CA1066414A - Apparatus and method for storing information on a videodisc - Google Patents
Apparatus and method for storing information on a videodiscInfo
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- CA1066414A CA1066414A CA313,007A CA313007A CA1066414A CA 1066414 A CA1066414 A CA 1066414A CA 313007 A CA313007 A CA 313007A CA 1066414 A CA1066414 A CA 1066414A
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Abstract
ABSTRACT OF THE DISCLOSURE
The present invention relates to the storage of information upon a videodisc. Prior art attempts at storing video information on a disc have involved processing procedures producing recorded intelligence information on a disc directly from a corresponding intelligence signal in amplitude varying with time format. The present invention provides an improved apparatus and method for reliably storing information on a videodisc in the form of a carrier frequency having frequency changes with time varying from the carrier frequency. In accordance with the invention, a video-disc having a light responsive coating covering the disc substrate is provided, and a light source produces a light beam of sufficient intensity to interact with the responsive coating of the videodisc as the disc rotates. An optical system defines an optical path between the light source and the responsive coating on the disc for focusing the light beam on the coating. A
light intensity modulator, responsive to the informa-tion signal, is positioned in the optical path for intensity modulating the light beam. The beam passing through the intensity modulator and optically focused on the responsive coating on the disc forms an informa-tion track of indicia representing the frequency modu-lated information signal. The information signal and altered coating of the disc contain intelligence in-formation in the form of a carrier frequency varying in frequency as a function of time.
The present invention relates to the storage of information upon a videodisc. Prior art attempts at storing video information on a disc have involved processing procedures producing recorded intelligence information on a disc directly from a corresponding intelligence signal in amplitude varying with time format. The present invention provides an improved apparatus and method for reliably storing information on a videodisc in the form of a carrier frequency having frequency changes with time varying from the carrier frequency. In accordance with the invention, a video-disc having a light responsive coating covering the disc substrate is provided, and a light source produces a light beam of sufficient intensity to interact with the responsive coating of the videodisc as the disc rotates. An optical system defines an optical path between the light source and the responsive coating on the disc for focusing the light beam on the coating. A
light intensity modulator, responsive to the informa-tion signal, is positioned in the optical path for intensity modulating the light beam. The beam passing through the intensity modulator and optically focused on the responsive coating on the disc forms an informa-tion track of indicia representing the frequency modu-lated information signal. The information signal and altered coating of the disc contain intelligence in-formation in the form of a carrier frequency varying in frequency as a function of time.
Description
. 106641~
INFOR~ATION ON A VIDEODISC
- TECHNICAL FIELD
The present inventlon relates to the storage Or lnrormation upon an information storage member, and more particularly to the writing of vldeo lnformation on a vldeodlsc.
BACKG ~lND OF ~XE PRIOR ART
Systems have heretofore been developed ~or 10 recordlng signals at video frequencles upon dl~cs, tapes, or other.media. Such system3 have utlllzed, .... among other things, optical recording upon photosensl-- tlve media, electron beam recordlng on thermo-plastlc ~urraces, and stlll other system~ prov~de an lnstan-taneously repro~.'ucible r~cord Or vid~o inrormatlon.
The prlor art can generally be dlvided into .. systems utlllzlng photographic sur~aces, systems utll-lzlng electron beam sensitive surraces, magnetic . .recording systems, and æs ln the present inventlon, 20 .systems in whlch a radlant energy beam ¢auses an lrre-.versible.çharge to a surrace, thereby "wrltlng" inrorma-... . .
tion thereon.
Photographic systems have been described ln 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, whlch teaches the photogr~.phic recordatlon Or video in~ormatlon on a rotatlng dlsc in a splral path.
_/_ , ~ ..
;6414 The patent to W. C. Hughes, et al, ~o. 3,283,310 is lllustrative Or recordation of lnformation on a thermo-plastic fllm surface, which utilizes an electron-lc beam writing apparatus such as was dlsclosed ln U. S.
Patent No. 3,120,991.
Yet other systems have emplbyed 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. 3,350,503. An alternative scheme utllizlng an electron beam on a photosensitive medium such as photographic film has been taught ln the patent to R. F. Dubbe, et al, No. 3,444,317.
In recent years, alternative methods have been disclosed for hlgh density recording whlch are based upon either removal of material or vaporization of materlal by laser beam bombardment. These methods have been dlscussed brlefly in the magazine "Electronics", of March 3, 1969, at page 110. Further, a "laser thermal mlcro image recorder" was descrlbed ln some detall by C. O. Carlson and H. D. Ives in a paper glven at the 1968 WESCON meetlng (Western Electronlc Show and Con-ventlon), whlch paper was published ln Volume 12 of WESCON Technical Papers for 1968, at page 1, of Sectlon 16/1. The authors have referred to artlcles in the December 23, 1966, issue o~ "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 ln the "Bell Systems Technlcal Journal", o~ March 1968, pages 385, 405.
These publlcatlons disclose a recording technique which utilizes a thln metalllc film coating upon a substrate. The thin metal film, under applied heat, melts rapidly and forms small globules within a recorded spot. A hlghly concentrated spot of laser 35 lllumlnation can apply sufficient heat in a short enough time so that a sultably modulated laser beam impinging upon a moving surface can produce a pattern of holes ln the metalllc surface which, when "read back , can reproduce the lnformatlon recorded.
~ r .
10664~4 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 thickness;
laser divergence and spot power, an appropriate system can be designed to record video frequencies with reasonably high resolution. However, the quality of the video signal reproduced from such recordings 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. Additionally, 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 ineffective 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 prlmarily to the inability of the laser beam modulator to 0 "write" precise analog signals in the coating of the disc.
BRIEF SUMMARY OF THE INVENTION
The present invention provides an improved apparatus and method for storing information on a videodisc, the stored information having the form of a lineal series of regions representing a frequency modulated informational signal~
Specifically, the invention relates to apparatus for storing information in the form of a frequency modulated signal compising: means for providing an initial information having its informational content in the form of voltage varying with time format;
frequency modulator means, responsive to the means providing .Y ., bc/3-o an initial information signal, for converting the voltage varying with time signal to a frequency modulated signal having its informational content in the form of a carrier frequency having frequency changes in time representing the information to be recorded; a disc-shaped information storage member including a substrate having a first surface and a light responsive coating covering the first surface for retaining indicia representative of the information signal; means for imparting uniform rotational motion to the storage member~ a light source for providing a light beam, the light beam being of sufficient intensity for interacting with the coating while the coating is in motion and positioned upon the moving information storage member, and for altering the coating to retain indicia respresentative of the information; optical means for defining an optical - path between the light source and the coating on the storage member, and for focusing the light beam upon the coating; and light intensity modulating means positioned in the optical path between the light source and the coating on the storage member, the light intensity modulating means operating over a range between a higher light transmitting state and a lower light transmitting state for intensity modulating the light beam with the information to be stored; wherein the light intensity modulating means is responsive to frequency modulated signal and changes between its higher light transmitting state and its lower light transmitting state during each cycle of the frequency modulated signal for modulating the light beam with the frequency modulated signal to be stored; the light intensity modulating means including electrically controllable means responsive to the frequency modulator means for varying ~ - 3a -bc/~o ; 10664i4 the intensity of the light beam above a predetermined intensity at which the focused beam physically alters the coating and below the predetermined intensity at which the focused beam fails to physically alter the coating, to form alternating discrete regions of altered and unaltered coating being representative of the frequency modulated signal.
In its method aspect, the invention relates to a method for recording information an information storage member using a laser beam, comprising the steps of: providing an intial electrical signal having its informational content in the form of a voltage varying with time format; changing the voltage varying with time signal to a frequency modulated electrical signal having its informational content in the form of a carrier frequency having frequency changes with time corresponding to the voltage variations with time; controlling the intensity of the transmission of a laser light beam upon a light responsive surface of a disc-shaped information storage member, using the frequency modulated signal as a control signal; and rotating the information storage member at a constant rate relative to the light beam while focusing the light beam upon the light responsive surface of the information storage member; the controlling step including using the frequency modulated signal for varying the intensity of the light beam above a predetermined intensity at which the focused beam physically alters the light responsive surface and below the predetermined intensity at which the focused beam fails to physically alter the light responsive surface, the alteration being representative of the frequency modulated signal; the controlling step further including using the transmitted light beam for irreversibly altering the light responsive surface of ~ - 3b -bc/.)8 the information storage member under the control of one portion of the frequency modulated signal, as the member moves at a constant rate, and lowering the intensity of tbe transmitted light beam to the light responsive surface of the information storage member for leaving the light responsive surface unaltered under the control of a second portion of the frequency varying signal, as the member moves at a constant rate.
In accordance with the invention, a videodisc is provided which has a light responsive coating covering the disc substrate for retaining indicia representing the frequency modulated information signal. A light source provides a light beam of sufficient intensity to interact with the responsive coating of the videodisc as the disc uniformly rotates. An optical system defines an optical path between the light source and the responsive coating on the disc for focusing the light beam on the coating. A light intensity modulator is positioned in the optical path and is responsive to bc/.
the frequency modulated information signal to intensity modulate the light beam. The light intensity modulator i~ effective to transmit light over a predetermined i~tensity range, the lntensity of the modulated light 5 ~eam changlng between a relatively high light level and a relatlvely lower light level durlng each cycle of the frequency modulated signal. The llght beam passing through the intensity modulator and optically focused on the responsive coatlng forms the indicia represent-lO ing the frequency modulated information signal.
In a preferred embodiment, an initial input slgnal has its information content ln the form of a voltage varylng with tlme. Such a slgnal ls sultable ror dlsplay on a standard televlslon monitor. In such 15 an embodlment, a frequency modulator, responsive to the voltage varying with time signal converts the slgnal to a frequency modulated signal for drlving the light lntensity modulator.
The relatively higher and lower llght trans-20 mittlng states of the light lntenslty modulator result in the varylng of the lntenslty of the resultant modulated llght beam above a predetermined lntensity at whlch the focused beam alters the responslve coatlng, and below the predetermlned lntenslty at whlch the 25 ~ocused beam fails to alter the responslve coatlng.
In order to establish an optlmum average in-tenslty level for the modulated light beam, the light intensity modulator may further include feedback circuitry for stabllizing the operating level Or the 3 electrlcal control of the light modulator to produce optlmum first and second prede~ermined intensities.
The ~eedback clrcultry may include a level ad~u~tment to selectlvely ad~ust the average power level of the light beam to a predetermined value. Toward thls end, 35 a light-sensor senses at least a portion of the light beam from the optical modulator to produce an elec-trical feedback signal representatlve of the average intenslty of the light beam such that when the average ~r level Or modulated light beam increases, the feedback ~ r .
~" 1066414 ~ignal tends to lower ~he operat~ng level of the optical modulator in order to keep the average intensity of the modulated l~ght beam stabilized at a c~nstant operatlng lntenslty level.
At the heart of the optical modulator i5 a Pockels cell driven by a Pockels cell driver. The Pockels cell driver has as lts input the frequency modulated information slgnal, and the Pockels cell drlver responds to the frequency modulated signal to provide corresponding driving signals to the Pockels cell devlce. The Pockels cell drlver is A.C. coupled to the Pockels cell devlce, and the a~orementioned ~eedback circultry ls D.C. coupled to the Pockels cell device.
The light source for produclng the wrltlng llght beam ls a writing laser, preferably an Argon ion laser, whlch produces a colllmated wrlting beam of polarlzed monochromatic light. The wrltlng laser pro-duces locallzed heatlng upon lmplngement of the llght 20 responslve coatlng of the videodisc.
The videodisc has a rigid disc-shaped sub-strate wlth a surface of opaque metalllzed coatlng.
The coatlng has sultable physical properties to permit looallzed heating by the writlng laser to, ln turn, cauae localized meltlng accompanled by wlthdrawal of the molten materlal toward the perlmeter of the molten area. Upon freezlng, a permanent aperture ls left ln the metallized coating. Under proper operating condl-tions, the optlcal modulator varles the intel~lty of the wrlting laser beam from an lntensity above the afore-mentloned predetermlned intenslty, at whlch the focused peam melts the metalllzed coating without vaporlzing lt to an lntenslty below the predetermined lntenslty, at whlch the focused beam falls to melt the ~,etallized coating.
The dlsc-shaped vldeodlsc ls uniformly rota-ted, and radial tranælational motion ls glven to the vldeodlsc relatlve to the writing beam for writing the lnformation in a spiral path on the disc. Both rotatlon-._ . .
1~6~
--6--al and translational notion are under control of an electrlcal synchronizer which maintains a con~tant re-latlonship between the rotational mot-lon of the disc and the relatlve translational motion between the writ-lng laser beam and tl~e disc.
A microscope objective lens for focusing thewrltlng beam on the videodisc surface may be maintained ln a spaced relationship to the videodisc by a hydro-dynamic air bearing created as the disc rotates rela-tive to the ob~ective lens support. The ob~ective lenshas an aperture larger in diameter than the diameter Or the reading light beam, and the optlcal system makes use of mirrors and lenses for diverglng the writing light beam from the laser source to at least fill the entrance aperture of the ob~ective lens.
The rotational driver includes a spindle which rotates the disc precisely in a circle, and the radial translating driver includes a lead screw mechan-lsm for effecting relative translational motion between the disc and t,le writing beam at a very constant veloclty along a radlus of the rotating dlsc.
Synchronlzatlon of the dlsc drlve wlth the translatlng drive creates a spiral track of predeter-mined pltch. If deslred, concentrlc clrcles can be created by alternately translating and wrlting. In a preferred embodiment employlng a splral track, the ~paclng between adJacent turns of the splral is 2 ~m, center to center. Assuming a spot diameter on the order of 1 ~m, thls would produce a guard area of 1 ~m between spots in adJacent tracks.
The mlcroscope ob~ective lens "flies" at a conBtant height above the disc on an alr bearing. The constant height ls desirable because of the shallow focal depth of the ob~ective lens A 40X dry mlcro-scope ob~ective lens has been found to be satisfactoryln terms of concentrating the energy of the laser beam at the disc surface to enable the writlng of the l~m spot.
The novel features which are belleved to be ~
r-k~ 1066414 ~ --7--characteristic of the invention~ both as to organiza-tion and method of operation, together with further ob~ects and adYantages thereof, will be better under-stood from the following description considered in connection with the accomp~nying drawings in which several preferred embodiments of the invention are illustrated by way of example. It is to be expressly understood, however, that the drawings are for the purpoæe of illustration and description only and are not lntended as a definition 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;
FIG. 2 ~s a view ~ the optical path through the ob~ective lens of FIG. l;
FIG. 3 is a representational lndlcation o~
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 stabillzlng circuit.
DETAILED DESCRIPTION OF THE INVENTION
Turnillg first to FIG. 1, the writing apparatus ~ lncludes a writing head 12 wh~ch is, in the preferred l; embodlmentJ a dry microscope ob~ective lens 14 mounted upon an alr bearing support member 16. A 40X lens has been ~ound to be satlsfactory. A disc 18 is speclally prepared and may be constructed according to the teach-lngs of the prior art, in whlch a substrate has coated thereon a very thin film of a metal with a reasonably low meltlng polnt and a high surface tension.
A crystal oscillator 20 controls the drlve elements. me disc 18 is rotated by a first, rotational drlve element 22 which is coupled to a spindle 24. A
second, translatlonal drive element 26 controls the posltlon of the writing head 12.
A translating carrlage 28, whlch is driven by the translational drive element 26 through a lead screw and travelling nut, moves the writing head 12 in the radlal direction relative to the rotating disc 18. The `- ~066414 . .
carriage 28 is provided with appropriate mirrors and lenses so that t~le remainder of the optics and elec-tronlcs neces~ary to the writing device may be perman-ently mounted.
In the preferred embodiment of the present lnvention, 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 i~ of the type displayable on a TV monitor. Accordingly, lt is a voltage varying with time signal. The FM modu-lator 36 is of standard design and converts the voltage varying with time signal to a frequency modulated signal having its informational content in the form of a carrier frequency having frequency changes with time correspond-ing to said voltage variations with time.
As is known, the Pockels cell 32 responds to applled signal voltages by rotating the plane of polar-lzation of the light beam. Since a linear polarizer transmits light only ln a predetermined polarlzation plane, a polarizer, such as a Glan prism 38 in the pre-ferred embodiment, is lncluded in the writing beam path to provide a modulated writing beam 40. The modulated wrltlng beam effectively follows the output of the FM
modulator 36.
The modulated writing beam 40 emerging from the Pockels cell-Glan prlsm combination 32, 38 is ap-plied to a first mirror 42 which directs the writlng beam 40 to the translating carriage 28. The first mirror 42 transmlts a portion of the wrlting beam 40 to a Pockels cell stabllizing circuit 44 which responds to the average intensity of the writing beam to maintain the energy level of the beam.
A ~ens 46 is inserted in the path of the writing beam 40 to diverge the substantially parallel beam so that it will spread to fill the entrance aperture of the ob~ective lens 14 for optimum resolution. A
dichroic mirror 48 is incluted in the path oriented to substantially transmit all of the writing beam 40 to a second, articulated mirror 50. me articulated mirror 50 then directs the beam through the lens 14 ant is capable of shifting the point of impingement of the beam 40 on the surface of the tisc 18.
A series of holes is formet in the metal coat-ing by the writing beam. One hole is fonmed 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. Obviousl~, since the informational content in the output signal of the FM modulator 36 is in the form of frequencg changes in time about a carrier fre-quenc~, and since the "hole", 'bo hole" sequence repre-sents the stored information, and since the disc 18 is rotating at a uniform speed, the "hole'~ ~bo hole"
sequence changes to represent the stored ~ideo informa-tion by the holes being foroed closer or farther apart nd the Ize of the hole beco~es l-rger or ~aller as the ~riting beao 40 changes under the control of the PM
modulated output signal fro- the FM modulator 36.
m e ob~ecti~e lens 14 and the associated air bearing 16 effectively fly on a cushlon of air at a Jubstantially flxed distance fro~ the surface of the disc 18. Th t distance is determined b~ the geometry of the bearing 16~ the linear ~elocity of the disc lô, and the force used to load the head agalnst the disc 18.
me fixed spacing is required because the focal toler-ance of a lens capable of resol~ing a 1 ~m spot is al~o of the order of 1 ~m .
A second~ relati~ely low-power laser 52 pro-~ides a nitoring beam 54. In the preferred embodi-ment, the reàding laser 52 i9 a helium-neon de~ice which enables the reading beam 54 to be dlstinguished from ~ 1066414 the wrlting beam 40 by wavelength. A polarizing, beam splltter cube 55 transmits the reading beam 54 to a mirror 58 t~Aat directs the beam 54 through a second diverging lens 60 that spreads the reading beam 54 to Pill the entrance aperture of the ob~ective lens 14.
A quarterwave plate 62 i8 placed in the opti-cal path and, in conjunction with the plane polarizing beam splitter 56, prevents light reflected from the disc 18 ~rom re-entering the laser 52 and upsetting its mode of oscillation. me 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 wlth respect to the polarlzing beam splitter 56 and ls there~ore 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 ad~ustment so that the paths of the writing and reading beams are substantlally ldentical, except that the reading beam "spot" implnges on the disc 18 downstream from the writing beam spot as explalned in greater detail below.
A filter 66 that is opaque to the argon ion beam ls interposed in the path of light reflected from the beam splltter 56. me He.Ne reading beam 54 that i8 returned ~rom the disc sur~ace is able to pass through the ~llter 66 and through a lens 68 onto a photodetector 7o-me reflected llght of the reading beam lm-plnges upon the photodetector 70. me photodetector 70 operates ln its standard manner and generates an elec-trical cur~ent representative of the light impinging thereupon. In this case, the photodetector generates the slgnal represented by the "hole", "no hole" con-~iguratlon formed ln the coatlng. me 'hole", no hole"
configuration is representative of the output of the FM modulator 36. me output of the FM modulator 36 ls a carrier frequency having frequency changes with tlme representing the video signal to be recorded. The "hole`', "no hole" configuration is representative of a ~ 1 066~4 carrier frequency having frequency changes with time repre~enting the stored video signal The output of the photodetector 70 is an electrical signal representing the stored carrier frequency having frequency changes wlth tlme representing the stored video signal.
~ ne output of the photodetector 70 is applied to a preamplifier 72 which provides a signal of suffi-clent amplitude and signal strength for subsequent utllization. A video discriminator 74 then provldes a video output signal which can be utillzed in several ways, two of which are shown, as examples only.
me discriminator 74 is of standard design and ~unctlon. It takes the frequency modulated signal from the photodetector 70 and ohanges it to a time dependent voltage signal having its informatlonal content in the ~orm of a voltage varying with time format suitable for display in the TV monitoF 76.
In a first application, the video output i8 applied to a TV monitor 76 and an oscilioscope 78. As i~ well known, the TV monitor is responsive to a voltage varylng with time signal. The information to be dis-played on the TV monitor ls repreS~ented by a voltage . change with time.
The TV monitor 76 shows the plcture fldellty o~ the recordlng, and the oscilloscope 78 indicates the 81gna1-to-noise ratlo of the record ~nd the quallty of the cuttingJ 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 30 adequate dlscrimination on the disc between a "hole" or "black" area and "no hole" or "white" area.
As an alternative utilization, the video out-put of the discriminator 74 is applied to a comparator 80. me other input of the comparator 80 is taken from 35 the video input signal which is directed through a delay line 81. A delay that is equal the accumulated delays o~ the writing system and the time elapsed between the instant of writing of the information and the time re-quired for that incremental ar~a of the disc to reach r ~j ` 1066414 the reading polnt must be imparted to the input videG
signal.
Ide~lly, the vldeo output signal of the dis-crimlnator 74 should be identical in all respects to the vldeo input signal, after the proper delay.
As previously mentioned, the output from the discrlminator 74 is a voltage varying with time signal.
me video in signal is also a voltage varying with time slgnal. Any differences 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 information, i8 le~s critical when other information ls 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. ~lhen the errors counted exceed the standard, the writing opera-tion 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't disc for a subse-quent recording.
Well-known techniques are avallable to trans-late the writing head assembiy 12 in the radial direc-tion with respe¢t to the rotating dlsc 18. While in FIG. 1 the rotatlonal and translatlonal drives 20, 22 are lndlcated as independent, the drives are synchron-ized to enable the wrltlng assembly 12 to translate a predetermined increment for each revo}ution of the disc 18j by means of the common crystal osclllator 20.
Turnlng next to FIG. 2 there ls shown, ln somewhat exaggerated form, the sllghtly dlffering opti-¢al paths of beam 40 from the wrltln~ laser 30 and the beam 54 from readlng lase~ 52. The wrltlng beam 40 ¢oincldes with the optical axls of the mlcroscope ob~ec-tlve 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 of the ob~ective, "downstream" from where the writlng beam 40 is "cutting".
The resulting delay between reading and writing allows ...
~ 10664~4 -13~
the molten metal to solidity so that the recording is read in its final ~tate. If lt were read too soon while the metal was still molten, it would not provide per-tinent lnformation for ad~usting the recording parameters.
This i~ best indicated in FIG. 3 where two points in the ~ame information channel are shown as displaced. The point A, which is the polnt of implnge-ment o~ the wrltlng beam 40, ls shown as belng on the optical axls of the ob~ective lens 14. Separated from point A, in the dlrectlon of medium motion, as lndicated by the arrow~ is the reading point B, whlch is at an angle ~ from the axis of the mlcroscope ob~ectlve lens 14. A dlstance between polnts A and B of two ~m has pr~
vlded a satisfactory monltorlng of the wrltlng operation.
Turning finally to FIG. 4, there is shown an idealized diagram o~ a Pockels cell stabillzlng clrcult 44, sultable for use in-the apparatus of FIG. 1. As 18 known, a Pockels cell rotates the plane of polariza-tlon of the applied light as a function of an applied voltage. There~ore, the Pockels cell i8 used to rotate plane polarlzed llght, and the rotated llght ls passed through a plane polarlzer, such as a Glan prism. me - llght issuing from the polarizer will be amplitude modulated in accordance with the applied voltage.
Stated another wsy~ the standard operatlng mode Or a Pockels cell 32 and Glan prism 38 is for use as a llght lntenslty modulating means. Each cycle ~rom the FM modulator drlves the Pockels cell through lts ~ull operating range of nlnety degrees. Wlthln thls operatlng range of nlnety degrees, one operating polnt passes al~ llght applled thereto and ldentlfled as a full light transmlttlng state. A second operatlng polnt passes no llght and 18 ldentlfled as a full llght block-lng state. The Pockels cell itself only rotates the plane of polarlzation. me Glan prism passes light in one plane of polarization and no light in the plane dls-placed nlnety degrees from that plane ln whlch all the llght passes.
Dependlng upon the indivldual Pockels cell, a i f ~` 10ti6~14 ~.......................... .
voltage change of approximately lOO volts wlll cause the cell to rotate the plane of polarization through 360 degrees. However, the transfer characteristic of an indivldual cell may drift spontaneously. correspond-ing to a voltage change of ~ 50 volts, and accordingly,a ~eedback loop i5 deslrable to maintain the cell within a use~ul, reasonably linear, operating range.
The stabilizing circuit 44 includes a photo-Bensltlve silicon diode 82, which iB positioned to recelve a portion of the writlng beam 40 reflected from the mlrror 42 of FIG. 1. The silicon diode 82 functions ln much the same fashion as a solar cell and is a source Or electrlcal energy when illuminated by incident radla-tlon. One terminal Or the silicon diode 82 is connected to common re~erence potential 84, indicated by the con-ventlonal ground symbol, and the oth~r terminal is con-nected to one input of a dlfferential ampllfier 86.
me sllicon cell 82 is shunted by a load 88 which enables a linear response mode.
me other input to the differential ampllfier 86 19 connected through an approprlate potentiomater 90 to the common reference 84. A source of power 92 is coupled to the potentlometer 90, whlch enables the oettlng o~ the dl~erential ampiifler 86 to establlsh the average light level transmltted by the Pockels cell 32.
Accordingly, a pair of output termlnals of the di~ferentlal ampllfier 86 are respectively connected through resistlve elements 94, 96 to the input terminals 3 o~ 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, whlle the dlfferentlal ampllfler 86 18 d.c. coupled t~ the Pockels cell 32.
In operation, the system is energized. The light from the writing beam implnging on the silicon diode 82 generates a dlfferential voltage at the lnput to the differentlal ampllfier 86. Inltiallyj the potentlometer 90 is ad~usted to produce llght at a pre-determlned average level of lnte-nslty. Thereafter, lf r-i ~` ~0664~4 the average level of intensity impinging on the silicon cell 82 either increases or decreases, a correctlng voltage will be generated ln the differential amplifier 86. m e correcting voltage applied to the Pockels cell 32 is of a polarity and magnitude adequate to restore the averaKe level Or intensity to the predetermined level.
m us there has been shown an improved vldeo dlsc recording assembly. A microscope ob~ective lens mounted on an air bearing "flies" at a predetermined distance from the surface of a metallized dlsc. The metallized coating is such that a laser beam can, under suitable modulation, dellver sufflcient energy to melt localized areas of the surface. Under surface tension, the molten metal retracts leaving a clear area of approx-lmately one micron in diameter.
A second, low-energy laser utilizing substan-tially the same optlcal path is directed through the samd microscope objective lens, but is brought to the surface of the disc at a slight dlstance "downstream"
from the point of writing. me reading beam 1~ returned through an approprlate optical system that excludes the reflected energy of the writing beam and enables an analysis of the information that has been written on the disc.
me playback information can, among other things, control the intensity of the wrlting beam to a~sure adequate "recordlng levels", determlne whether an unacceptable number of errors have been made in the recordlng process.
INFOR~ATION ON A VIDEODISC
- TECHNICAL FIELD
The present inventlon relates to the storage Or lnrormation upon an information storage member, and more particularly to the writing of vldeo lnformation on a vldeodlsc.
BACKG ~lND OF ~XE PRIOR ART
Systems have heretofore been developed ~or 10 recordlng signals at video frequencles upon dl~cs, tapes, or other.media. Such system3 have utlllzed, .... among other things, optical recording upon photosensl-- tlve media, electron beam recordlng on thermo-plastlc ~urraces, and stlll other system~ prov~de an lnstan-taneously repro~.'ucible r~cord Or vid~o inrormatlon.
The prlor art can generally be dlvided into .. systems utlllzlng photographic sur~aces, systems utll-lzlng electron beam sensitive surraces, magnetic . .recording systems, and æs ln the present inventlon, 20 .systems in whlch a radlant energy beam ¢auses an lrre-.versible.çharge to a surrace, thereby "wrltlng" inrorma-... . .
tion thereon.
Photographic systems have been described ln 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, whlch teaches the photogr~.phic recordatlon Or video in~ormatlon on a rotatlng dlsc in a splral path.
_/_ , ~ ..
;6414 The patent to W. C. Hughes, et al, ~o. 3,283,310 is lllustrative Or recordation of lnformation on a thermo-plastic fllm surface, which utilizes an electron-lc beam writing apparatus such as was dlsclosed ln U. S.
Patent No. 3,120,991.
Yet other systems have emplbyed 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. 3,350,503. An alternative scheme utllizlng an electron beam on a photosensitive medium such as photographic film has been taught ln the patent to R. F. Dubbe, et al, No. 3,444,317.
In recent years, alternative methods have been disclosed for hlgh density recording whlch are based upon either removal of material or vaporization of materlal by laser beam bombardment. These methods have been dlscussed brlefly in the magazine "Electronics", of March 3, 1969, at page 110. Further, a "laser thermal mlcro image recorder" was descrlbed ln some detall by C. O. Carlson and H. D. Ives in a paper glven at the 1968 WESCON meetlng (Western Electronlc Show and Con-ventlon), whlch paper was published ln Volume 12 of WESCON Technical Papers for 1968, at page 1, of Sectlon 16/1. The authors have referred to artlcles in the December 23, 1966, issue o~ "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 ln the "Bell Systems Technlcal Journal", o~ March 1968, pages 385, 405.
These publlcatlons disclose a recording technique which utilizes a thln metalllc film coating upon a substrate. The thin metal film, under applied heat, melts rapidly and forms small globules within a recorded spot. A hlghly concentrated spot of laser 35 lllumlnation can apply sufficient heat in a short enough time so that a sultably modulated laser beam impinging upon a moving surface can produce a pattern of holes ln the metalllc surface which, when "read back , can reproduce the lnformatlon recorded.
~ r .
10664~4 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 thickness;
laser divergence and spot power, an appropriate system can be designed to record video frequencies with reasonably high resolution. However, the quality of the video signal reproduced from such recordings 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. Additionally, 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 ineffective 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 prlmarily to the inability of the laser beam modulator to 0 "write" precise analog signals in the coating of the disc.
BRIEF SUMMARY OF THE INVENTION
The present invention provides an improved apparatus and method for storing information on a videodisc, the stored information having the form of a lineal series of regions representing a frequency modulated informational signal~
Specifically, the invention relates to apparatus for storing information in the form of a frequency modulated signal compising: means for providing an initial information having its informational content in the form of voltage varying with time format;
frequency modulator means, responsive to the means providing .Y ., bc/3-o an initial information signal, for converting the voltage varying with time signal to a frequency modulated signal having its informational content in the form of a carrier frequency having frequency changes in time representing the information to be recorded; a disc-shaped information storage member including a substrate having a first surface and a light responsive coating covering the first surface for retaining indicia representative of the information signal; means for imparting uniform rotational motion to the storage member~ a light source for providing a light beam, the light beam being of sufficient intensity for interacting with the coating while the coating is in motion and positioned upon the moving information storage member, and for altering the coating to retain indicia respresentative of the information; optical means for defining an optical - path between the light source and the coating on the storage member, and for focusing the light beam upon the coating; and light intensity modulating means positioned in the optical path between the light source and the coating on the storage member, the light intensity modulating means operating over a range between a higher light transmitting state and a lower light transmitting state for intensity modulating the light beam with the information to be stored; wherein the light intensity modulating means is responsive to frequency modulated signal and changes between its higher light transmitting state and its lower light transmitting state during each cycle of the frequency modulated signal for modulating the light beam with the frequency modulated signal to be stored; the light intensity modulating means including electrically controllable means responsive to the frequency modulator means for varying ~ - 3a -bc/~o ; 10664i4 the intensity of the light beam above a predetermined intensity at which the focused beam physically alters the coating and below the predetermined intensity at which the focused beam fails to physically alter the coating, to form alternating discrete regions of altered and unaltered coating being representative of the frequency modulated signal.
In its method aspect, the invention relates to a method for recording information an information storage member using a laser beam, comprising the steps of: providing an intial electrical signal having its informational content in the form of a voltage varying with time format; changing the voltage varying with time signal to a frequency modulated electrical signal having its informational content in the form of a carrier frequency having frequency changes with time corresponding to the voltage variations with time; controlling the intensity of the transmission of a laser light beam upon a light responsive surface of a disc-shaped information storage member, using the frequency modulated signal as a control signal; and rotating the information storage member at a constant rate relative to the light beam while focusing the light beam upon the light responsive surface of the information storage member; the controlling step including using the frequency modulated signal for varying the intensity of the light beam above a predetermined intensity at which the focused beam physically alters the light responsive surface and below the predetermined intensity at which the focused beam fails to physically alter the light responsive surface, the alteration being representative of the frequency modulated signal; the controlling step further including using the transmitted light beam for irreversibly altering the light responsive surface of ~ - 3b -bc/.)8 the information storage member under the control of one portion of the frequency modulated signal, as the member moves at a constant rate, and lowering the intensity of tbe transmitted light beam to the light responsive surface of the information storage member for leaving the light responsive surface unaltered under the control of a second portion of the frequency varying signal, as the member moves at a constant rate.
In accordance with the invention, a videodisc is provided which has a light responsive coating covering the disc substrate for retaining indicia representing the frequency modulated information signal. A light source provides a light beam of sufficient intensity to interact with the responsive coating of the videodisc as the disc uniformly rotates. An optical system defines an optical path between the light source and the responsive coating on the disc for focusing the light beam on the coating. A light intensity modulator is positioned in the optical path and is responsive to bc/.
the frequency modulated information signal to intensity modulate the light beam. The light intensity modulator i~ effective to transmit light over a predetermined i~tensity range, the lntensity of the modulated light 5 ~eam changlng between a relatively high light level and a relatlvely lower light level durlng each cycle of the frequency modulated signal. The llght beam passing through the intensity modulator and optically focused on the responsive coatlng forms the indicia represent-lO ing the frequency modulated information signal.
In a preferred embodiment, an initial input slgnal has its information content ln the form of a voltage varylng with tlme. Such a slgnal ls sultable ror dlsplay on a standard televlslon monitor. In such 15 an embodlment, a frequency modulator, responsive to the voltage varying with time signal converts the slgnal to a frequency modulated signal for drlving the light lntensity modulator.
The relatively higher and lower llght trans-20 mittlng states of the light lntenslty modulator result in the varylng of the lntenslty of the resultant modulated llght beam above a predetermined lntensity at whlch the focused beam alters the responslve coatlng, and below the predetermlned lntenslty at whlch the 25 ~ocused beam fails to alter the responslve coatlng.
In order to establish an optlmum average in-tenslty level for the modulated light beam, the light intensity modulator may further include feedback circuitry for stabllizing the operating level Or the 3 electrlcal control of the light modulator to produce optlmum first and second prede~ermined intensities.
The ~eedback clrcultry may include a level ad~u~tment to selectlvely ad~ust the average power level of the light beam to a predetermined value. Toward thls end, 35 a light-sensor senses at least a portion of the light beam from the optical modulator to produce an elec-trical feedback signal representatlve of the average intenslty of the light beam such that when the average ~r level Or modulated light beam increases, the feedback ~ r .
~" 1066414 ~ignal tends to lower ~he operat~ng level of the optical modulator in order to keep the average intensity of the modulated l~ght beam stabilized at a c~nstant operatlng lntenslty level.
At the heart of the optical modulator i5 a Pockels cell driven by a Pockels cell driver. The Pockels cell driver has as lts input the frequency modulated information slgnal, and the Pockels cell drlver responds to the frequency modulated signal to provide corresponding driving signals to the Pockels cell devlce. The Pockels cell drlver is A.C. coupled to the Pockels cell devlce, and the a~orementioned ~eedback circultry ls D.C. coupled to the Pockels cell device.
The light source for produclng the wrltlng llght beam ls a writing laser, preferably an Argon ion laser, whlch produces a colllmated wrlting beam of polarlzed monochromatic light. The wrltlng laser pro-duces locallzed heatlng upon lmplngement of the llght 20 responslve coatlng of the videodisc.
The videodisc has a rigid disc-shaped sub-strate wlth a surface of opaque metalllzed coatlng.
The coatlng has sultable physical properties to permit looallzed heating by the writlng laser to, ln turn, cauae localized meltlng accompanled by wlthdrawal of the molten materlal toward the perlmeter of the molten area. Upon freezlng, a permanent aperture ls left ln the metallized coating. Under proper operating condl-tions, the optlcal modulator varles the intel~lty of the wrlting laser beam from an lntensity above the afore-mentloned predetermlned intenslty, at whlch the focused peam melts the metalllzed coating without vaporlzing lt to an lntenslty below the predetermined lntenslty, at whlch the focused beam falls to melt the ~,etallized coating.
The dlsc-shaped vldeodlsc ls uniformly rota-ted, and radial tranælational motion ls glven to the vldeodlsc relatlve to the writing beam for writing the lnformation in a spiral path on the disc. Both rotatlon-._ . .
1~6~
--6--al and translational notion are under control of an electrlcal synchronizer which maintains a con~tant re-latlonship between the rotational mot-lon of the disc and the relatlve translational motion between the writ-lng laser beam and tl~e disc.
A microscope objective lens for focusing thewrltlng beam on the videodisc surface may be maintained ln a spaced relationship to the videodisc by a hydro-dynamic air bearing created as the disc rotates rela-tive to the ob~ective lens support. The ob~ective lenshas an aperture larger in diameter than the diameter Or the reading light beam, and the optlcal system makes use of mirrors and lenses for diverglng the writing light beam from the laser source to at least fill the entrance aperture of the ob~ective lens.
The rotational driver includes a spindle which rotates the disc precisely in a circle, and the radial translating driver includes a lead screw mechan-lsm for effecting relative translational motion between the disc and t,le writing beam at a very constant veloclty along a radlus of the rotating dlsc.
Synchronlzatlon of the dlsc drlve wlth the translatlng drive creates a spiral track of predeter-mined pltch. If deslred, concentrlc clrcles can be created by alternately translating and wrlting. In a preferred embodiment employlng a splral track, the ~paclng between adJacent turns of the splral is 2 ~m, center to center. Assuming a spot diameter on the order of 1 ~m, thls would produce a guard area of 1 ~m between spots in adJacent tracks.
The mlcroscope ob~ective lens "flies" at a conBtant height above the disc on an alr bearing. The constant height ls desirable because of the shallow focal depth of the ob~ective lens A 40X dry mlcro-scope ob~ective lens has been found to be satisfactoryln terms of concentrating the energy of the laser beam at the disc surface to enable the writlng of the l~m spot.
The novel features which are belleved to be ~
r-k~ 1066414 ~ --7--characteristic of the invention~ both as to organiza-tion and method of operation, together with further ob~ects and adYantages thereof, will be better under-stood from the following description considered in connection with the accomp~nying drawings in which several preferred embodiments of the invention are illustrated by way of example. It is to be expressly understood, however, that the drawings are for the purpoæe of illustration and description only and are not lntended as a definition 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;
FIG. 2 ~s a view ~ the optical path through the ob~ective lens of FIG. l;
FIG. 3 is a representational lndlcation o~
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 stabillzlng circuit.
DETAILED DESCRIPTION OF THE INVENTION
Turnillg first to FIG. 1, the writing apparatus ~ lncludes a writing head 12 wh~ch is, in the preferred l; embodlmentJ a dry microscope ob~ective lens 14 mounted upon an alr bearing support member 16. A 40X lens has been ~ound to be satlsfactory. A disc 18 is speclally prepared and may be constructed according to the teach-lngs of the prior art, in whlch a substrate has coated thereon a very thin film of a metal with a reasonably low meltlng polnt and a high surface tension.
A crystal oscillator 20 controls the drlve elements. me disc 18 is rotated by a first, rotational drlve element 22 which is coupled to a spindle 24. A
second, translatlonal drive element 26 controls the posltlon of the writing head 12.
A translating carrlage 28, whlch is driven by the translational drive element 26 through a lead screw and travelling nut, moves the writing head 12 in the radlal direction relative to the rotating disc 18. The `- ~066414 . .
carriage 28 is provided with appropriate mirrors and lenses so that t~le remainder of the optics and elec-tronlcs neces~ary to the writing device may be perman-ently mounted.
In the preferred embodiment of the present lnvention, 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 i~ of the type displayable on a TV monitor. Accordingly, lt is a voltage varying with time signal. The FM modu-lator 36 is of standard design and converts the voltage varying with time signal to a frequency modulated signal having its informational content in the form of a carrier frequency having frequency changes with time correspond-ing to said voltage variations with time.
As is known, the Pockels cell 32 responds to applled signal voltages by rotating the plane of polar-lzation of the light beam. Since a linear polarizer transmits light only ln a predetermined polarlzation plane, a polarizer, such as a Glan prism 38 in the pre-ferred embodiment, is lncluded in the writing beam path to provide a modulated writing beam 40. The modulated wrltlng beam effectively follows the output of the FM
modulator 36.
The modulated writing beam 40 emerging from the Pockels cell-Glan prlsm combination 32, 38 is ap-plied to a first mirror 42 which directs the writlng beam 40 to the translating carriage 28. The first mirror 42 transmlts a portion of the wrlting beam 40 to a Pockels cell stabllizing circuit 44 which responds to the average intensity of the writing beam to maintain the energy level of the beam.
A ~ens 46 is inserted in the path of the writing beam 40 to diverge the substantially parallel beam so that it will spread to fill the entrance aperture of the ob~ective lens 14 for optimum resolution. A
dichroic mirror 48 is incluted in the path oriented to substantially transmit all of the writing beam 40 to a second, articulated mirror 50. me articulated mirror 50 then directs the beam through the lens 14 ant is capable of shifting the point of impingement of the beam 40 on the surface of the tisc 18.
A series of holes is formet in the metal coat-ing by the writing beam. One hole is fonmed 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. Obviousl~, since the informational content in the output signal of the FM modulator 36 is in the form of frequencg changes in time about a carrier fre-quenc~, and since the "hole", 'bo hole" sequence repre-sents the stored information, and since the disc 18 is rotating at a uniform speed, the "hole'~ ~bo hole"
sequence changes to represent the stored ~ideo informa-tion by the holes being foroed closer or farther apart nd the Ize of the hole beco~es l-rger or ~aller as the ~riting beao 40 changes under the control of the PM
modulated output signal fro- the FM modulator 36.
m e ob~ecti~e lens 14 and the associated air bearing 16 effectively fly on a cushlon of air at a Jubstantially flxed distance fro~ the surface of the disc 18. Th t distance is determined b~ the geometry of the bearing 16~ the linear ~elocity of the disc lô, and the force used to load the head agalnst the disc 18.
me fixed spacing is required because the focal toler-ance of a lens capable of resol~ing a 1 ~m spot is al~o of the order of 1 ~m .
A second~ relati~ely low-power laser 52 pro-~ides a nitoring beam 54. In the preferred embodi-ment, the reàding laser 52 i9 a helium-neon de~ice which enables the reading beam 54 to be dlstinguished from ~ 1066414 the wrlting beam 40 by wavelength. A polarizing, beam splltter cube 55 transmits the reading beam 54 to a mirror 58 t~Aat directs the beam 54 through a second diverging lens 60 that spreads the reading beam 54 to Pill the entrance aperture of the ob~ective lens 14.
A quarterwave plate 62 i8 placed in the opti-cal path and, in conjunction with the plane polarizing beam splitter 56, prevents light reflected from the disc 18 ~rom re-entering the laser 52 and upsetting its mode of oscillation. me 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 wlth respect to the polarlzing beam splitter 56 and ls there~ore 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 ad~ustment so that the paths of the writing and reading beams are substantlally ldentical, except that the reading beam "spot" implnges on the disc 18 downstream from the writing beam spot as explalned in greater detail below.
A filter 66 that is opaque to the argon ion beam ls interposed in the path of light reflected from the beam splltter 56. me He.Ne reading beam 54 that i8 returned ~rom the disc sur~ace is able to pass through the ~llter 66 and through a lens 68 onto a photodetector 7o-me reflected llght of the reading beam lm-plnges upon the photodetector 70. me photodetector 70 operates ln its standard manner and generates an elec-trical cur~ent representative of the light impinging thereupon. In this case, the photodetector generates the slgnal represented by the "hole", "no hole" con-~iguratlon formed ln the coatlng. me 'hole", no hole"
configuration is representative of the output of the FM modulator 36. me output of the FM modulator 36 ls a carrier frequency having frequency changes with tlme representing the video signal to be recorded. The "hole`', "no hole" configuration is representative of a ~ 1 066~4 carrier frequency having frequency changes with time repre~enting the stored video signal The output of the photodetector 70 is an electrical signal representing the stored carrier frequency having frequency changes wlth tlme representing the stored video signal.
~ ne output of the photodetector 70 is applied to a preamplifier 72 which provides a signal of suffi-clent amplitude and signal strength for subsequent utllization. A video discriminator 74 then provldes a video output signal which can be utillzed in several ways, two of which are shown, as examples only.
me discriminator 74 is of standard design and ~unctlon. It takes the frequency modulated signal from the photodetector 70 and ohanges it to a time dependent voltage signal having its informatlonal content in the ~orm of a voltage varying with time format suitable for display in the TV monitoF 76.
In a first application, the video output i8 applied to a TV monitor 76 and an oscilioscope 78. As i~ well known, the TV monitor is responsive to a voltage varylng with time signal. The information to be dis-played on the TV monitor ls repreS~ented by a voltage . change with time.
The TV monitor 76 shows the plcture fldellty o~ the recordlng, and the oscilloscope 78 indicates the 81gna1-to-noise ratlo of the record ~nd the quallty of the cuttingJ 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 30 adequate dlscrimination on the disc between a "hole" or "black" area and "no hole" or "white" area.
As an alternative utilization, the video out-put of the discriminator 74 is applied to a comparator 80. me other input of the comparator 80 is taken from 35 the video input signal which is directed through a delay line 81. A delay that is equal the accumulated delays o~ the writing system and the time elapsed between the instant of writing of the information and the time re-quired for that incremental ar~a of the disc to reach r ~j ` 1066414 the reading polnt must be imparted to the input videG
signal.
Ide~lly, the vldeo output signal of the dis-crimlnator 74 should be identical in all respects to the vldeo input signal, after the proper delay.
As previously mentioned, the output from the discrlminator 74 is a voltage varying with time signal.
me video in signal is also a voltage varying with time slgnal. Any differences 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 information, i8 le~s critical when other information ls 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. ~lhen the errors counted exceed the standard, the writing opera-tion 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't disc for a subse-quent recording.
Well-known techniques are avallable to trans-late the writing head assembiy 12 in the radial direc-tion with respe¢t to the rotating dlsc 18. While in FIG. 1 the rotatlonal and translatlonal drives 20, 22 are lndlcated as independent, the drives are synchron-ized to enable the wrltlng assembly 12 to translate a predetermined increment for each revo}ution of the disc 18j by means of the common crystal osclllator 20.
Turnlng next to FIG. 2 there ls shown, ln somewhat exaggerated form, the sllghtly dlffering opti-¢al paths of beam 40 from the wrltln~ laser 30 and the beam 54 from readlng lase~ 52. The wrltlng beam 40 ¢oincldes with the optical axls of the mlcroscope ob~ec-tlve 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 of the ob~ective, "downstream" from where the writlng beam 40 is "cutting".
The resulting delay between reading and writing allows ...
~ 10664~4 -13~
the molten metal to solidity so that the recording is read in its final ~tate. If lt were read too soon while the metal was still molten, it would not provide per-tinent lnformation for ad~usting the recording parameters.
This i~ best indicated in FIG. 3 where two points in the ~ame information channel are shown as displaced. The point A, which is the polnt of implnge-ment o~ the wrltlng beam 40, ls shown as belng on the optical axls of the ob~ective lens 14. Separated from point A, in the dlrectlon of medium motion, as lndicated by the arrow~ is the reading point B, whlch is at an angle ~ from the axis of the mlcroscope ob~ectlve lens 14. A dlstance between polnts A and B of two ~m has pr~
vlded a satisfactory monltorlng of the wrltlng operation.
Turning finally to FIG. 4, there is shown an idealized diagram o~ a Pockels cell stabillzlng clrcult 44, sultable for use in-the apparatus of FIG. 1. As 18 known, a Pockels cell rotates the plane of polariza-tlon of the applied light as a function of an applied voltage. There~ore, the Pockels cell i8 used to rotate plane polarlzed llght, and the rotated llght ls passed through a plane polarlzer, such as a Glan prism. me - llght issuing from the polarizer will be amplitude modulated in accordance with the applied voltage.
Stated another wsy~ the standard operatlng mode Or a Pockels cell 32 and Glan prism 38 is for use as a llght lntenslty modulating means. Each cycle ~rom the FM modulator drlves the Pockels cell through lts ~ull operating range of nlnety degrees. Wlthln thls operatlng range of nlnety degrees, one operating polnt passes al~ llght applled thereto and ldentlfled as a full light transmlttlng state. A second operatlng polnt passes no llght and 18 ldentlfled as a full llght block-lng state. The Pockels cell itself only rotates the plane of polarlzation. me Glan prism passes light in one plane of polarization and no light in the plane dls-placed nlnety degrees from that plane ln whlch all the llght passes.
Dependlng upon the indivldual Pockels cell, a i f ~` 10ti6~14 ~.......................... .
voltage change of approximately lOO volts wlll cause the cell to rotate the plane of polarization through 360 degrees. However, the transfer characteristic of an indivldual cell may drift spontaneously. correspond-ing to a voltage change of ~ 50 volts, and accordingly,a ~eedback loop i5 deslrable to maintain the cell within a use~ul, reasonably linear, operating range.
The stabilizing circuit 44 includes a photo-Bensltlve silicon diode 82, which iB positioned to recelve a portion of the writlng beam 40 reflected from the mlrror 42 of FIG. 1. The silicon diode 82 functions ln much the same fashion as a solar cell and is a source Or electrlcal energy when illuminated by incident radla-tlon. One terminal Or the silicon diode 82 is connected to common re~erence potential 84, indicated by the con-ventlonal ground symbol, and the oth~r terminal is con-nected to one input of a dlfferential ampllfier 86.
me sllicon cell 82 is shunted by a load 88 which enables a linear response mode.
me other input to the differential ampllfier 86 19 connected through an approprlate potentiomater 90 to the common reference 84. A source of power 92 is coupled to the potentlometer 90, whlch enables the oettlng o~ the dl~erential ampiifler 86 to establlsh the average light level transmltted by the Pockels cell 32.
Accordingly, a pair of output termlnals of the di~ferentlal ampllfier 86 are respectively connected through resistlve elements 94, 96 to the input terminals 3 o~ 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, whlle the dlfferentlal ampllfler 86 18 d.c. coupled t~ the Pockels cell 32.
In operation, the system is energized. The light from the writing beam implnging on the silicon diode 82 generates a dlfferential voltage at the lnput to the differentlal ampllfier 86. Inltiallyj the potentlometer 90 is ad~usted to produce llght at a pre-determlned average level of lnte-nslty. Thereafter, lf r-i ~` ~0664~4 the average level of intensity impinging on the silicon cell 82 either increases or decreases, a correctlng voltage will be generated ln the differential amplifier 86. m e correcting voltage applied to the Pockels cell 32 is of a polarity and magnitude adequate to restore the averaKe level Or intensity to the predetermined level.
m us there has been shown an improved vldeo dlsc recording assembly. A microscope ob~ective lens mounted on an air bearing "flies" at a predetermined distance from the surface of a metallized dlsc. The metallized coating is such that a laser beam can, under suitable modulation, dellver sufflcient energy to melt localized areas of the surface. Under surface tension, the molten metal retracts leaving a clear area of approx-lmately one micron in diameter.
A second, low-energy laser utilizing substan-tially the same optlcal path is directed through the samd microscope objective lens, but is brought to the surface of the disc at a slight dlstance "downstream"
from the point of writing. me reading beam 1~ returned through an approprlate optical system that excludes the reflected energy of the writing beam and enables an analysis of the information that has been written on the disc.
me playback information can, among other things, control the intensity of the wrlting beam to a~sure adequate "recordlng levels", determlne whether an unacceptable number of errors have been made in the recordlng process.
Claims (19)
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
1. Apparatus for storing information in the form of a frequency modulated signal comprising: means for providing an initial information signal having its informational content in the form of a voltage varying with time format; frequency modu-lator means, responsive to said means providing an initial infor-mation signal, for converting said voltage varying with time signal to a frequency modulated signal having its informational content in the form of a carrier frequency having frequency changes in time representing said information to be recorded; a disc-shaped information storage member including a substrate having a first surface and a light responsive coating covering said first surface for retaining indicia representative of said information signal; means for imparting uniform rotational motion to said storage member; a light source for providing a light beam, said light beam being of sufficient intensity for interacting with said coating while said coating is in motion and positioned upon said moving information storage member, and for altering said coating to retain indicia representative of said information;
optical means for defining an optical path between said light source and said coating on said storage member, and for focusing said light beam upon said coating; and light intensity modulating means positioned in said optical path between said light source and said coating on said storage member, said light intensity modulating means operating over a range between a higher light transmitting state and a lower light transmitting state for intensity modulating said light beam with said information to be stored; wherein said light intensity modulating means is responsive to said frequency modulated signal and changes between its higher light transmitting state and its lower light transmitting state during each cycle of said frequency modulated signal for modu-lating said light beam with the frequency modulated signal to be stored; said light intensity modulating means including electrically controllable means responsive to said frequency modulator means for varying the intensity of said light beam above a predetermined intensity at which the focused beam physically alters said coating and below said predetermined intensity at which the focused beam fails to physically alter said coating, to form alternating discrete regions of altered and unaltered coating being represen-tative of said frequency modulated signal.
optical means for defining an optical path between said light source and said coating on said storage member, and for focusing said light beam upon said coating; and light intensity modulating means positioned in said optical path between said light source and said coating on said storage member, said light intensity modulating means operating over a range between a higher light transmitting state and a lower light transmitting state for intensity modulating said light beam with said information to be stored; wherein said light intensity modulating means is responsive to said frequency modulated signal and changes between its higher light transmitting state and its lower light transmitting state during each cycle of said frequency modulated signal for modu-lating said light beam with the frequency modulated signal to be stored; said light intensity modulating means including electrically controllable means responsive to said frequency modulator means for varying the intensity of said light beam above a predetermined intensity at which the focused beam physically alters said coating and below said predetermined intensity at which the focused beam fails to physically alter said coating, to form alternating discrete regions of altered and unaltered coating being represen-tative of said frequency modulated signal.
2. The apparatus as claimed in Claim 1, wherein: said initial information signal has its informational content in the form of a voltage varying with time signal suitable for display on a standard television monitor; said light source comprises a writing laser for producing a collimated writing beam of polarized monochromatic light; said substrate defines a smooth flat rigid disc, said first surface being a planar surface; said coating is a thin opaque metallized coating having suitable physical properties to permit localized heating responsive to the impingement of light from said writing laser, said heating causing localized melting accompanied by withdrawal of the molten material toward the perimeter of the melted area, leaving upon freezing a permanent aperture in the thin metallized coating; and said electrically controllable means is responsive to said frequency modulator means for varying the intensity of said writing beam above said predetermined intensity at which the focused beam melts said metallized coating without varporizing It and below said predetermined intensity at which the focused beam fails to melt said metallized coating.
3. The apparatus as claimed in Claim 1, wherein: said storage member is disc-shaped; said means for imparting 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 focused light beam radially across said first surface of said disc-shaped storage member; and electrical synchronizing means for maintaining a constant relationship between said rotational motion and said translational motion.
translational drive means synchronized with said rotational drive means for relatively moving said focused light beam radially across said first surface of said disc-shaped storage member; and electrical synchronizing means for maintaining a constant relationship between said rotational motion and said translational motion.
4. The apparatus as claimed in Claim 1, 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 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 light beam issuing from said electrically controllable means to produce an electrical feedback signal representative of the intensity of the beam issuing from said electrically controllable means and applying the feedback signal to said electrically controllable means to stabilize its operating level.
5. The apparatus as claimed in Claim 4, wherein: said light-sensing means produces an electrical feedback signal which is representative of the average intensity of the light beam issuing from said electrically controllable means, the operating level of said light intensity modulating means being stabilized to issue the light beam at a substantially constant average power level.
6. The apparatus as claimed in Claim 2, wherein said optical means includes: an objective lens;
and hydrodynamic air bearing means for supporting said lens above said first surface of said information storage member.
and hydrodynamic air bearing means for supporting said lens above said first surface of said information storage member.
7. The apparatus as claimed in Claim 6, wherein: said collimated beam of light has substantially parallel light rays; said objective lens has an entrance aperture larger in diameter than the diameter of said light beam as provided by said light source; and said optical means further includes mirror means for folding said light beam path provided by said light source, and a diverging lens for spreading the substantially parallel light beam from said light source to at least fill said entrance aperture of said objective lens.
8. The apparatus as claimed in Claim 1, wherein said light source produces a polarized laser beam, and said electrically controllable means includes:
means for rotating the plane of polarization of said laser beam from said source under control of said frequency modulated signal; and a linear polarizer the output of which is a modulated laser beam corresponding to said frequency modulated signal.
means for rotating the plane of polarization of said laser beam from said source under control of said frequency modulated signal; and a linear polarizer the output of which is a modulated laser beam corresponding to said frequency modulated signal.
9. The apparatus as claimed in Claim 5, wherein said feedback apparatus includes level adjustment means for selectively adjusting the average power level of said light beam to a predetermined value.
10. The apparatus as claimed in Claim 5, wherein: said electrically controllable means comprises A Pockels cell driver and a Pockels cell device, said Pockels cell driver responding to said frequency modulated signal to provide corresponding driving signals to said Pockels cell device; and wherein said Pockels cell driver is A.C. coupled to said Pockels cell, and said stabilizing feedback apparatus is D.C.
coupled to said Pockels cell.
coupled to said Pockels cell.
11. A method for recording information on an information storage member using a laser beam, comprising the steps of: providing an initial electrical signal having Its informational content in the form of a voltage varying with time format; changing said voltage varying with time signal to a frequency modulated electrical signal having its informational content in the form of a carrier frequency having fre-quency changes with time corresponding to said voltage variations with time; controlling the intensity of the transmission of a laser light beam upon a light responsive surface of a disc-shaped information storage member, using said frequency modulated signal as a control signal; and rotating the information storage member at a constant rate relative to said light beam while focusing said light beam upon said light responsive surface of said information storage member; said controlling step including using said frequency modulated signal for varying the intensity of said light beam above a predetermined intensity at which the focused beam physically alters said light responsive surface and below said predetermined intensity at which the focused beam falls to physically alter said light responsive surface, said alteration being representative of said frequency modulated signal; said control-ling step further including using said transmitted light beam for irreversibly altering said light responsive surface of said information storage member under the control of one portion of said frequency modulated signal, as said member moves at a constant rate, and lowering the intensity of the transmitted light beam to said light respnsive surface of said information storage member for leaving said light responsive surface unaltered under the control of a second portion of said frequency varying signal, as said member moves at a constant rate.
12. The method as claimed in Claim 11, wherein said light beam is held stationary and said storage member is moved at said constant rate relative to the stationary beam.
13. The method as claimed in Claim 11, wherein: said initial information signal has its informational content in the form of a voltage varying with time signal suitable for display on a standard television monitor; said controlling step includes producing a modulated collimated writing laser beam of polarized monochromatic light for impinging upon said light responsive surface of the storage member, said surface being a thin planar opaque metallized coating having suitable physical properties to permit localized heating responsive to the impingement of light from said writing laser, said heating causing localized melting accompanied by withdrawal of the molten material toward the perimeter of the melted area, leaving upon freezing a permanent aperture in the thin metallized coating; and said controlling step includes using said frequency modulated signal for varying the intensity of said writing beam above said predetermined intensity at which the focused beam melts said metallized coating without vaporizing it and below said predetermined intensity at which the focused beam fails to melt said metallized surface.
14. The method as claimed in Claim 12, wherein the storage member is disc-shaped and said storage member moving step 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 focused light beam radially across said surface of said disc-shaped storage member to maintain a constant relationship between said rotational motion and said translational motion.
15. The method as claimed in Claim 13, wherein said controlling step further includes stabilizing the level of modulation of said light beam to produce the intensity levels of said light beam above and below said predetermined intensity; sensing at least a portion of the laser writing beam after modulation of the beam to produce an electrical feedback signal representative of the intensity of the beam; and utilizing the feedback signal in said controlling step to effect stabilization of the level of modulation of said writing beam.
16. The method as claimed in Claim 15, 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 modulation being stabilized to issue the modulated writing beam at a substantially constant average power level.
17. The method as claimed in Claim 13, wherein said controlling step includes: rotating the plane of polarization of said laser beam under control of said frequency modulated signal; and linearly polarizing the rotating beam to produce a modulated laser beam corresponding to said frequency modulated signal.
18. The method as claimed in Claim 15, wherein said controlling step includes selectively adjusting the average power level of said modulated writing beam to a predetermined value.
19. The method as claimed in Claim 15 wherein said controlling step comprises: amplifying said frequency modulated signal to provide corresponding driving signals to a Pockels cell device; A.C. coupling the amplified frequency modulated signal to the Pockels cell; and D.C. coupling said feedback signal to said Pockels cell.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA313,007A CA1066414A (en) | 1977-10-27 | 1978-10-10 | Apparatus and method for storing information on a videodisc |
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA289,623A CA1069214A (en) | 1973-02-20 | 1977-10-27 | Videodisc mastering system |
| CA313,007A CA1066414A (en) | 1977-10-27 | 1978-10-10 | Apparatus and method for storing information on a videodisc |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1066414A true CA1066414A (en) | 1979-11-13 |
Family
ID=25668592
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA313,007A Expired CA1066414A (en) | 1977-10-27 | 1978-10-10 | Apparatus and method for storing information on a videodisc |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1066414A (en) |
-
1978
- 1978-10-10 CA CA313,007A patent/CA1066414A/en not_active Expired
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