CA1069214A - Videodisc mastering system - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
The present invention relates to a videodisc containing stored information in the form of frequency modulated intelligence. Prior art videodiscs have been made using recording procedures which record video signals in an amplitude varying with time format. As a result, small inaccuracies in the recording process produce low quality signals when read by playback appar-atus. Additionally, handling of the videodiscs of the prior art can easily destroy or adversely affect the quality of the reproduced signal. The present invention overcomes the deficiencies by providing an improved videodisc having its information stored in a unique manner which permits faithful recording and reproduction of a video signal in the face of minor processing in-accuracies and handling of the disc. In accordance with the invention, the videodisc contains information in the form of a carrier frequency having frequency changes with time varying from the carrier frequency. The videodisc has an upper surface carrying a lineal series of indicia positioned in track-like fashion on the sur-face. The indicia represents a frequency modulated intelligence signal. The lineal series of indicia includes regions of alternately high and low directional reflectivity normal to the surface of the disc. The regions of high directional reflectivity normal to the surface are areas of a metal coating on the disc, while the regions of low direction reflectivity normal to the surface are apertures in the metal coating.

Description

~a~sz~4 INFORMATION STORAGE MEMBER
TECHNICAL FIELD
The present invention relates to informatlon storage membersg and in particular to videodlscs ~or the permanent storage of video information.
BACKGROUMD OF THE PRIOR ART
Videodiscs have heretofore been produced by systems developed for recording signals at video fre-quencies upon discs, tapes, or other medla. Such ; 10 systems have utilized, among other things, optical re-cording upon photosensitive media, electron beam record-lng on thermo-plastic surfaces, and still other systems provide an instantaneously reproducible record of video information. ~ -~
The prior art can generally be di~7ided into information storage mediums utilizing photographic sur-faces, electron beam sensitive surfaces, magnetic record-ing surfaces, and as used in the present lnvention, a -medium having a surface`in which a radiant energy beam causes an irreversible change, thereb~ "writing" in~or-mation thereon.
- Photographic techniques have been described in the patents to~P. C. Goldmark, et al, No. 3,234,326g which teaches re~cording on a continuous web such as a tape or film, or the`patent to W. R. Johnson, No.
3,361,873, which teaches the photographic recordation of ~ ~
video information on a rotating disc in a spiral path. ~ ~ -The patent to W. C. Xughesg et al, No. 3,283,310 is illustrative o~ recordatlon of information on a~the~

~692~L4 plastic film sur~ace, which u~ilizes an electronic beam writing apparatus such as was disclosed in U. S.
Patent No. 3,120,991.
Yet other techniques have employed an electron beam to record information on a special storage medium.
One such system has been disclosed in the patent to D. P. Gregg, No. 3,350,503. An alternative scheme utilizing an electron beam on a photosensitive medium such as photographic film has been taught in ~he patent to R. F. Dubbe, et al~ No. 3,444,317.
In recent years, alternative methods have been disclosed 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 33 1969, at page 110. Further~ a "laser thermal micro image recorder" was described in some detall by C. O. Carlson and H. D. Ives in a paper given at the 1968 WESCON meeting (~estern Electronic Show and Con-vention), which paper was published in Volume 12 ofWESCON Technical Papers for 1968, at page 1 of Section 16/1. 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 Technical Journal"
of March 1968) pages 385, 405.
These publications disclose a recording technique which utilizes a thin metallic film coating 30 upon a substrate. The thin metal ~ilm, under applied heat, melts rapidly and forms small globules within a recorded spot. A highly concentrated spot of laser illumination can apply sufficient heat in a short enough time so that a suitably modulated laser beam impingi~g ùpon a moving surface can produce a pattern o~ holes in the metallic surface which, when "read back", can reproduce the information recorded.
As pointed ~t in the Carlson and Ives papér,~
supra, the size of the recorded spot or hole can be =: . .
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3692~4 much smaller than the diameter ~ the imaged laser beam.
By an appropriate choice of metal film materialg film thicknessg laser divergence and spot power9 an appro-priate system can be designed to record video frequencies with reasonably high resolution. However, the quality of the video signal reproduced from such recordlngs has not been good due to a low signal-to-noise ratio resulting ~rom the direct recording of the video signal onto the recording medium. Additionallyg 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 amplltude varying analog video signal. Llkewiseg reading back t~e re-corded information in the form of a track of holes or spots on the surface of the disc is ineffective to faith-fully recreate an analog video signal from the recorded track on the disc. The lack of fidelity in the repro-duced video signal can be attributed primarily to the inability of the laser beam modulator to "write" precise analog signals in the coating of the disc.
BRIEF SUMMARY OF THE INVENTION
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The present invention provides an improved information storage medium for storing video information~
the stored information having the form of a lineal series of indicia representing a frequency modulated information signal.
In accordance with the invention~ a videodisc is provided for retaining indicia representing-the fre-quency modulated information signal. The frequency modulated signal has its informational contents in the ~orm of a carrier frequency having frequency changes with time varying from the carrier frequçncy. The upper surface of the disc carries the lineal series of indicia positioned in track-like fashion upon the surface. The indic~a similarly represe~nts a frequency modulated signal having its informational content in the form of a carrier frequency having frequenc~ changes with time varying from the carrier frequency.
Preferably~ the lineal series of indicia .
.:' ,' ,' 1~69Z14 includes regl~ns of alternately hlgh and 10T~ directional reflectivity normal to the surface. That is, when viewed from a point above the surface of the disc~ an impinging light beam directed normal to the surface would reflect alternately with high and low levels of`
re~lected light in a direction opposite that of the impinging light beam.
The videodisc may have a glass disc substrate, the upper surface of which is covered with a light responsive coating. The coat~ng is altered by a suit-able writing apparatus so as to carry the lineal series of indicia positioned in track-like fashion, preferably along a spiral path.
The regions of alternately high and low direc-tional reflectivity normal to the surface may be,respectively, light reflective and light absorbing areas.
Each region of high directional reflectivity normal to the sur~ace is preferably a metal coating. Each region ~ o~ low directional reflectivity normal to the surface - 20 consists of apertures in the metal coating.
Also described herein is a suitable writing apparatus ~or recording the ~requency modulated in~orma-tion signal on the videodisc and an approp~ria~ reading apparatus ~or retrieving the information ~re~ on the disc.
The writing apparatus includes a light source providing a light beam of sufficient intensity to interact with the responsive coating of the videodisc as the disc uniformly rotates. An optical system de-3 fines an optical path between the light source and theresponsive coating on the disc for ~ocusing the light beam on the coating. A light lntensity modulator is positioned in the optical path and is responsive to the frequency modulated information signal to intensity modulate the light beam. The light intensity modulator is e~ective to transmit light over a predetermined intensity range3 the intensity of the modulated light beam changing between a relatively high light level and a relatively lower light level during each cycle o~ the "
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frequency modulated signal. The light beam passing through the intensity modulator and optlcally focused on the responsive coating ~orms the indicla representing the frequency modulated in~ormation signal.
The reading apparatus includes a polarized collimated reading beam o~ light, and an optical system between the light source and the videodisc ~or focusing the reading beam upon the videodisc As the disc rotates, the alternate regions o~ indicia generate re-flections~ the intensity o~ which varies in accordance with the recorded in~ormation. A light sensor is re-sponsive to the re~lection ~or generating a correspond- -ing frequency modulated electrical signal comparable to that used in the writing apparatus. That is, the requency modulated output signal has its in~ormational content in the ~orm of a carrier frequency whlch has a frequency varying with time.
The writing apparatus pre~erably uses an Argon ion laser beam ~or selectively melting, without vapor-izing, the metalized coating on the disc. The coating has suitable physical properties to permit localized heating by the writing lasers tOg in turn, cause local-ized melting accompanied by withdrawal of the molten --material toward the perimeter of the molten area. Upon

2~ freezingJ a permanent aperture is le~t in the metalized coating. Under proper operating conditions, the in-tensity-o~ the writing laser beam varies from an in-tensity above a predetermined intensity level at which the ~ocused beam melts the metalized coating without vaporizing it, to an intensity below the predetermined intensity~ at which the focused beam ~ails to melt the metalized coating.
I~ desired, concentric circles can be created on the videodisc by alternately translating and writing with the wri-ting laser beam. In a pre~erred embodiment employing a spiral track, the spacing between adjacent turns o~ the spiral is 2~, center to center. Assuming a spot diameter on the order of l~m, this would produce a guard are~ of one 1~ between spots in ad~acent tracks.

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~69Z14 The novel features which are believed to be characteristic of the invention~ both as to organization and method of operationg together with further objects and advantages thereof~ will be better understood from the following descrlp~ion considered in connection with the accompanying drawings in which several pre~erred embodiments of the invention are illustrated by way of example. It is to be expressly understood, hol~ever, that the drawings are for the purpose of illustration 10 and description only and are not intended as a defini-tion of the limits of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a generalized diagram of the appar-atus of the present invention;
1~ 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 spac~ng between a point of impingement of ~he writing beam and of the reading beam; and FIG. 4 is a diagram of a novel Pockels cell stabilizing circuit.
C . DETAILED DESCRIPTION OF INVENTION
Turning first to FIG. 1, the writing appara-tus ~ includes a writing head 12 which is, in the pre-25 ferred 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 prepared and may be constructed according to the teachings of the prior art, in which a substrate ! 30 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 2~. A
second, translational drive element 26 controls the position of the writing head 12.
A translating carriage 28, which is driven by the translat;ional drive element 26 through a lead screw and travelling nut, moves the writing head 12 in the .'":;'' ,, ~
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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 elec-tronics necessary to the writing device may be perman-ently mounted.
In the preferred embodiment of the presentinventiong the beam of a polarized cutting laser 30g which is an argon ion laser, Ls passed through a Pockels cell 32 which is driven by the Pockels cell driver 34.
An FM modulator 36 receives the video signal th~t 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. Accordinglyg it is a voltage varying with time signal. The FM modu-lator 36 is of standard design and con~erts the voltage varying with time signal to a frequency modulated slgnal having its lnformational content in the form of a carrier frequency having frequency changes with time ` 20 corresponding to said voltage variations with time.
As is known3 the Pockels cell 32 responds to applied signal voltages by rotating the plane of polar-ization of the light beam. Slnce a linear polarizer ~
~ransmits light only ln a predetermined polarization `
plane, a polarizer, such as a Glan prism 38 in the pre-ferred embodiment, is included in the writing beam path to provide a modulated writing beam 40. The modulated writing beam effectively follows the output of the FM
modulator 36.
; 30 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 lntensity of the writing beam to maintain the energy `
level of the beam.
A lens 46 is inserted in the path of the writ-ing beam 40 to diverge the substantially parallel beam , --~ _L_ 11 ~6~

- 50 that It wlll spre~d t3 ftll the entrance aperture of the obJectlve lens 14 for optlmum resolutlon. A dlchrolc mlrror 48 Is Included In the path orlenled ~o subs~sntl~lly transmtt all of the wrltlng beam 40 to a s,econd~ artlculeted mTrror ~O. A mlrror may be 0nployed In the present Inventlon. The artlcul~ted mlrror SO th3n dlrects the bsam through the lens 14 and Is capable of shlftlng the~ polnt of Implng6m~nt of ths bsam 4O on the ~ur~ace of tha dls;c t8.
A serles of holss ts formsd tn tha me~t c~atlng by o tho wr!ttng beom. One hole Is formed fvr s3ch cycle of me FM mcdut~ted slgnal represanted by the modulat~d wrltlng b~
40. SInce the modulated ~rlttng beam tr3cks the output o~
tne FM moduletor 36, ~he holes ~ormed 1n the coatln3 ~l~a track th~ ou~put of ~he FM modulator. Obvlously, slnce the InformDtlonal content tn the output slgnal ot the FM modut~tor 36 15 In the form o~ frequency ch~nges In tlme abDut a carrler fr~quency, and slnce the "hole", "no hote" sequence repra-sen~s the storod tnformatlon, and slnce the dlsc 18 Is ro-totlng ~ u unlfor~ spsed, the "hole", "no hole" sequence ;-change~ to repres0n~ the stored vldso tnformatlon by ths holes belng fonmed closer or farther apart and the sl2e of the hote becomes larger or smatler as the wrtttng be~m 40 changes under the controt of the FM modulated output slgnal from the FM modulator 36.
The obJ~ctlve lens t4 and tho ossocl~ted ~tr beorlng 16 effectt~ely fly on ~ cushlon of alr ~t a sub-stsnt1~11y flxed dls~ance from the surf~ce o~ the dlsc t8.
Tho~ dlstance Is detenmlned by the geometry of the beartng 16, th~ tlnear v010clty of ~ho dlsc 18, and the force us~d tD
load the heod ~ga1nst the dlsc 18. The flxeJ sp~clng Is r0qulred bocause the ~ocal.tDlerance of a lens capabls of r~solvlng ~ l~m spot Is ~Iso of the order of l~m.
A second, relatlvely low-power laser 5~ provldes ~
monltorTng ~eom 54~ In the pre~err~J embodlment, the reodlng laser 52 Is ~ hellum-neon devlce whlch .

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g enables the reading beam 5~ to be distinguishe~ ~rom ~he writlng beam 40 by wavelength, A polariæing~ beam splitter cube 56 transmits the reading beam 54 to a mirror 58 that directs the beam 5L~ through a second diverging lens 60 that spreads the reading beam 54 to fill the entrance aperture of the ob~ective lens 14.
A quarterwave plate 62 is placed in the opti-cal path and~ in con~unction w~th the plane polarizing beam splitter 56, prevents light reflected ~rom the disc 18 fro~ re-entering the laser 52 and upsetting its mode of oscillation. The quarterwave plate 52 rotates the plane of polarization of the beam by ~5 degrees on each pass so that the re~lected beam is rotated 90 - degrees with respect to the polarizlng beam spl~tter 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 43 and is capable o~ 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 ~rom 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 through the filter 66 and through a lens 68 onto a photodetec-` tor 70.
The reflected light of the reading beam lm-pinges upon the photodetector 70. The photodetector 70operates in its standard manner and generates an elec-trical current representative of the light impinging thereupon. In this case, the photodetector generates the signal represented by the "hole", "no hole" con-figuration ~ormed in the coating. The "hole", "nohole" con~i~uration is representative o~ the output o~ the F~ modulator 36. The output of the FM modulator 36 is a carrier frequenc~y havlng frequency changes Wlth time representing the vldeo slgnal to be recorded. The ..
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"hole", '1no hole" configuration is representative of a carrier frequency having fre~uenc~ c anges with time representing the stored video signal. The output of the photodetector 70 is an electrical signal representing the stored carrier frequency having frequency changes with time representing the stored video signal.
The output of the photodetector 70 is applled to a preamplifier 72 which provides a signal o~ suffi-cient amplitude and signal strength for subsequent utllization. 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 ~unction. It takes the frequency modulated signal from 15 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 - 20 a~pplied to a TV monitor 76 and an oscilloscope 78. As ~`s well knowng the TV monitor is responsive to a vol-tàge varying with time signal. The in~ormation to be displayed on the TV monitor is represented by a voltage ` ~ change with time.
The TV monitor 76 shows the picture fidelity o~ the recording, and the oscilloscope 78.indicates the signal-to-noise ratio of the record and the quality of ; the cutting, whether it is light or heavy. Not shown, an appropriate feedback loop could be provided through the Pockels cell stabilizing circuit 44 to assure an adequate discrimination on the disc between a "hole" or "black" area and "no hole11 or "white" area.
As an alternative utilization, the ~lideo out-put of the discriminator 74 is applied to a comparator 35 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 .

`69~4 required for that incremental area of the disc to reach the reading point must be imparted to the input video signal.
Ideally, the video output signal of the dis-criminator 74 shoul~ be identical in all respects to the - video input signal~ after the ~roper delay.
As previously mentionedf 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 di~ferences noted represent errors which might be caused by imperfections in the disc's surface or malfunctions of the writing circuits. This applica-tion, while essential if recording digital informationg is less critical when other information is recorded.
The output o~ 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 standard~ the writing opera-tion can be terminated. I~ necessary, a new disc can be written. Any disc with excessive errors can then be ~eprocessed to serve as a "new" disc for a subsequent recording.
Well-known techniques are available to trans-late the writing head assembly 12 in the radial direc-tion with respect to the rotating disc 18. l~1hile inFIG. 1 the rotational and translational drlves 20, 22 are indicated as independent, the drives are synchronized to enable the writing assembly 12 to translate a pre-determined increment for each revolution of the disc 18, by means of the common crystal oscillator 20.
Turning next to FIG. 2 there is shown, in somewhat exaggerated form, the slightly differing opti-cal paths of beam 40 from the writing laser 30 and the beam 54 from reading laser 52. The writing beam 40 : 35 coincldes with the optical axis of the microscope objec- :
tive lens 14. The reading beam 54, in contrast, makes an angle ~ with the axis so that it falls some distance X, equal to ~ times the focal length o~ the objective, "downstream" ~rom where the writing beam 40 is "cutting".

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The result~ng dela~ 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 molteng it would not provide per-tinent information for adjusting khe recording parameters.
This is best indicated ln FIG. 3 where twopoints in the same information channel are shown as displaced. The point A~ which is the point of impinge-ment of the writing beam ~0~ 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 objectlve lens 14. A distance between points A and B of two~m has provided a satisfactory monitoring of the writing oper-ation.
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 polarization o~ the applied light as a function of an applied vol-tage. 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 lssuing ~rom the polarizer will be amplitude modulated 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 ~he FM modulator drives the Pockels cell through its ~ull operating range ~ 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 operatlng point passes no light and is identified as a ~ull li~ht blocking state. The Pockels cell itself only rotates the plane o~ polarization. The Glan prism passes light in one plane o~ polarization and no light in the plane displaced ninety degrees from that plane in which all .
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; Depending upon the lndivldual 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 characterlstic of an individual cell may drift spol~taneously, corresponding to a voltage change of ~ 50 volts~ and accordingly~ a feedback loop is desirable to maintain the cell within a use~ul, reasonably linear~ 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 ~2 of FIG 1. 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 radiation. One terminal of the silicon diode 82 is connected to common reference potential 84, indicated by the conventional ground symbol, and the other terminal is connected to one input of a dlfferential 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 potentiometer 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

3 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 di~ferentlal amplifier 86 is d.c.
coupled to the Pockels cell 32.
In operatlon~ the system is energized. The light from the writing beam impinging on the silicon diode 82 generates a differential voltage at the input to the differential amplifier 86. Initially, the :

. -~L~69Z~4-14-potentiometer 90 is adjusted to produce light at apredetermined average level of intensity. Therea~ter, if the average level of intensity impinging on the silicon cell 82 either lncreases or decreasesg a correct-ing voltage will be generated in the differential am-plifier 86. The correcting voltage applied to the Pockels cell 32 is o~ a polarity and magnitude adequate to restore the average level of intensity to the pre-determined level.
Thus there has been shown an improved video disc recording assembly. A microscope objective lens mounted on an air bearing "flies" at a predetermined distance ~rom the surface of a metallized disc. The metallized coating is such that a laser beam can, under suitable modulation, deliver su~icient energy to melt localized areas of the surrace. Under surface tension, the molten metal retracts leaving a clear area of appro~
im~tely one micron in diameter.
A second~ low-energy laser utilizing substan-tially the same optical path is directed through thesame microscope ob~ective lenst but ls brought to the surface ~ 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 in~ormation that has been written on the disc.
The playback information can, among other things, control the intensity of the writing beam to ; 30 assure adequate "recording levels", determine whether an unacceptable number of errors have been made in the recording process.

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Claims (6)

1. An information storage medium for storing a fre-quency modulated signal, said medium comprising: an information storage member having an upper surface, said surface interrupted by a lineal series of surface discontinuities positioned in track-like fashion along said surface; said series of surface discontinuities and the surface portions therebetween comprising, respectively, variable length regions of alternately low and high directional reflectivity normal to said surface to thereby represent a frequency modulated signal having its informational content in the form of a carrier frequency having frequency changes with time varying from said carrier frequency.

2. The information storage medium as claimed in Claim 1, comprising: a glass disc substrate, one surface of said substrate defining said upper surface; and a light responsive coating carried by said surface; said coating altered to define said lineal series of surface discontinuities positioned in track-like fashion.

3. The information storage medium as claimed in Claim 1, wherein said regions of alternately high and low directional reflectivity are positioned in a spiral path.

4. The information storage medium as claimed in Claim 1, wherein said regions of alternately high and low directional reflectivity normal to said surface are, respectively, light reflective and light absorbing areas.

5. The information storage medium as claimed in Claim 4, wherein each region of high directional reflectivity normal to said surface is a metal coating.

6. The information storage medium as claimed in Claim 5, wherein each region of low directional reflectivity normal to said surface is an aperture in said metal coating.