CA1067206A - Apparatus and method for checking information recorded on a videodisc - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

The present invention relates to instantaneous monitoring of information recorded on a videodisc. In the prior art, it has not been possible to accurately determine the quality of a recorded videodisc until after completing the recording process. The present invention overcomes this problem by providing an im-proved apparatus and method for instantaneously checking information recorded on a videodisc. In accordance with the invention, the instantaneous monitoring system includes a comparator for comparing a video output information signal developed from reading a videodisc with a video input signal used to write information on the videodisc. Further included is an F.M. modulator for converting the voltage varying with time signal to a frequency modulated signal. A writer is provided for storing the frequency modulated signal upon a light responsive videodisc in the form of alternate regions of high and low directional reflectivity normal to the surface of the disc. A sensor, responsive to the reflected light, is used for recreating the frequency modulated signal, and a modulator responsive to the output of the sensor produces a voltage varying with time output video information signal. The comparator output is used to verify the quality of the information being stored on the videodisc.

Description

INSTANTANEOUS MONITORING OF INFORMATION
RECORDED ON A VIDEODISC
TECHNICAL FIELD
The present invention relates to monltoring of information stored on an info:mation storage member, and more particularly to the reading of information stored on a videodisc and comparing the retrieved information with the information used to effect writing o~ the - in~ormation on the disc.
BACKGROUMD OF THE PRIOR ART
Systems have heretofore been developed for recording signals at video ~requencies upon discs, tapes or other media. Such systems have utilized, among other things, optical recording upon photosensitive media, electron beam recordlng on thermo-plastic sur-~aces, and still other systems provide an instantane-cu91y reproducible record o~ video information.
The prior art can generally be divided into systems utilizing photographic surfaces, systems utIl-izing electron beam sensitive surfaces, magnetic recording systems, and as in the present invention, systems in which a radiant energy beam causes an irre-versible change to a surface, thereby "writing" informa-tion thereon.
Photographic systems have been described in the patents to P. C. Goldmark, et al, No. 3,234,326, which teaches recording on a continuous web such as a tape or film, or the patent to W. R. Johnson, No. 3,361,873, whi~h teaches the photographic recordation of video ~ .

~067Z06

-2-lnformation on a rotating disc in a spiral path.
The patent to W. C. Hughes, et al, No. 3,283,310 is illustrative of recordation o~ information on a thermo-plastic film surface, which utilizes an elec-tronic beam writing apparatus such as was disclosed inU. S. Patent No. 3,120,991.
Yet other systems have employed an electron beam to record information on a special storage medlum.
One such system has been disclosed in the patent to D. P. Gregg, No. 3J350,503. An alternative scheme utilizing an electron beam on a photosensitive medium such as photographic film has been taught in the patent to R. F. Dubbe, et al, No. 3,444,317.
In recent years, al~ernative methods have been disclo~ed for high density recording which are based upon either removal of material or vaporization of material by laser beam bombardment. These methods have been discussed briefly in the magazine "Electronics"
of March 3, 1969, at page llO. Furtherg a "laser therml micro image recorder" was described in some detail by C. O. Carlson and H. D. Ives in a paper given at the 1968 WESCON meeting (Western Electronic Show and Con-vention), which paper was published in Volume 12 of WESCON Technical Papers for 1968, at page l of Section ?g 16/l. The authors have referred to articles in the December 23, 1966, issue of "Science", Volume 54, No.
3756, at pages 1550 and 1551, the Proceedings of the Fall Joint Computer Conference of 1966, pages 711-716, and an article in the "Bell Systems Technlcal Journal"

3~ of March 1968, pages 385, 405.
These publications disclose a recording tech-nique which utilizes a thin metallic film coating upon a substrate. The thin metal film, under applied heat, melts rapidly and forms small globules within a re-corded spot. A highly concentrated spot of laser illum-ination can apply sufficient heat in a short enough time so that a suitably modulated laser beam impinging upon a moving surface can produce a pattern of holes in the metallic surface which, when "read back", can ~067Z~

reproduce the information recorded.
As pointed out in the Carlson and Ives paper, supra, the size of the recorded spot or hole can be much smaller than the diameter of the imaged laser beam.
By an appropriate choice of metal film material, film thickne.ss, laser divergence and spot power, an appro-priate system can be designed to record video frequen-cies with reasonably high resolution. However, the quality of the video signal reproduced from such record-ings has not been good due to a low signal-to-noise ratio resulting from the direct recording of the video signal onto the recording medium. Additlonally, the use of a modulated laser beam to selectively produce a pattern of holes on the metallic surface of a videodisc is not conducive to faithfully represent an amplitude varying analog video signal. Likewise, reading back the recorded information in the form of a track of holes or spots on the surface of the disc is ineffec-tive to faithfully recreate an analog video signal from the recorded track on the disc. The lack of fidelity in the reproduced video signal can be attributed pri-marily to the inability of the laser beam modulator to "writel' precise analog signals in the coating of the disc. Finally, the quality of the recorded signal could only be known after completion of the re-cording process.
BRIEF SUMMARY OF THE INVENTION
The present invention provides an improved apparatus and method for instantaneously checking in-formation recorded on a videodisc.
The instantaneous monitoring system includes acomparator for comparing a video output information signal developed from reading a videodisc with a video input signal used to write information on the videodisc.
Further included is a modulator for converting the input signal in the form of a voltage varying with tine signal to a frequency modulated signal. A writer is provided for storing the frequency modulated signal upon a light responsive surface of the videodisc in the form of ~067Z06 alternate regions of hlgh and low reflectivity normal to the disc surface. A sensor responsive to the light reflected from the regions is used for recreating the frequency modulated signal corresponding to the video input lnformation, and a demodulator responsive to the output of the sensor produces a voltage varying with time output video information signal. The comparator output can be used to verify the quality of the infor-mation being stored on the videodisc.
In a preferred embodiment, the monitoring system has a delay apparatus in the input video signal path for imparting a delay in the input video signal equal to the accumulated values of the delay from frequency modulation of the input video signal through frequency demodulation of the signal from the sensor.
Included in the delay time is the time necessary for the videodisc to move from the point of storage of frequency modulated signal by the writer to the point of impingement by a reading light beam.
The storing of frequency modulated signals is effected by a high intensity laser beam source producing a writing beam. A light intensity modulator, having electrical control circuitry responsiv~ to the level of optical modulation varies the intensity of the modulated writing beam from above a predetermined intensity at ~hich the writing beam alters the light responsive videodisc and below the predetermined intensity at which the writing beam fails to alter the light responsive videodisc. The alterations being representative of the frequency modulated input signals.
The light intensity modulator includes a feed-back apparatus for stabilizing the operating level of the electrical control circuitry to produce the light intensity levels above and below the predetermined intensity. The light intensity modulator further includes a light-sensing unit for sensing at least a portion of the writing beam issuing from an optical modulator to produce an electrical feedback signal representative ~ the average intensity o~ the modulated ~C~67206 writing beam and for applying the feedback signal to the electrical control circuitry to stabilize lts oper-ating intensity level. The light intensity modulator operating level is stabilized to issue khe writing beam at a substantially constant average power level.
The videodisc is disc-shaped to which uniform rotational moti~ is provided by a rotational driver synchronized with a translational driver. An electrical synchronizer maintains a constant relationship between the rotational and translational motion. The drivers provide the relative motion for a focused light beam to move radially across the videodisc surface.
The novel features which are believed to be characteristic of the invention, both as to organization and method of operation, together with further ob~ects and advantages thereof, will be better understood from the following description considered in connection with the accompanying drawings in which several preferred embodiments of the invention are illustrated by way of ~ample. It is to be expressly understood, however, that the drawings are for the purpose of illustration and description only and are not intended as a definition of the limits of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a generalized diagram of the appara-tus of the present invention;
FIG. 2 is a view of the optical path through the objective lens of FIG. l;
FIG. 3 is a representational indication of the relative spacing between a point of impingement of the writing beam and of the reading beam; and FIG. 4 is a diagram of a novel Pockels cell stabilizing circuit.
DETAILED DESCRIPTION OF THE INVENTION
Turning first to FIG. 1, the writing apparatus includes a writing head 12 which is, in the preferred embodiment, a dry microscope objective lens 14 mounted upon an air bearing support member 16. A 40X lens has been found to be satisfactory. A disc 18 is specially ~L067Z~)6 .

prepared and may be constructed according to the teach-ings of the prior art, in which a substrate has coated thereon a very thin film of a metal with a reasonably low melting point and a high surface tension.
A crystal oscillator 20 controls the drive elements. The disc 18 is rotated by a first, rotational drive element 22 which is coupled to a spindle 24. A
second, translational drive element 26 controls the position of the writing head 12.
A translating carriage 28, which is driven by the translational drive element 26 through a lead screw and travelling nut, moves the writing head 12 in the radial direction relative to the rotating disc 18.
The carriage 28 is provided with appropriate mirrors and lenses so that the remainder of the optics and electronics necessary to the writing device may be permanently mounted.
In the preferred embodiment of the present invention, the beam of a polarized cutting laser 30, which is an argon ion laser, is passed through a Pockels cell 32 which is driven by the Pockels cell driver 34. An FM modulator 36 receives the video signal that is to be recorded and applies the appropriate control signals to the Pockels cell driver 34.
As described hereinafter, the video in signal is of the type displayable on a TV monitor. According-ly, it is a voltage varying with time signal. The FM
modulator 36 is of standard design and converts the voltage varying with time signal to a frequency modu-lated signal having its informational content on the form of a carrier frequency having frequency changes with time corresponding to said voltage variations with time.
As is known, the Pockels cell 32 responds to applied signal voltages by rotating the plane of polar-ization of the light beam. Since a linear polarizer transmits light only in a predetermined polarization plane, a polarizer, such as a Glan prism 38 in the pre-ferred embodiment, is included in the writing beam path 1C~67;Z06 to provide a modulated writing beam 40. The modulated writing beam effectively follows the output of the FM
modulator 36.
The modulated writing beam 40 emerging from the Pockels cell-Glan prism combination 32, 38 is applied to a first mirror 42 which directs the writing beam 40 to the translating carriage 28. The first mirror 42 transmits a portion of the writing beam 40 to a Pockels cell stabilizing circuit 44 which responds to the aver-age intensity of the writing beam to maintain the energylevel of the beam.
A lens 46 is inserted in the path of the writ-ing beam 40 to diverge the substantially parallel beam so that it will spread to fill the entrance aperture of the objective lens 14 for optimum resolution. A
dichroic mirror 48 is included in the path oriented to substantially transmit all of the writing beam 40 to a second, articulated mirror 50. The articulated mirror 50 then directs the beam through the lens 14 and is capable of shifting the point of impingement of the beam 40 on the surface of the disc 18.
A series of holes is formed in the metal coating by the writing beam. One hole is formed for each cycle of the FM modulated signal represented by the modulated writing beam 40. Since the modulated writing beam tracks the output of the FM modulator 36, the holes formed in the coating also track the output of the FM
modulator. Obviously, since the informational content in the output signal of the FM modulator 36 is in the form of frequency changes in time about a carrier frequency, and since the "hole", "no hole" sequence represents the stored information, and since the disc 18 is rotating at a uniform speed, the "hole", "no hole"
sequence changes to represent the stored video informa-tion by the holes being formed closer or farther apartand the size of the hole becomes larger or smaller as the writing beam 40 changes under the control of the FM

~067206 ` -8 -modulated output signal from the FM modulator 36.
The objective lens 14 and the associated air bearing 16 effectively fly on a cushion of air at a substantiall~J fixed distance from the s~rface of the disc 18. That distance is determined by the geometry of the bearing 16g the linear velocity of the disc 18, and the force used to load the head against the disc 18. The fixed spacing is required because the focal tolerance of a lens capable of resolving a 1 ~m spot is also of the order of 1 ~m.
A second, relatively low-power laser 52 provides a monitoring beam 54. In the preferred embodiment, the reading laser 52 is a helium~neon device which enables the reading beam 54 to be distinguished from the writing 15 beam 40 by wavelength. A polarizingJ beam splitter cube 56 transmitB the reading beam 54 to a mirror 58 that directs the beam 54 through a second diverging lens 60 that spreads the reading beam 54 to fill the entrance aperture of the objective lens 14.
A ~uarterwave plate 62 is placed in the optical pa~h and, in conjunction with the plane polarizing beam splitter 56, prevents light reflected from the disc 18 from re-entering the laser 52 and upsetting its mode of oscillation. The quarterwave plate 62 rotates the plane of polarization of the beam by 45 degrees on each pass so that the reflected beam is rotated 90 degrees with respect to the polarizing beam splitter 56 and is therefore not passed by it.
A second mirror 64 in the reading beam 54 path directs the beam into the dichroic mirror 48 and is capable of limited adjustment so that the paths of the writing and reading beams are substantially identical, except that the reading beam "spot" impinges on the disc 18 downstream from the writing beam spot as explained in greater detail below.
A filter 66 that is opaque to the argon ion beam is interposed in the path of light reflected from the beam splitter 56. The He.Ne reading beam 54 that is returned from the disc surface is able to pass - ~672()6 _g_ through the filter 66 and through a lens 68 onto a photo-detector 70.
The reflected light of the reading beam impinges upon the photodetector 70. The photodetector 70 operates in its standard manner and generates an electrical current representative of the light impinging thereupon. In this case, the photodetector generates the signal represented by the "hole", "no hole" configuration formed in the coating. The "hole", "no hole" configuration is represen-tative of the output of the FM modulator 36. The outputof the FM modulator 36 is a carrier frequency having fre-quency changes with time representing the video signal to by recorded. The "hole", "no hole" configuration is representative of a carrier frequency having frequency changes with time representing the stored video signal.
The output of the photodetector 70 is an electrical signal representing the stored carrier frequency having fre-quency changes with time representing the stored video signal.
The output of the photodetector 70 is applied to a preamplifier 72 which provides a signal of sufficient amplitude and signal strength for subsequent utilization.
A video discriminator 74 then provides a video output signal which can be utilized in several ways, two of which are shown, as examples only.
The discriminator 74 is of standard design and function. It takes the frequency modulated signal from the photodetector 70 and changes it to a time dependent voltage signal having its informational content in the form of a voltage varying with time format suitable for display in the TV monitor 76.
In a first application, the video output is applied to a TV monitor 76 and an oscilloscope 78. As is well known, the TV monitor is responsive to a voltage varying with time signal. The information to be displayed on the TV monitor is represented by a voltage change with time.
The TV monitor 76 shows the picture fidelity of the recording, and the oscilloscope 78 indicates the 10672()6 -10=
signal-~o-noise ratio of the record and the quality of the cuttingj whether it is light or heavy. Not shown, an appropriate ~eedback loop could be provided through the Pockels cell stabilizing circuit ~4 to assure an adequate discrimination on the disc between a "holel' or black' area and 'no hole' or "white' area.
As an alternative utilization, the video output of the discrlminator 74 is applied to a comparator 80.
The other input of the comparator 80 is taken from the video input signal which is directed through a delay line 81. A delay that is equal the accumulated delays of the writing system and the time elapsed between the instant of writing of the information and the time required for that incremental area of the disc to reach the reading point mus~ he imparted to the input video signal.
Ideally, the video output signal of the dis-criminator 74 should be identical in all respects to the video input signal, after the proper delay.
As previously mentioned, the output from the discriminator 74 is a voltage varying with time signal.
The video in signal is also a voltage varying with time signal. Any differences noted represent errors whlch might be caused by imperfections in the disc's surface or malfunctions of the writing circuits. This application, while essential if recording digital information, is less critical when other information is recorded.
The output of the comparator circuit 80 can be quantized and counted~ so that an acceptable number of errors can be established for any disc. When the errors counted exceed the standardJ the writing operation can be terminated. If necessary, a new disc can be written.
Any disc with excessive errors can then be reprocessed to serve as a "new" disc for a subsequent recording.
Well-known techniques are available to translate the write head assembly 12 in the radial direction with respect to the rotating disc 18. While in FIG. 1 the rotational and translational drives 20, 22 are indicated as independent, the drives are synchronized to enable the writing assembly 12 to translate a predetermined 106720~

increment for each revolution of tne disc 18, by means of the common crystal oscillator 20.
Turning next to FIG. 2 there is shown, in some-what exaggerated form, the slightly differing optical paths of beam 40 from the writing laser 30 and the beam 54 ~rom reading laser ~2. The writing beam 40 coincides with the optical axis of the microscope objectiv~ lens 14.
The reading beam 54, ln contrast, makes an angle ~ with the axis so that it falls some distance X, equal to times the focal length of the objective~ "downstream"
~rom where the writing beam 40 is "cutting". The result-ing delay between reading and writing allows the molten metal to solidify so that the recording is read in its final state. If it were read too soon while the metal was still molten, it would not provide pertinent informa-tion for adjusting the recording parameters.
This is best indicated in FIG. 3 where two points in the same information channel are shown as dis-placed. The point A, which is the point of impingement of the writing beam 407 is shown as being on the optical axis of the objective lens 14. Separated from point Ag in the direction of medium motion, as indicated by the arrow, is the reading point B, which is at an angle ~ from the axis of the microscope obJective lens 14.
A distance between points A and B of two ~m has provided a satisfactory monitoring of the writing operation.
Turning finally to FIG. 4, there is shown an idealized diagram of a Pockels cell stabilizing circuit 44, suitable for use in the apparatus of FIG. 1. As is known, a Pockels cell rotates the plane of polari-zation of the applied light as a function of an applied voltage. Therefore, the Pockels cell is used to rotate plane polarized light, and the rotated light is passed through a plane polarizer, such as a Glan prism. The light issuing from the polarizer will be amplitude modu-lated in accordance with the applied voltage.
Stated another way; the standard operating mode of a Pockels cell 32 and Glan prism 38 is for use as a light intensity modulating means. Each cycle from the FM modulator drives the Pockels cell through its full operating range of ninety degrees. Within this operating range of ninety degrees~ one operating point passes all light applied thereto and identified as a full light transmitting state. A second operating point passes no light and is identified as a full light block-ing state. The Pockels cell itself only rotates the plane of polarization. The Glan prism passes light in one plane of polarization and no light in the plane dis-placed ninety degrees from that plane in which all thelight passes.
Depending upon the individual Pockels cell, a voltage change of approximately 100 volts wlll cause -_ the cell to rotate the plane of polarization through 360 degrees. However, the transfer characteristic of an individual cell may drift spontaneously, corresponding to a voltage change of + 50 volts, and accordingly, a feedback loop is desirable to maintain the cell within a usefulg reasonably linearJ operating range.
The stabilizing circuit 44 includes a photo-sensitive silicon diode 82, which is positioned to receive a portion of the writing beam 40 reflected from the mirror 42 of FIG. l. The silicon diode 82 functions in much the same fashion as a solar cell and is a source of electrical energy when illuminated by incident radia-tion. One terminal of the silicon diode 82 is connected to common reference potential 84, indicated by the con-ventional ground symbolg and the other terminal is con-nected to one input of a differential amplifier 86.
The silicon cell 82 is shunted by a load 88 which enables a linear response mode.
The other input to the differential amplifier 86 is connected through an appropriate potentiometer 90 to the common reference 84. A source of power 92 is coupled to the potentlometer 90, which enables the setting of the differential amplifier 86 to establish the average light level transmitted by the Pockels cell 32.
Accordingly, a pair of output terminals of the ~067Z0 differential amplifier 86 are respectively connected through resistive elements 94, 96 to the input terminals of the Pockels cell 32 of FIG. 1. It is noted that the Pockels cell driver 34 is a.c. coupled to the Pockels cell 32, while the differential amplifier 86 is d.c.
coupled to the Pockels cell 32.
In operation, the system is energized. The light from the writing beam impinging on the silicon dlode 82 generates a differential voltage at the input to the dl~ferential amplifier 86. Initially, the poten-tlometer 90 is adjusted to produce light at a predeter-mined average level Or intensity. Thereafter, 1~ the average level of intensity impinging on the silicon cell 82 either increases or decreases, a correcting voltage will be generated in the differential amplifier 86. The correcting voltage applied to the Pockels cell 32 ~s of a polarity and magnitude adequate to restore the average level of intensity to the predetermined level.
Thus there has been shown an improved video disc recording assembly. A microscope ob~ective lens mounted on an air bearing "flies" at a predetermined distance from the surface of a metallized disc. The metallized coating is such that a laser beam can~ under suitable modulation, deliver sufficient energy to melt localized areas of the surface. Under surface tension, the molten metal retracts leaving a clear area of approx-imately one micron in diameter.
A secondJ low-energy laser utilizing substan-tially the same optical path is directed through the same microscope objective lens, but is brought to the surface of the disc at a slight distance "downstream" from the point of writing. The reading beam is returned through an appropriate optical system that excludes the reflected energy of the writing beam and enables an analysis of the lnformation that has been written on the disc.
The playback information can~ among other things, control the intensity of the writing beam to assure adequate "recording levels", determine whether an un-acceptable number of errors have been made in the re c ord ing pr oc e ,, c,, ~

Claims (12)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE PROPERTY
OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. Apparatus for monitoring the storage of video information upon an information storage member comprising: means for providing an input video information signal to be recorded in the form of a voltage varying with time; modulator means for converting said voltage varying with time signal to a frequency modulated signal with a carrier frequency having frequency changes in time correspond-ing to said voltage variations with time; means for storing said frequency modulated signal upon a light responsive member in the form of a lineal series of regions of alternately high and low directional reflectivity normal to said surface;
a light source for providing a light beam to illuminate said alternate regions; means for imparting relative motion between said illuminating beam and said regions for generating reflections from said high directional reflective regions;
sensing means responsive to said light reflected from said high directional reflective regions for recreating a frequency modulated signal corresponding to said reflections representa-tive of said video information; demodulator means responsive to the output of said sensing means for producing an output video information signal having a voltage varying with time format suitable for display on a standard television monitor; and a comparator for comparing said output video information signal with said input video information signal.

2. The apparatus as claimed in Claim 1, wherein said apparatus further comprises delay means in the signal path of said input video information signal, said delay means imparting a time delay to said input video information signal equal to the accumulated values of the delay from frequency modulation of said input video information signal through frequency demodulation of said signal from said sensing means, and including the delay of travel time of the point on said storage member moving from the point of storing said input video information signal by said storing means to the point of impingement of said light beam.

3. The apparatus as claimed in Claim 2, wherein said means for storing includes a writing light beam source producing a writing light beam, and intensity modulating means having electrically controllable means responsive to said modulator means for varying the intensity of said writing light beam above a predetermined intensity at which the writing light beam alters said light responsive member and below said predetermined intensity at which the writing light beam fails to alter said light responsive member, said alteration being representative of said frequency modulated signal.

4. The apparatus as claimed in Claim 1, wherein:
said storage member is disc-shaped; said means for impart-ing relative motion comprises rotational drive means for producing uniform rotational motion of said disc; and wherein said apparatus further comprises: translational drive means synchronized with said rotational drive means for relatively moving said light beam radially across the surface of said disc-shaped storage member; and electrical synchronizing means for maintaining a constant relationship between said rotational motion and said translational motion.

5. The apparatus as claimed in Claim 3, wherein said light intensity modulating means further includes feedback apparatus for stabilizing the operating level of said electrically controllable means to produce the intensity levels of said writing light beam above and below said predetermined intensity, said light intensity modulating means including light-sensing means for sensing at least a portion of the writing beam issuing from said electrically controllable means to produce an electrical feedback signal representative of the intensity of the writing beam issuing from said electrically controllable means and applying the feedback signal to said elec-trically controllable means to stabilize its operating level.

6. The apparatus as claimed in Claim 5, wherein said light-sensing means produces an electrical feedback signal which is representative of the average intensity of the writing light beam issuing from said electrically controllable means, the operating level of said light intensity modulating means being stabilized to issue the writing light beam at a substantially constant average power level.

7. A method for monitoring the storage or video information upon an information storage member comprising the steps of: providing an input video in-formation signal to be recorded in the form of a voltage varying with time; converting said voltage varying with time signal to a frequency modulated signal with a carrier frequency having frequency changes in time corresponding to said voltage variations with time; storing said fre-quency modulated signal upon a light responsive surface of the storage member in the form of regions of alter-nately high and low directional reflectivity normal to said surface; illuminating said alternate regions with a reading laser beam generating a beam of polarized mono-chromatic light; providing relative motion between said illuminating beam and said alternate regions for gener-ating reflections from said light reflective regions;
sensing said light reflected from said regions and re-creating a frequency modulated signal corresponding to said reflections representative of said video informa-tion; demodulating said recreated frequency modulated signal for producing an output video information signal to be displayed suitable for display on a standard television monitor, said video information signal to be displayed being in the form of a voltage varying with time; and comparing said output video information signal with said input video information signal.

8. The method as claimed in Claim 7, including the step of delaying said input video information signal to impart a time delay to said input video information signal equal to the accumulated values of the delay from frequency modulation of said input video information signal through frequency demodulation of said signal in said demodulating step, said delay including the delay of travel time of the point on said storage member moving from the point of storing said input video in-formation signal in said storing step to the point of impingement and reflection of said reading laser beam.

9. The method as claimed in Claim 7, wherein said step of storing includes: providing a writing laser beam; modulating said writing laser beam using said fre-quency modulated signal for varying the intensity of said writing laser beam above a predetermined intensity at which the beam alters said light responsive surface and below said predetermined intensity at which the beam fails to alter said light responsive surface, said alteration being representative of said frequency modulated signal.

10. The method as claimed in Claim 9, wherein the storage member is disc-shaped and said step of pro-viding relative motion includes: producing uniform rotational motion of said disc; and synchronizing said rotational motion with movement of said storage member for effecting relative movement of said writing and reading laser beams radially across the surface of said disc-shaped storage member to maintain a constant rela-tionship between said rotational motion and said trans-lational motion.

11. The method as claimed in Claim 9, including the step of stabilizing the level of modulation of said writing beam to produce the intensity levels of said writing light beam above and below said predetermined intensity; sensing at least a portion of said writing laser beam after modulation of the writing beam to produce an electrical feedback signal representative of the intensity of the writing beam; and utilizing the feedback signal in said storing step to effect stabiliza-tion of the level of modulation of said writing beam.

12. The method as claimed in Claim 11, wherein said step of sensing at least a portion of said writing beam produces an electrical feedback signal which is representative of the average intensity of the modulated writing beam, the operating level of light beam modula-tion being stabilized to issue the modulated writing beam at a substantially constant average power level.