CA1069215A - Video disc beam tracking apparatus - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

This invention relates to an improvement in a video signal playback device which derives video signals from an information track on a video disc using a light beam source for impinging upon an information track on the surface of the video disc. In prior art devices of this type, the circuitry necessary to sense whether or not a light beam is on or off track is rather complicated, since it has been necessary to have at least two sensors (and in some instances three sensors), together with the associated electronics to perform the necessary feedback function for maintaining accurate tracking of the reading beam as the reader and disc move relative to one another radially of the disc. The present invention overcomes this difficulty by providing a simplified single beam tracking system including circuitry which is responsive to the position of the impinging light beam relative to a prescribed edge portion of the informa-tion track. When the light beam impinges the prescribed edge portion of the track, the output from the circuitry is nil.
The circuitry provides an output voltage having a first polarity when the light beam impinges less than the prescribed edge portion, and a second polarity when the light beam impinges more than the prescribed edge portion. When the output voltage signal is applied to the tracking control device, a closed loop tracking system results, accurately keeping the light beam impinging on the prescribed edge portion of the track.

Description

1~69Z~S

VIDEO DISC PLAYER
ACKGROUND OF 'mE PRIOR ART
Systems have hereto~ore been developed for - reproducing s~gnals at video ~requencies from in~orma-tion recorded on discs, tapes~ or other media. Such systems have utilized~ among other thlngs~ optical recordings upon photosensitive discs, electron beam recording on thermo plastic sur~aces and, in prior patents assigned to the assignee of the present inven-~ion, systems utllizing a rota~ing dlsc which isresponsive to impinging radiation ~ reflect or transmit radiation corresponding to and representative o~ the In~ormation ~ored on the surface of the disc.
For example, in U. S. Patent No. 3,530,258~ -lssued ~o David Paul Gregg and Keith 0. Johnson on September 22, 1970, there was shown and described a system in which a video signal transducer included a servo controlled pair o~ flexible, ~ibre optic elements.
An air bearing supported an objective lens system. A
A light source of radiant energy was positioned below the disc and the transducer was responsive to trans-mitted light.
;~ ~ Other patents have shown the use o~ a radiant source which directed an energy beam to the ~ur~ace o~ the d~ac and provided a transducer that w~s respon-slve to r~3~1ected energy. One o~ the maJor problems to be enc~untered in the recording and reproduction of video in~ormationJ arises directly from a consideration o~ the energy level3 involved in 3uch a process and ' 1~69Z15 the restraints imposed by the consideratlons o~ size, weight and operating conditlons.
To be commercially desirable as a home instru-ment, the system should be able to store and reproduce a "program" o~ at least 15 to 30 minutes in length.
The record disc should be o~ an easily handled s~ze, comparable to the phonograph records currently ln use.
If the playback turntable was operated at 1800 rpmg some 5~,000 re~olutions would provlde 30 mlnutes o~
playback. Assuming a 1 micron track width and 1 mlcron spaclng between ad~acent tracks, a circular band approximately ~.25 inches wide is required. Assuming that the smallest radius at which in~ormation can be stored is approximately three inches, the resultant disc is about 15 inches in diameter. The duration o~
the program or the speed o~ the turntable can change the dimens.ions o~ the recorded area, as can the width of the individual track and the spacing between :. ad~acent tracks.
Assuming that the video lnformation has baen - recorded in so~e digital fashion, the presence or absence o~ a signal can be detected at an appropri&~e information rate. 'I~ the width oP the track is approx-imately one micron, and that the space between adjacent tracks is also one micron, the quantity o~ energy necessary to impart information ~rom the disc can be determined. It is necessary to provide suf~icient radiant energy to "illuminate" a "spot" ~ approxi-mately one micron in diameter and, at the same time, provide su~ficient radiant energy at the detector, so that the "presence" or "absence" of a signal can be '.
distinguished. .
It has been di~3covered3 in attempting to utilize the transmitted radiation techniques of the prior art~ that the provlsion o~ an inordinately large amount o~ radiation into the system is required in order to "transmit" a su~iciently use~ul increment o~ .
; energy ~or detection through the record. It has also been determined that a substantlal magnification is - ' ,:
,'..

~069Z~LS

required to enable a state-of-the-art transducer to respond to a one micron diameter radiant spot.
If a ligh-t source illuminates the entire field which can be scanned by the detector under control of the servo system, it will be seen that an extraordinary light intensity must be provided before the light transmitted through or reflected from the disc will be of sufficient intensity to register upon the photosensitive device.
According to the present invention there is provided an apparatus for use in a video disc playback system, the system including a video disc reader and a turntable adapted to receive and rotate a video disc, the reader having a light beam source for impinging upon and reflecting from an information track on a surface of the video disc and a light sensor for receiving light reflected from the disc, the reader and disc being arranged for relative movement radially of the disc. The apparatus in-cludes circuit means responsive to the position of the impinging light beam relative to a prescribed edge portion of the infor-mation track, the circuit means including a comparator and a bias means. The comparator has a pair of inputs and a single output. The bias means is connected to one of the inputs and the light sensor is connected to the other of the inputs. The output of the comparator represents the difference between the input from the light source and from the bias means input to produce an output electrical parameter having a first polarity when the light beam impinges less than the prescribed edge - :
portion and a second polarity when the light beam impinges more than the prescribed edge portion.
An import:ant aspect of the present invention is the ~ - 3 -~Ot69Z~5 ability to direct the illuminating radiation to a par-ticular spot and to return the information from the spot thus illum-inated to a detector system. The prior art has suggested the use of a pair of transducers in conjunction with a summing amplifier to provide signal information and a differen-tial amplifier to provide feedback servo information for error correction. However, given the limitations of the extremely low radiation levels, the diffraction limited characteristics of the image and the extreme sensitivity of the system to noise and vibration, such an approach is not entirely satis-factory. A different "curve following" technique described in the U.S. patent to W.D. Munro, U.S. Patent No. 2,83B,683, issued June 10, 1958, has suggested an alternative solution.
In a specific embodiment therefore, a single "~

'"' "
,:

- 3a -13~ ' :' kh~

: : . . . . . - ~ - . : : ., .. .. . . , . ,- .. . . . . .

_4_ photosensitive pickup is used as one input to a dif~erential amplifler, and a second input is provided from a ~ixed bias source. The bias is adJusted to balance the input o~ the photodetector when it is illuminated by the reflected spot that is approximately hal~ way into the information track~ ~or exa~ple, on the periphery side of the track. I~ the intensity o~
the radiation upon the detector increases in a system where the track is "darker" than the band between "trackstl, then a servo sigm~l is developed to drive the mirror in a first direction, tending to move the spot toward the track and toward the center. Simi-larly, if the radiation decreases, the relatively higher magnitude of the bias causes an error signal to be generated which moves the mirror and the "spot"
in a respectlvely opposite directiong away ~rom the track and toward the periphery.
Since, in the preferred embodimentg one revo-lution o~ the disc represents one "frame" ~ the T.V.
picture, an error in trackingg where the track is "lost", merely results in either the sklpplng or the repeating of a frame, both of which are undetectable by the human observer.
In alternative embodiments, it is possible to use the earlier prior art technique of the photo-detector pair.
A second, articulated mirror may be provided which rotates in a second direction, orthogonal to the direction used for the radial tracking of the image.
Such tracking may be considered to be in the circum-ferential direction and would aid in the synchroniæa-tion and timing o~ the recorded information with respect to the timing frequencies generated in the reproducer circuits. As is known, television circuits, and especially color television circults, require extremely ~ccurate time synchronization in order to maintain color fidelity. Therefore, any error in synchronism between the local oscillator of the reproduction apparatus and the timing information ~.. .

. .

1~69Z~S

recorded on the disc, may be resolved and eliminated through the use ~ mirror motion in the second direc-tion.
It has been *ound that any errors resulting from eccentriclty o~ the dlsc can be simply corrected.
It will be noted that the tracking circuit which maintains the radiant spot on the appropriate spiral track will undergo some periodic signal ~luctua~ion that is related to eccentricity. It can then be shown that the change in ~nstantaneous veloclty in the cir-cumferential direction also changes -tn substantially similar rashiong but lags b~ one-quarter revolutlon o~
the disc. Therefore, it is possible either to sense the velocity changes from the recorded timlng informa-tion and from this derive ~ correcting signal to drivethe tracking servos, or to sense the eccent~ioity *rom the tracking servo and use that slgnal with an appropriate phase shift to drive the "timing'; servo to correct for ~elocity changes due to eccentrioity. In an alternative embodiment, a single axis articulated mirror corrects ~or tracking and electronic circuits compensate ~or timing errors.
In yet another improvement, it has been dis-covered that the bias ~orce needed to maintain the air bearing that supports the objective lens at a predeter~
mined distance from the disc surface varies as a *unc-tion o* the surface velocity of the disc. Since the sur*ace velocity is directly related to the relative radial location o~ the air bearing, a simple mechanical cam assembly is employed to modi~y the bias *orce on the air bearing as a ~unction o~ radial location o~
the playback assembly.
Accordingly~ it is an object of the present invention to provide an improved playback assembly *or a di~c upon which video in~ormation has been recorded.
It i9 yet another ob~ect of the invention to provide an improved tracking clrcuit ~or optically scanning a video disc.
It is yet another object of the invention to ~ .

: . . - - ~ . . .... : . ..

~69Z~S

provide an improved scanning assembly for v-ldeo diæc which includes an optical system for directing a radiant energy spot to the disc and to detect reflected radiant energy therefrom and to direct this reflected 5 energy to a photosensitive transducer.
It is yet an additional object of the inven-tion to provide an improved articulated mirror assembly in the optical path between a light source and the ~-surface of the video dlsc, which mirror assembly can be used to direct the location of the spot relative to the disc surface within certain limits.
It ~ yet an additional obJect of the inventlon to provide an articulated m~rror assembly in an optical path which permitsJ with small incremental motions of the mirror~ to var~J widel-y the location of a transmitted spot of radiant energy on the surface of the disc and, at the same time, transmit to a de tector s~Jstem the returned radiant energy.
It is yet a different object of the invention to provide a video disc playback assembly which directs a radiant spot to the surface of the disc and directs the returning radiation to a photosensitive detector and which detects returning raaiation from the disc surface.
It is yet an additional ob~ect o~ the invention to provide a radiation detector for a video disc play-back assembly which applies an input to a differential amplifier, the second input to which i8 a fixed biasg for generating an error signal to control the optical system directing a radiant spot to the disc surface and returning a reflected spot therefrom to the detec-tor.
The novel features which are believed to be characteristic of the invention~ both as to organiza-tlon and method of operation, together with furtherob~ects and advantages thereof will be better under-stood from the following description consldered in connection with the accompan~ing drawings in which several preferred embodiments of the invention are ~6921S

illustrated by way of example. It is to be expressly understood, howeverg that the drawings are ~or the purpose o~ illustration only and are not intended as a definition o~ the limits o~ the invention.
BRIEF DESCRIPTIOM OF THE DRAWINGS
FIG. 1 is an idealized side view of a play-back assembly according to the present invention;
FIG. 2 is a more detailed block diagram of the elements ~n the optical playback systemg FIG. 3 is an idealized view o~ an alternative articulated mirror assembly;
FIG. 4 is a block diagram o~ a suitable de-tector and tracking circuit o~ the prior art;
FIG. ~ is a block diagram of an optical de-tector of the prior art suitable ~or use in the present invention;
FIG. 6 is an enlarged side view of the optical head and air bearlng assembly9 FIG. 7 is a top idealized view ~ a cam and ~ollower assembly ~or controlling the bias on the air bearing assembly; and FIG. 8 is a side view ~ another alternative articulated mirror arrangement use~ul in the system of the present invention.

Turning ~irst to FIG. 1, there is shown, in side view~ a playback assembly 10 suitable for use in the present invention. The playback assembly 10 include~ a laser element 12 which moves with the playback assembly 10. It is, however, within the ~tate-of-the-art to provide a stationary laser which is coupled opticall~ to the movable assembly 10. Prefer-ably~ the laser 12 provides coherent, polarized light.
A read head 14 is mounted in arm 16 of the playback assembly 10.
A video disc 20, which has video information recorded u,pon it is mounted on a turntable 22~ which i~ adapted to rotate the disc 20 at a relatively high ~peed, In the pre~erred embodiment, the turntable ', "

.. . . . . . . ... . .. . . . .

~06~Z~

speed is set at 1~00 rpm.
Suitable video discs have been described ~nd claimed in the patents to Greg~, Johnson, supra~
The playback assembly 10 is mounted on a ~ rotatable element 24 which, in the view o~ FIG. 1, - translates the readlng head in the radial direction relative to t'ne disc 20 and in an arc that ~s gener-ally orthogonal to the plane o~ the drawing.
The laser 12 generates a reading beam 26 10 which generally passes from the laser 12 through an optical system to the pla~back head 14. The be-am is - then directed to the surface of the dlsc 20 and re-turns through the playback head 14 along the same optical path until a read assembly 28 is encountered.
The read assembly 28 is mounted on the arm 16.
In operation, the laser directs a reading light beam 26 to the sur~ace o~ the disc 20 through the optical system. The information recorded upon the disc interacts with the impinging beam and a reflected beam is prod~lced which contains the recorded ln~orma_ tion. The reflected light beam is returned to t~he optical system which "analyzes'` the returned beam to determine whether the beam is properly tracking the signal channel-. ^
I~ the electronics determine that the laser spot is not being directed to a predetermined area o~
the information channel, appropriate servo signals are derived which, when applied to the read head 14, cau~e the point of impingement of the laser beam to 3o shift in the radial direction to retain alignment with the track that is being read.
In an alternative embodimentg the driver ~or the rotatable element 24 ~or the playback assembly 10 can also be controlled by the servo signals which changes the position o~ the laser spot. In yet other embodiments, a motor can be coupled to the turntable driver to provide a predetermined increment of radial motion for ea.ch revolution o~ the turntable 22. In ~ny case, th~ playback head D can be made to track ~ ;

, ~
:; ' ' ' :'`

,~. . .. . : , .
, ~
. ~ . . . .
.. : ~. . . ~ :

~69~5 the lnrormation channel recorded on the disc 20 wi~h a "coarse ad~ustment being-applied to the driver o~
the rotatable element 24 and a "fine" ad~ustment being applied to an articulated mirror, described in greater detail below.
Turning next to FIG. 2, there is shown a diagram of the elements of the reading system, The reading laser beam 26 is applied to a beam spl~tting prism 30. The prism 30 is rotated slightly with 10 respect to the optical path. A lens 32 is provided to better form the beam 26 at the surface 20 and to optimize the resolving power of the system, The trans-mitted portion of the beam 26 is applied through a quarter wave plate 36 and is then directed through the reading head 14 to the disc 20.
A returning beam 38 containing the information from the disc 20 follows substantially the identical path. At the quarter wave plate 36J the returning beam is now given an addltional quarter wave shlft for a total polarization of one-half wavelength. The returning beam 38 reaches the beam splitter 30 and is reflected therefrom to a suitable optical system 40.
~ight from the laser 12 that is initially reflected in the prism 30 and re-re~lected from the base of the prism willj due to the slight rotation of the prism 30, be aimed at a point that wholly misses the detector 40. Moreover, the cumulative effect of the quarter wave plate.which polarizes the returning beam by ~/~
substantially attenuates any transmitted component.
What is transmitted is cross polarized with respect ` to the laser 12.
The read head 14 includes a fluid-bearing ~ -member 50 which is ad~acent to and supportive o~ a microscope obJective lens 52. A l-lmited amount of vertical ad~ustment is available in the objectiYe lens 52. Direc~ing the illumination to the ob~ective lens 52 is an articulated mirror 54 which is mounted ad~acent to and cooperates with a second or fixed mirror 56 that is substantially parallel with the - - . ' .

- . .
: , ' - - . . . . - . . .

1~692~5 - .
articulated mlrror 54. The fixed mirror receives - the reading beam 26 and directs ~t to the articulated mlrror 54.
The reading beam 26 undergoes at least one reflection from the artlculated mirror 5~ be~ore the beam is applied to the objective lens 52. Two such reflection~ are illustrated in the embodlment o~
FIG. 2. Similarly, the beam path is such that a re-flected beam 38 returning from the surface o~ the disc 20 would also undergo two re~lections ~rom the articu-lated mirror 54 and two re~lections from the ~ixed mirror 56 be~ore proceeding into the optical pa~h including an additional fixed mirror 57 which ulti- -mately leads to the read assembly 28.
In the embodiment illustrated, the artlculated mirror 54 ~s mounted on a point pivot 58 that is - centrally located with respect to the mirror 54. The mirror 54 may have an oblong shape with the long axis in the plane of the drawing and the short axis orthog-onal to the plane o~ the drawing. As showng a mirrordriver 60 ~s connected to one end o~ the mirror 5~
and is operable to lmpart motion about the central pivot 58.
If the driver 60 rotates the mirror 54 in the clockwise direction, as viewed in FIG. 2, the point o~
impingement o~ the read beam 26 will be shifted to the ; le~t. This would represent a deflectlon of the beam in a first radial direction. If the drlver 58 rotates -:
the mirror 54 in the counter-clockwise direction, then the point of impingement of the transmitted beam 26 will be shi~ted to the rightJ as seen in FIG. 2, or ;
in a second, opposite radial direction.
It will be obvious that the reflected beam 38 and the reading beam 26 trace identlcal paths between the sur~ace of the disc 20 and the beam splitter 30.
The articulated mirror 54 serves to "steer" the read-ing spot to a desired location and then "reads" only the illuminated area, transmitting that lnformation back to the read assembly 28. ;

' : - - . . . .
~, - . . . .. . . .

~C1692~5 - In alternatlve embodiments, the articulated mirror 54 and the stationary mirror 56 can be adjusted and repositioned to provide a greater plurality of reflections between the two mirrors before the beam continues either to or from the disc surface 20. In such an arrangement, the magnitude of mirror deflection required to steer the reading spot appropriately can be greatly reduced. The drlver 60 therefore, need only impart small, incremental motions to the articu~
lated mirror 54.
In an alternative embodiment, as shown in FIG. 3, a first articulated mirror 54' is provided - which is mounted on a central pivot member 58', and is driven about an axis orthogonal to the plane of the FIGURE and in the clockwise and counter-clockwise direction by a first driver 60' that is coupled to the - mirror 54' at the end of a long axis.
- A second driver 60" is coupled to one end o~ a third mirror 54" for imparting rotational motion to the third mirror 5~" about the long axis that is in the plane of ~he FIGURE.
In operation, the ~irst driver 60' permits ~ranslation of the beams in the "radial" direction to - permit "fine" tracking of the information channel.
The second driver 60" is used to translate the beam in the circumferential direction, to provide time - synchronization, if desired, and to compensate ~or eccentricity.
In other embodiments, the problem of time synchronization can be handled mathematically, as a step in the process of electronically compensating for eccentricity of the disc 20 and in such embodiments, onl~ the single articulated mirror ~s used.
Turning next to FIG. ~, there is shown a preferred embodiment of the optical detector assembly 40 which utilizes some of the electronics of the Munro patent, supra. As shown in FIG. 4, the returned optical lmage 38 is directed to impinge upon a photo-cell 70 when cL channel is being tracked properly, - - - . . .
.! - , . , .
i'' . ..,' " ' - ~6~
-12~
~- with the spot on the outer half of the track9 a pre--determined output signal is ~enerated. The output of the pho~ocell 70 is applied to a comparator 72. An ad~ustable bias 74 is applled to the other input o~
the comparator 72 and is adjusted to provide a null when the predetermined output signal is being applied.
The error signals resulting from drift can be inte-grated, and the output of the integrator can be applied to an appropriate circuit to urge the movable playback assembly 10 relative to the center o~ the disc 20.
The error signal is also-used to apply a slgnal directly to the mirror driver 60 of FIG. 2 to urge the beam to ~ollow the track.
If, however~ the track is not being followed properlyJ depending, o~ course~ upon the character-istics of the disc sur~ace, a condition will be presented in which the energy impinging upon the photocell 70 will be di~ferent than the bias provided - by bias circuit 74~ and accordingly, the error signal of appropriate polarity will be provided to correct the position o~ the light spot relative to the in~orma-~lon channel. The integrator output then is applied to the movable playback assembly 10~ and if the bias signal is greater, a forcing ~unction is generated tending to send the spot toward the periphery o~ the disc. If the received signal is greater, the spot .
is directed to the center o~ the disc. As the spot ~ollows the spiral track properly, the di~ferential output tends toward the null.
3 In FIG. 5, there is illustrated the prior art optical detector electronics utilized and shown as FIG. 10 in the previously issued Gregg~ et al., U. S. ;
Patent No. 3J530,2581 assigned to the ass~gnee of the present invention. For convenience, the same reference `; 35 numbers are used in Gregg, et al and herein. A pair of photo detectors 96, 98 are employed which, in com-blnation, provide an additive information signal and, ~ ~
when differenced, an error signal which controls servo ~ i elements that redirect the reading elements. As :

- ~

applied to the present invention, the radlal error s~gnal could be applied to either o~ the drivers 60, 60' of the ar~iculated mirror assemblies o~ FIGS. 2 and 3g respectively.
As shown in FIG. 5~ a dual photo detector has two sections 96~ 98 whose outputs are applied to re-spective ampli~iers lOOg 101. The outputs o~ the amplifiers 1009 101 are summed in a summing networ~
106. The output ~rom the summing network represents the sum signal from the two photo detector sections 96, 98 and constitutes khe modulated signal output of the transducer.
The signal amplltude ~rom the first photo detector section is applied to a detector 102~ and this detector produces a negative unidirectional signal representative thereof. The signal amplitude from the second photo detector section is applied to a detector 103, and the latter detector produces a negative unidirectional signal in response thereto.
The two signals are added algebraically in a summing network 105 which produces an error signal.
In the present example 3 the resulting error signal is ampli~ied in an amOlifier 104, and it is applied to the circuits of FIG. 3 and driver 60'.
25 The error signal applied to the driver 60' causes the ~-mlrror 54' to shift the beams in a radial direction with respect to the disc 20~ as explained above. The direction and amount of the shift depends on the ~ polarity and amplitude of the error signal, so as to ; 30 maintain the spot in perfect registry with the record-ing track on the record 20.
The output signal from the summing network 106 is appliecl to appropriate video detection and repro-ducing circuitry such as is illustrated in FIGS. 17 and 18 o~ Gregg et al, supra~ and described therein.
; The DC component of the oUtput of the amplifier 104, when properly processed~ may be used in several ways to move the pick-up arm of ~IG. 1 across the disc 20 at very nearly the rate which makes the signal ~0692~5 approach ~ero~ One method is to integrate this com-ponent over short intervals until it reaches a pre-determined value~ at which it triggers a solenoid.
This solenoid, in turn9 actuates a light-duty ~riction ratchet which then turns the pick-up arm through a very small angle as is taught ln CTregg et alg supra.
Another method also suggested in Gregg et al, suprag is to use an inexpensive electric clock movement with a reduction gear to drive the arm continuously across the disc at a rate just slightly above 2 microns for each 1/30 second or re~101ution of the disc. In this caseg the integrated signal Or the first method ls used to interrupt the motor voltage ~casionally.
To assist the processg the arm 16 o~ FIG. 1 may be biased slightly towards the center ~ the disc 20.
In FIGo 6g there is shown an enlarged side view -of the lens and air bearing assembly of the playback head 14. The movable arm 16 connects to the playback head 14 through a pair ~ parallel leaf springs 120g 20 122. The spring force of the lea~ springs 120, 122 is generally insuf~icient to maintain the springs in the horizontal position with the playback head 14 -unsupported by the fluid bearing that is generated by the rotating disc 20. Within the read head 14 ls the fluid bear-lng member 50 and the microscope type ob~ec-tive lens 52. Also contained in the read head 14 are the ~ixed and articulated mirrors 5~g 56, 57 necessary to dlrect the beam o~ light ~rom the source to the lens 52 and back from the surface of the disc 20.
3 A support post 12~ extends outward of the read head 14 toward the inner end of the arm 16. Mounted to this support post 124 is a bias ~ring 126g the other`end of which is fastened to a lever 128. The lever 12~ is coupled to the arm 16 andg through a flexible cable 130g connects to a cam and follower assembly 132g to be described in connection with FIG.
7, below.
Also included, but not described in detailg are ~; appropriate interlocklng solenoid assemblies operating '` ' .
. . .
, -; , . . :, ~ - . - -, .. . . ;

..
.: . -: . ~: `. , ~ . ~ .; ..... .

~6369;~5 in conjunction with the cam and follower assembly to maintain the read head 14 out o~ contact with the disc 20 as the arm 16 swings ou~ o~ engagement with the disc 203 and which ac~ to prevent damage ifg for any reason, the disc 20 should ~low appreciably whlle belng tracked by the read head 14.
The bias spring 126g when compressed, acts like a solld rod~ enabling the lever 128 to directly cam the read head 14 upward and away ~rom the disc 20, if this con~iguratlon is desired. Alternatively~ ~hen the read head 14 is in positlon over the disc~ the lever 128 rotates in the opposite direction~ relleving the compresslon on the spring 126. Under normal cir-cumstancesg the welght of the read head 14 is suppor-; 15 ted by the fluld bearing member 50 on the disc~ there-by enabling the leaf springs 120~ 122 ~o be substan-tially pa~llel and horizontal.
According to the present invention, an addl-tlonal bias is provided through the use o~ the bias spring 126 to maintain a substantially constant separation between the read head 14 and the fluid bearing member 50 and the surface of the disc 20. The relat~ve surface velocity changes as the moving arm 16 prbgresses toward the center of the disc and the fluid bearing is less able to support the read head.
; Therefore, at the outset9 the lever 128 is rotated in the downward direction~ applying a stretch to the sprlng 126 which~ in turn, lmparts a downward force to the support arm 124~ thereby increasing the bias on the fluid bearing 50 while the fluid pressure is at ~` its greatest.
As the arm 16 moves inwardly o~ the disc 20 and the surface velocity is reduced; a cam ~ollower arrangement gradually rotates the lever 128 in the upward direction~ reducing the tension o~ the spring ` 126, therl3by lessening the bias on the read head 14.
By seleating an appropriate cam contour, the bias on ~` the ~luid bearing 50 can be maintained at an optimum value for constant separation from the disa 20 for the ;, . :
., ' .
:

~ l-q~92l5 surface velocity of the disc at any radial location, Turning now to FIG. 7, t`~lere is shown one ~orm of cam and follower assembly 132 that can drlve the lever 128 through the flexible cable 130 (also shown in FIG. 1). A cam 140 is cut so that at the outermost position of the arm 16g a follower 1~2 rests on a high lobe which maintains the head 14 in an "up" positiong safely out of contact wlth the edge of the rotating disc 20.
As the arm 16 tracks inwardly~ the follower 142 immediately proceeds to the innermost polnt on the cam 140 surfaceg apply'Lng maximum bias to the read head 14. As the arm then continues inwardly in the radial directlon, the follower 142 gradually rides outwardly from the center ~ the cam 140g thereby reducing the blas forces on the read head 14.
It ls clear th~t techniques are readlly avall-able for transmitting simple mechanical motlon from the cam follower assembly 132 to the arm 16g and the specific details are unnecessary ln the present appli-cation.
In FIG. 8, there is shown an alternatlve con-figuration for the articulated mirror assembly that is mounted on the read head 14. In this alternative embodlmentg a fi~ed mirror 150 and an articulated mlrror 152 are arranged on converglng planes. An incoming beam in the horlzontal direction implnges upon the articulated mirror 152, and through multiple reflection between the ~ixed mlrror 150 and the articu-lated mirror 152J the beam is ultimately rotatedthrough 90 and is directed downward lnto the readlng assembly. Slmllarly~ the returning beam retraces the same path. The mirror 152 is articulated to rotate about an axis that is in the plane of the drawing to deflect the transmitted beam in a direction that is perpendicular to the plane of the drawing.
The angle o~ incidence of the mirror 150 and the angle o~ convergence between the mirrors 150 and 152 are controlled so that the incoming beam makes a '"' . . - . . . , , ~ :: .

-17- 1 ~ 6~ ~ 5 plurallty of reflections of~ of the two mirrors before being directed into the discn More~ver, slnce the pair of mirrors, in addition to providing a l'folded"
light path, also rotates the beam through 90g a separate 4~ mirror can be omitted 3 thereby increasing the intensity o~ available light to the disc. Of course~ this would permit at least one extra reflection between the mirror pair without ~n any way degrading the quality of the light beam. The same number of internal reflections as in the embodiment of FIG. 2 could be employed with less light loss in the mirror system.
Thusg there has been shown an improved video disc reading assembly which steers the illum~nating radiation to the information track on the surface of the disc and steers the return signal from the track to an optical detector. An articulated mirror enables the steering of both the transmitted and the returned light beam.
An improved optical detector is utilized in combination with a fixed bias source so that a single ~tector provides both the information signal and the servo slgnals necessary to trac~ the in~ormation channel.
A novel air bearing assembly has also been disclosed~ which enables a microscope lens to travel at a fixed distance above the disc supported on a fluid bearing~ and means are provided to impart a variable bias to the fluid bearing as a function relative velocity between the disc and the bearing member.
What is claimed as new is:

i~'' . ;

Claims (10)

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

1. Apparatus for use in a video disc playback system, the system including a video disc reader and a turntable adapted to receive and rotate a video disc, the reader having a light beam source for impinging upon and reflecting from an information track on a surface of the video disc and a light sensor for receiving light reflected from the disc, the reader and disc arranged for relative movement radially of the disc, said apparatus comprising: circuit means responsive to the position of said impinging light beam relative to a prescribed edge portion of said information track, said circuit means including a comparator and a bias means, said comparator having a pair of inputs and a single output, said bias means connected to one of said inputs and said light sensor connected to the other of said inputs, the output of said comparator representing the difference between the input from said light sensor and from said bias means input to produce an output electrical par-ameter having a first polarity when said light beam impinges less than said prescribed edge portion and a second polarity when said light beam impinges more than said prescribed edge portion.

2. Apparatus as claimed in Claim 1, wherein:
said light sensor is a photocell providing a voltage output proportional to the light received from said information track; said bias means is an adjustable voltage bias means; and said output electrical parameter from the comparator is an electrical voltage proportional to the difference between said input from the light sensor and said bias means input.

3. Apparatus as claimed in Claim 2, wherein:
said bias means is adjusted to provide a null when said light beam impinges said prescribed edge portion.

4. Apparatus as claimed in Claim 1, including optical means for directing said light beam along a prescribed optical path from said source means to said surface of said disc.

5. Apparatus as claimed in Claim 4, wherein said optical means comprises: a pair of mirrors disposed in said optical path, said mirrors being arranged in opposing space relation to one another to cause said light beam to be reflected at least once from each of said mirrors; and means for articulating at least one of said mirrors to steer said light beam to a precisely selected location on said surface of said disc.

6. Apparatus as claimed in Claim 4, including means for effecting relative movement between the reader and the disc radially of the disc.

7. Apparatus as claimed in Claim 6, wherein said circuit means includes an integrator circuit for receiving the output of said comparator, and said integrator circuit producing an output signal for control of said means for effecting relative radial movement.

8. Apparatus as claimed in Claim 7, wherein: said means for articulating at least one of said mirrors receives an output signal directly from said comparator; and said means for effecting relative radial movement receives an output signal from said integrator.

9. Apparatus as claimed in Claim 5, wherein said mirrors are planar and have their respective planar reflecting sur-faces lying in parallel planes.

10. Apparatus as claimed in Claim 5, wherein said mirrors are planar and have their respective planar re-flecting surfaces lying in converging planes, the angle of convergence between said reflecting surfaces being such as to cause multiple reflections in the direction opposite the direction of convergence.