US3588584A

US3588584A - Apparatus for positioning a light spot onto a character mask

Info

Publication number: US3588584A
Application number: US748367A
Authority: US
Inventors: Matthew P Tubinis
Original assignee: Xerox Corp
Current assignee: Xerox Corp
Priority date: 1968-07-29
Filing date: 1968-07-29
Publication date: 1971-06-28
Anticipated expiration: 1988-06-28

Abstract

AN ELECTROOPTICAL CHARACTER DISPLAY AND RECORDING DEVICE WHICH INCLUDES A DIGITAL CONTROL SYSTEM FOR ACCURATELY POSITIONING A LIGHT BEAM FROM A CATHODE-RAY TUBE ONTO A SYMBOL CONTAINED ON A MASK, SAID MASK ALSO INCLUDING A PLURALITY OF REFERENCE MARKS ALONG THE HORIZONTAL AND VERTICAL EDGES THEREOF, THE NUMBER OF MARKS TRAVERSED BY SAID BEAM ALONG ITS SCAN PATH REPRESENTING THE BEAM POSITION ON THE MASK AND MEANS FOR RECORDING SAID SYMBOL.

Description

United States Patent COUNTING Inventor Matthew P. Tublnls Penfield, N.Y.

Appl. No. 748,367

Filed July 29, 1968 Patented June 28, 1971 Assignee Xerox Corporation Rochester, N.Y.

APPARATUS FOR POSITIONING A LIGHT SPOT ONTO A CHARACTER MASK 13 Claims, 6 Drawing Figs.

11.8. CI 315/10, 250/201, 250/217, 250/237, 315/85, 315/18, 328/124, 340/173 Int. Cl....'. .l .Q G0lj1/20, G1 1c 1 1/26, 1101 31/26 Field of Search 250/217 (CRT), 201, 219 (R0), 237; 315/10, 8.5,18; IMO/324.1, 173 (LM), 173 (CRT); 328/123, 124

(56] References Cited UNITED STATES PATENTS 2,855,539 10/1958 Hoover Jr, 250/217X 2,929,956 3/1960 Jacobs ct a1. 250/217X 3,299,418 1/ 1967 Treseder 340/324.l

' FORElGN PATENTS 1,192,975 4/1959 France 328/124 Primary Examiner-James W Lawrence Assisiant Examiner-T. N. Grigsby Anomeys- Paul M. Enlow, Ronald Zibelii, James J. Ralabate and Norman E. Schrader CONTROL 8. HORIZONTAL DE F L ECTION CIRCUIT GENERATOR VERTICAL DEFLECTION GENERATOR PmEminJuuzwl 3,588,584

SHEET 1 OF 4 CONTROL a HORIZONTAL v COUNTING DEFL ECTION 204 cmcun GENERATOR VERTICAL 20a DEFLECTION 2 GENERATOR 2 FIG.

A T TORNEV PATENTEnJunzelsn 3588.584

SHEET 2 BF 4 .904 3 2 CLQCK ZERO 3 0 2 0 LEVEL r 302 m .308 I .338 BINARY D/ g m/ COUNTER A HORIZ. RESET I i DEFLECT GATE 8 i 1 8 $893953? .252 0-) GATE MEMORY 1 5 844 3/8 s20 COMPARISON a/a .322 cmcurr CONTROL 3/4 J HORIZONTAL DEFLECTION .222

ml, DETECTOR 326 24 2/2 340' H BINARY D CRT H RESET 'l/ 220 DEF'LECT.

GATE

r300 CHARACTER R JC DECODER 342 .234 .336 T L MPAR N TUNBLANK cm: cmcun' CONTROL 'smo 356' 354 VERTICAL 5 DEFLECTION 330 3 3 COUNTER PAIENIEnJuueslsn 35 ,5 21.

SHEET 3 III 4 TI Ea. I 2/6 I I I 426 I I I I T3 I l-Ecc. 404 k 406 08 4M I L To .356 r T7 I |D%9E!ECT k (OK I 424 I 202 I I I F I +Ecc. 42 I .36? GA a I HORIZONTAL DEFLECTION I GENERATOR I I. .I

I I r0 IVERTICAL IDEFLECT YOKE 202 VERTICAL DEF ECTION GENERATOR SAME AS ABOV E) PATENTEU JUH28 297i SHEET 0F 4 APPARATUS FOR POSITIONING A LIGHT SPOT ONTO A CHARACTER MASK BACKGROUND OF THE INVENTION An electrooptical character display system, as the term is used herein, refers to a device for forming a beam of light in the shape of a character or for positioning the light beam onto a mask containing alphanumerical information and including electrically controlled beam deflection means for selectively positioning the beam of light at a predetermined position on the mask, a viewing screen or on a photosensitive member for recordation thereon. The most common example of the term is a cathode-ray tube (CRT), but the term, as well as the present invention, also applies to related devices such as those employing mirror galvanometers or piezoelectrically or magnetorestrictively driven mirrors for beam deflection or employing electrooptical effects in electrically active crystals. The characters referred to may be simply spots of light which are displayed at preselected positions, but more commonly will comprise a set of numerals, letters and symbols so that the display device may write words, numbers or some mixture thereof. Such devices are particularly useful for the writing and instantaneous display of information from high speed digital computers. With improvements in cathode-ray tube technology and with an ever increasing need for greater speed in recording output information from computers and high speed telecommunications circuits, recording devices of the indicated kind are beginning to displace electromechanical printers such as teletypewriters or high speed line-at-a-time computer printers-This is particularly so where the information is to be recorded in a miniaturized format, for which electromechanical printers are quite unsuited. However, electrooptical display devices have suffered from a serious defect as compared to electromechanical devices. The latter characteristically prints characters at accurately spaced intervals along a straight line, but this precise character alignment and positioning has been difficult to achieve with electrooptical printers. For example, if a perfectly linear relation is established between an angle of deflection and a beam voltage, the displacement of the beam on a flat display or recording surface will vary as the tangent of the control voltage rather than the voltage itself. Even this situation is not achieved in a practice because the beam deflection angle itself is not strictly linearly related to the control voltage. Pincushion distortion also effects alignment as the horizontal and vertical traces on the CRT face are forced in along the axis and out along the diagonal. Finally, the high speed electronic circuits which are required to generate the control voltages corresponding to a great many character positions tend to vary with time in an unpredictable fashion, as may the deflection system itself. A partial solution to this problem may be had by employing nonlinear compensating circuits together with other highly stabilized electronic circuits. However, the required circuits are expensive and difficult to adjust and still fail to provide the ultimate in character positioning accuracy.

SUMMARY OF THE INVENTION The present invention relates to electrooptical display devices and, more particularly, to optical feedback means for providing a fast and accurate method for eliminating positional errors in the output display.

The present invention includes a mask, having. symbols printed thereupon, interposed between the output of a cathode-ray tube and a photodetector. The mask also includes a number of marks positioned along its horizontal and vertical edges which eliminates the linearity problem caused by pincushion distortion. The light beam from the cathode-ray tube scans the mask along the edges, the photodetector sensing the number of marks passed along the scan path. Theelectrical output of the detector is fed back to a digital control system which compares the position of the light beam on the mask with a desired position, represented by an encoded input signal. The position of the beam is adjusted by the control system until the beam strikes the desired symbol on the mask.

It is, accordingly, the principal object of the invention to provide an electrooptical display and recording means for precise character positioning and recording.

It is a further object of the invention to provide an electrooptical display means for fast and accurate character positioning by electrooptical a novel digital control system to position a cathode-ray tube beam.

It is still a further object of the invention to provide an electrooptical display means for precise character positioning which utilizes a mask with indexing marks along the horizontal and vertical edges thereof, the mask also compensating for pincushion distortion.

Other objects and features of the present invention will become apparent from the following detailed description when taken in conjunction with the drawings in which:

FIG. I is a block diagram of a preferred embodiment of the invention;

FIG. 2 shows the novel digital control system utilized in the invention;

FIG. 3 is a schematic diagram of the horizontal and vertical deflection signal generators; and

FIGS. 4(a) and 4(b) illustrate the optical mask system utilized in the invention.

FIG. 5 illustrates another embodiment of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT In FIG. 1 there is shown a cathode-ray tube 200 including deflection means 202 which is focused through a mask 204 having symbols, such as alphanumerical information, thereon and optical system 206 onto photodetector 208. Deflection means 202 would ordinarily be a magnetic yoke or a set of internal electrostatic plates. For the purposes of the present invention, cathode-ray tube 200 and deflection means 202 can be replaced by any suitable fixed light source operating in con junction with an electrically controllable means for deflecting a beam of light from the light source. The illuminated character on mask 204 is recorded by focusing the light passing through optical system 206 on a viewing screen or on any form of photosensitive material which can convert the light beam into a permanent image, a xerographic drum being illustrative of such a photosensitive member. Since recorders employing xerographic techniques are in wide spread use and have been extensively described in the patent and technical literature, no further description of the photosensitive means or of the auxiliary equipment, such as charging, developing and paper handling equipment for use therewith will be given. An example of a recording technique is disclosed in copending U.S. Pat. application Ser. No. 583,542, filed Oct. 3, 1966. The described elements of FIG. 1 are typical of electro-optical display devices. The deflection means 202 will initially incrementally displace the light beam or spot along the mask 204 in a horizontal direction. After the beam is positioned in its proper location, the deflection means will incrementally displace the light beam to the proper vertical position. The light output from cathode-ray tube 200 passing through mask 204 is focused by optical system 206 onto the photodetector 208. The electrical output of photodetector 208 is fed to digital control circuit 214, the output thereof controlling horizontal deflection generator 218, vertical deflection generator 222 and the cathode-ray tube grid via lead 224 in order to unblank the beam in a manner which will be more fully explained in connection with the subsequent FIGS. Mask 204 has a plurality of slits along its horizontal and vertical edges and alphanumeric material thereon, the slits being utilized to position the beam to the desired position according to the data input information and also to correct for pincushion distortion. In an alternate embodiment, transparent material may be used in place of the slits for indexing purposes. In the mode of operation of the invention generally applicable to FIG. I, the data input to the control system 214 initially causes a signal to be transmitted to horizontal deflection generator 218 and vertic'al deflection generator 222 via

leads

216 and 220, respectively. The output of the

deflection generators

218 and 222 energizes the deflection coil 202, the coil deflecting the beam to a predetermined position. Utilizing apparatus described in greater detail in FIG. 3, the horizontal deflection generator is energized initially, causing the beam to move along the upper horizontal edge of mask 204, the beam striking the slits positioned along its scan length. The number of slits the beam passes as it traverses its scan path is sensed by photodetector 208, this information being fed back to the control circuit 214 via lead 212. The control means 214 continues to generate an output signal to horizontal deflection generator 218 until the desired position on mask 204 is attained. The beam is then returned to the initial position and a similar procedure is then followed with reference to the vertical deflection generator 222. The beam is caused to move along the vertical edge of the mask striking the slits positioned along its scan length. The number of slits struck by the beam is sensed by the photodetector and fed to control circuit 214 via lead 212. When the proper vertical position is reached, the horizontal deflection voltage is reapplied, positioning the beam at the proper point on the mask. The control circuit 214 also controls the unblanking of cathode-ray tube 200 via lead 224 and operates essentially to unblank the beam during the time when the beam scans the horizontal and vertical edges of the mask and when the beam reaches its proper position on the mask.

Referring now to FIG. 2, there is shown in greater detail the control circuitry 214 described in FIG. 1.

Upon receipt of the input data at character decoder 300, an output pulse therefrom is directed to AND gate 302 which allows clock 304 to drive binary counter 306. The output of binary counter 306 drives digital-analog converter 308 to provide an analog voltage for horizontal deflection generator 218 via lead 216. Switch 310 is positioned in its lower position allowing the signal from the digital analog converter to be applied to the deflection circuit. The voltage steps necessary to sweep the beam would be finely divided requiring approximately ten steps to drive the beam between the slits in line with each character. As the spot passes each slit in the horizontal direction, photodetector 210 detects the light at each slit and produces a count on line 212. This signal drives the horizontal deflection counter 314 when switch 322 is in the position shown. The horizontal position memory 316 is preset to some number corresponding to the next character position to be printed. A continual comparison is made between the horizontal deflection counter 314 and the horizontal position memory 316 so that when the numbers stored in each are identical, a negative pulse corresponding to the logic level is generated by the comparison circuit 318 and applied to gate 302, the gate now being closed to prevent any further pulses from entering the counter 306. At this time, a pulse also generated by comparison circuit 318, energizes switch control means 320. The switch control enables contact 3100 of switch 310 to be connected to lead 420, the zero level voltage source 312 being applied to horizontal deflection generator 218 (FIG. 3) via lead 420 which operates to return the beam to its initial resting position. Switch control 320 operates simultaneously upon switch 322 and places the switch at contact 322b. Pulses from clock 304 are applied to binary counter 324 via AND gate 326. The output of the binary counter 324 is connected to digital-analog converter 328, the output thereof driving the vertical deflection generator of the cathode-ray tube 200 via lead 220 and causing the beam to be driven along the vertical edge of mask 204. Photodetector 208 senses the number of slits exposed along the beam scan path and provides pulses representing this number to the vertical deflection counter 330 via lead 212 and contact 322b. Comparator circuit 334 compares this number with the number contained in the data input and decoded by character decoder 300. When these two numbers are equal, a negative output pulse (logical 0) is generated by the comparator circuit 334, closing gate 326, thereby inhibiting any further clock pulses to pass to binary counter 324. The output of comparator circuit 334 is also applied to unblank control means 336, the output thereof feeding switch control means 320, causing the output of counter 306 to be applied to lead 216 via digitalanalog converter 308 and contact 310b and allowing the voltage to be applied to the horizontal deflection circuit, thereby moving the beam to a position which exposes the proper character on mask 204. The output of unblanking control means 336 is also applied to the grid circuit of the cathode-ray tube 200 via lead 224 which unblanks the beam when it is properly positioned. The cathode spot or beam is rastered about this position in some manner and the

counters

306 and 324 are then reset to zero by applying the output pulse from comparator circuit 334 to reset

means

338 and 340 via delay means 342. The output of the delay means 342 is also applied to gate 344 which advances horizontal position memory 316 one count for printing the next character.

Referring now to FIG. 3, there is shown a schematic diagram of a horizontal deflection generator used in the present invention.

The basic operation of the generator shown in FIG. 3 is as follows: The voltage output generated by counter 306 (FIG. 2) and transmitted by input line 216, controls the input to transistor T1. When T1 is conducting, capacitor 404 charges towards Ecc, resulting in a ramp output from the Darlington pair comprising transistors T2 and T3. The output of transistor T3 is applied to a voltage attenuator made up of

resistors

406, 408, 410 and 412 which drives linear voltage amplifier 414. The output of amplifier 414 drives a current amplifier 416 to deflect the cathode-ray tube beam along a horizontal axis. As explained hereinbefore, as the beam is detected passing a slit, a pulse is transmitted to horizontal deflection counter 314 (FIG. 2). When a predetermined number of pulses has been detected, source 312 applies a ground on lead 420 via contact 310a causing transistor T1 to be reversed bias. The transistor no longer conducts and the charge on capacitor 404 is retained at the proper horizontal deflection voltage.

At the time when lead 420 applies the ground potential to the base of transistor T1, a negative pulse from comparison circuit 318 is applied to gate 350 via lead 352. Gate 350, initially at a zero voltage, corresponding to a logical l, is disabled and the negative pulse generated at its output drives transistor T4 into conduction, forward biasing diode 424, limiting the output of amplifier 414 to its initial value. The cathode-ray tube beam is therefore returned to its rest position and the vertical deflection cycle now is initiated as described hereinbefore. When the desired vertical position has been reached, a pulse generated by comparator 334 is transmitted to gate 350 via lead 353, driving T4 to its initial condition allowing the voltage on capacitor 404 to position the beam to the correct horizontal position at the same instant that the correct vertical voltage is attained at the output of vertical deflection generator 222. A modulating signal can be applied to voltage amplifier 414 for rastering the spot about the desired position. The output of transistor T5 can be used by focusing circuitry not disclosed herein for correcting the spot size on the cathoderay tube surface. Transistor T6 is a shunting control switch can be used to apply a correcting voltage to the amplifier in case the drift of the spot exceeds the desired level. Transistor T7 is a capacitor dumping circuit which is utilized to forward bias diode 426, providing a discharge path for capacitor 404 after the beam is positioned properly. Transistor T7 is driven to conduction by the negative pulse generated by comparator 334 and applied to lead 356 via delay circuit 354.

Referring now to FIG. 4a, mask 204 includes a plurality of slits or other indexing marks 500 extending along the horizontal edge and a plurality of slits 502 extending along the vertical edge of the mask. The light beam is initially at rest at location 504 and when voltage is applied to the horizontal deflection generator 218, the beam is initially deflected along the horizontal edge in the direction of arrow 506. When the beam reaches a slit corresponding to the desired symbol, the beam is reset to position 504 by the apparatus hereinbefore disclosed in reference to FIGS. 2 and 3. Subsequently, the beam is deflected downwards in the direction of arrow 508. Assuming that the vertical position has been reached, and that it is desired to position the beam at point 510, the reset mode of the horizontal deflection generator is removed and both the horizontal and vertical deflection voltages are applied simultaneously to deflection coils 202 illustrated in FIG. I. The beam will then be positioned at 510. As is obvious from the particular character mask 204 used, the horizontal deflection dictates only where the selected character will be located in the final optical display, while the vertical deflection actually determines what character will be displayed.

Referring now to FIG. 4b, there is shown in greater detail a portion of the mask 204 shown at location 512. Illustratively, the slit width C is approximately 0.004 inches, the character width d is approximately 0.017 inches and the approximate spacing between characters e is 0.005 inches. The system disclosed has a positional accuracy of about one part in three hundred to the middle of the character for a spot size diameter of between 2 and 3 mils.

FIG. 5 illustrates an arrangement wherein a plurality of

photosensitive members

600, 602, 604, and 606 are utilized as detectors in place of a single photodetector. The photosensitive members are positioned on mask 204 over corresponding indexing marks. As the light beam generated by cathode-ray tube 200 initially scans the portion of the horizontal edge of mask 204 illustrated, each of the

photosensitive members

600 and 602 generate an electrical pulse as the beam impinges thereon. The output pulses are transmitted to the horizontal deflection counter 314 (FIG. 2) via OR gate 608, contact 322a of switch 322' and lead 610. After the horizontal scan is completed, the beam returns to an initial position 504. The beam is now deflected along the vertical edge of mask 204,

photosensitive members

604 and 606 each generating a pulse as the beam impinges thereon. The outputs of

members

604 and 606 are coupled to vertical deflection counter 330 (FIG. 2) via OR gate 608', contact 322k of switch 322 and lead 612. The double throw switch 322' replaces switch 322 utilized in the embodiment shown in FIG. 2. Switch control 320 now operates to change the contact positions shown in FIG. 5, after the horizontal scan is completed, i.e., contact 322a would be disconnected from lead 610 and contact 322b is connected to lead 612.

The output leads of the photosensitive members along each edge alternately maybe soldered together and then connected to the respective contacts of switch 322', eliminating the necessity of OR gates 608 and 608'.

While the present invention has been described with reference to its preferred embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the true spirit and scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from its essential teachings.

lclaim:

I. Apparatus for controlling the position of a light spot including means for deflecting said light spot along horizontal and vertical directions in response to signals applied thereto, wherein the improvement comprises:

a mask positioned to receive said light spot, said mask having a plurality of discrete, spaced index marks along its horizontal and vertical edges to indicate the horizontal and vertical position of said light spot, said index marks passing light incident thereon;

photodetector means positioned with respect to said marks so that light passed by each of said index marks produces an output pulse therefrom; and

control means responsive to output pulses from said photodetector means for comparing the position of said light spot on said mask as determined by said output pulses with a predetermined light spot position, said deflection means being responsive to the output of said control means to. adjust the position of said light spot to said predetermined position.

2. The apparatus as defined in claim 1 wherein said mask includes alphanumerical symbols thereon, said symbols passing light incident thereon.

3. The apparatus as defined in claim 2 further including photosensitive recording means for recording light passed by said mask, the light passed by said mask being in the shape of the symbol located at said predetermined position.

4. The apparatus as defined in claim 3 wherein said control means further includes means for resetting said light spot to an initial position after said light spot has been deflected to a predetermined position along one of said mask edges and means for redeflecting said light spot to said predetermined position after said light spot is deflected to a predetermined position along the other mask edge.

5. The apparatus of claim 1 in which the source of said light spot comprises a cathode-ray tube.

6. The apparatus of claim 5 further including means connected to said control means for unblanking said cathode-ray tube during the time when said light spot is deflected along said horizontal and vertical directions and when said light spot reaches said predetermined position on said mask.

7. The apparatus as defined in claim 6 further including means to reset said control system after said predetermined position is attained. I I

8. Apparatus for optically displaying an alphanumerical symbol on a mask including a cathode-ray tube, means to form a light spot on the face of said cathode-ray tube, a horizontal deflection drive circuit for controlling the horizontal position of said light spot, and a vertical deflective drive circuit for controlling the vertical position of said light spot, wherein the improvement comprises:

a mask positioned near the face of said cathode-ray tube and having a plurality of discrete, light transmitting spaced index marks along its horizontal and vertical edges to indicate the horizontal and vertical position of said light spot and light transmitting alphanumerical symbols thereon;

photodetector means positioned with respect to said mask so that light transmitted by each of said index marks produce an output pulse therefrom;

control means responsive to the output pulses of said photodetector means for comparing the actual position of said light spot on said mask as determined by said output pulses with a predetermined light spot position, said control means generating signals representing the difference between said actual and predetermined light spot positions, said horizontal and vertical drive circuits being responsive to said difference signals whereby said light spot is positioned to said predetermined position; and

means for unblanking said cathode-ray tube during the times when light is transmitted by said horizontal and vertical edge index marks and when said light spot is at said predetermined position.

9. The apparatus as defined in claim 8 wherein said light spot exposes a selected alphanumerical symbol on said mask when said light spot is deflected to said predetermined position and further including photosensitive recording means for recording the light transmitted by said selected symbol.

10. The device as defined in claim 9 wherein said control means comprises first and second position counters coupled to the output of said photodetector means, and first and second comparator circuits, the output of said first counter being compared with a predetermined first position in said first comparator and the output of said second counter being compared with a predetermined second position in said second comparator, the output of said first and second comparators controlling said drive circuits.

11. Apparatus for optically displaying an alphanumerical symbol on a mask including a cathode-ray tube, means to form a light spot on the face of said cathode-ray tube, a horizontal deflection drive circuit for controlling the horizontal position of said light spot, and a vertical deflection drive circuit for controlling the vertical position of said light spot, wherein the improvement comprises:

a mask positioned near the face of said cathode-ray tube and having a plurality of discrete, light transmitting slits along its horizontal and vertical edges to indicate the horizontal and vertical position of said light spot and light transmitting alphanumerical symbols thereon;

photodetector means positioned with respect to said mask so that light transmitted by each of said slits produce an output pulse therefrom;

means for unblanking said cathode-ray tube during the times when light is transmitted by said horizontal and vertical edge slits and when said light spot is at said predetermined position.

12. Apparatus for optically displaying an alphanumerical symbol on a mask including a cathode-ray tube, means to form a light spot on the face of said cathode-ray tube, a horizontal deflection drive circuit for controlling the horizontal position of said light spot, and a vertical deflection drive circuit for controlling the vertical position of said light spot, wherein the improvement comprises:

control means responsive to the output pulses of said photodetector means for comparing the actual position of said light spot on said mask as determined by said output pulses with a predetermined light spot position, said control means generating signals representing the difference between said actual and predetermined light spot positions, said horizontal and vertical drive circuits being responsive to said difference signals whereby said light spot is positioned to said predetermined position, said control means comprising a horizontal deflection counter for counting the pulses generated by said photodetecting means as said light spot is driven along the horizontal edge of said mask, said pulses representing the actual horizontal position of said light spot;

first comparator circuit connected to said horizontal deflection counter;

horizontal position memory connected to said first comparator;

vertical deflection counter for counting the pulses generated by said photodetecting means as said light spot is driven along the vertical edge of said mask, said pulses representing the vertical position of said light spot;

second comparator circuit connected to said vertical deflection counter;

character decoder, the input of which is connected to a source of input data said character decoder converting said input data into horizontal and vertical position information the output of said character decoder being coupled to said second comparator circuit;

a first binary counter coupled to the output of said first comparator;

a second binary counter coupled to the output of said second comparator circuit;

means for coupling the output of said first binary counter to said horizontal deflection drive circuit;

means for coupling the output of said second binary counter to said vert ical deflection drive circuit; means for switching the output of said photodetector means from energizing said horizontal deflection counter to said vertical deflection counter after a predetermined horizontal position is attained; and means for unblanking said cathode-ray tube during the times when light is transmitted by said horizontal and vertical edge index marks and when said light spot is at said predetermined position. 13. The apparatus as defined in claim 12 further including means for resetting said first and second binary counters after said light spot is positioned at said predetennined position.