribed horizontal reference which lies, for example, at the center of the VIT plate 96. For a properly positioned raster scan (assuming good scan linearity) the electron beam should pass the slot 101 at a predetermined time after the occurrence of the horizontal sync pulse H. In the present 1 the pulses of graph 4A. The purpose of generating the gating signal on line 130A is to extract the pulse P from the vertical interval'test signal and preventlhe occur-o rence of extraneous pulses except at the approximate center of the scanlines where this interrupt pulse is expected. The output of thegate 110, i.e., the pulses of graph 4E, are utilized to sample the sawtooth voltage shown in graph 4F and produced by the ramp generator 170. The ramp is triggered by the leading edge of the horizontal sync pulse H and has a prescribed rise time. Therefore, the ramp contains intrinsic information concerning the timing'of horizontal scanlines The voltage to which the sawtooth rises after 22.5 microseconds is selected as a nominal correction voltage V.,. Thus, if the raster is properly positioned with respect tothe window 95 the sampled voltage will be V,,, a voltage which will not change the centering ofgun 70. Simi-.

ing of electron gun which tends to return the correction voltage to the nominal value, V The holding capacitor in circuit is charged through a relatively low resistance so that it has a relatively fast charge time and is discharged through a relatively high resistance so that it has a relatively slow discharge time, of the order of many video fields. In this manner, the error signal accumulated during the appropriate lines of the interval is applied during the active field portions without substantial degradation. 7

The invention has been described with reference to a particular embodiment but it will be appreciated that variations within the spirit and scope of the invention will occur to those skilled in the art. For example, the means for comparing the timing of the interrupt signal to the scanline timing could take various alternate forms. Also, the

slots

101 and 102 could have alternate shapes, such as a wedge shape. If desired, a correction of vertical scan position could also be achieved using the principles of the invention.

I claim:

1. In an electron beam recording system which includes a film transport for moving a film past the scanning window; an electron sensitive plate means positioned adjacent said window; and means for scanning a modulated electron beam in a repetitive scanline pattern over a reference position which includes said plate and said window, the rate of said scanning being synchronized with the motion of the film, the plate means having an output which is the function of the intensity of the' beam that is incident on an active area thereof; an improved subsystem for stabilizing the position of said beam with respect to said film, comprising:

interrupt means located at a prescribed position on said plate for causing an interruption signal in the output of said plate means when said electron beam is scanned over said prescribed position;

. means for comparing the timing of said interrupt signal to the timing of individual scanlines and for generating a correction signal which reflects the comparison; and e means for varying the reference position of said scanline pattern in accordance with said correction signal.

s 2. The subsystem'as defined in claim 1 wherein said interrupt means comprises a slot in said plate.

3. The subsystem as defined in claim l wherein said interrupt means comprises a vertical slot in said plate.

4. The subsystem as defined by claim 1 wherein said repetitive scanline pattern is a horizontal scanline pattern synchronized by horizontal synchronizing signals and wherein the timing of interrupt signals is compared to the timing of horizontal synchronizing signals.

5. The subsystem as defined by claim 4 wherein said comparing means comprises:

a. means for generating a ramp voltage synchronized by said horizontal synchronizing signals; and i b. sample and hold means responsive to said interrupt signal and the ramp voltage output of said ramp generating means for sampling and holding the value of said ramp at the time of occurrence of said interrupt signal, the output of said sample and hold means constituting said correction signal.

6.'The subsystem as defined by claim 5 further comprising means for activating said comparing means only 3,854,005 9 a .s during horizontal scanlines which are scanning said 8. The subsystem as defined in claim 6 wherein said P d F d b l 6 h d interrupt means comprises a slot in said plate.

f as F me y C i W erem Sal 9. The subsystem as defined in claim 6 wherein said correction signal 18 applied to the horizontal reference control terminal of the electron beam scanning means. 5 mterrupt means Comprises a Vertical Slot Said P

Claims

1. In an electron beam recording system which includes a film transport for moving a film past the scanning window; an electron sensitive plate means positioned adjacent said window; and means for scanning a modulated electron beam in a repetitive scanline pattern over a reference position which includes said plate and said window, the rate of said scanning being synchronized with the motion of the film, the plate means having an output which is the function of the intensity of the beam that is incident on an active area thereof; an improved subsystem for stabilizing the position of said beam with respect to said film, comprising: interrupt means located at a prescribed position on said plate for causing an interruption signal in the output of said plate means when said electron beam is scanned over said prescribed position; means for comparing the timing of said interrupt signal to the timing of individual scanlines and for generating a correction signal which reflects the comparison; and means for varying the reference position of said scanline pattern in accordance with said correction signal.

2. The subsystem as defined in claim 1 wherein said interrupt means comprises a slot in said plate.

3. The subsystem as defined in claim 1 wherein said interrupt means comprises a vertical slot in said plate.

5. The subsystem as defined by claim 4 wherein said comparing means comprises: a. means for generating a ramp voltage synchronized by said horizontal synchronizing signals; and b. sample and hold means responsive to said interrupt signal and the ramp voltage output of said ramp generating means for sampling and holding the value of said ramp at the time of occurrence of said interrupt signal, the output of said sample and hold means constituting said correction signal.

6. The subsystem as defined by claim 5 further comprising means for activating said comparing means only during horizontal scanlines which are scanning said plate.

7. The subsystem as defined by claim 6 wherein said correction signal is applied to the horizontal reference control terminal of the electron beam scanning means.

8. The subsystem as defined in claim 6 wherein said interrupt means comprises a slot in said plate.

9. The subsystem as defined in claim 6 wherein said interrupt means comprises a vertical slot in said plate.