Note: Descriptions are shown in the official language in which they were submitted.
~L~7 3~
' DYNAMIC FOCUSING OF VIDEO DISC OBJECTIVE LENS
BACKGROUND OF THE PRIOR ART
Systems have heretofore been developed for reproducing
signals at video frequencies from information recorded on discs,
tapes, or other media~ Such systems have utilized, among other
things, optical recordings upon photosensi-tive discs, electron
beam recording on thermo plastic surfaces and, in prior patents
assigned to the assignee of the present invention, systems
utilizing a rotating disc which is responsive to impinging
radiation to reflect or transmit radiation corresponding to
and representative of the information stored on the surface of
the disc.
For example, in U. S. Patent No. 3,530,258, issued
- to David Paul Gregg and Keith O. Johnson on September 22, 1970,
there was shown and described a system in which a video signal
transducer included a servo controlled pair of flexible, fibre
optic elements. An air bearing supported an objective lens
system. A light source of radiant energy was positioned below
the disc and the transducer was responsive to transmitted light.
Other patents have shown the use of a radiant source
which directed an energy beam to the surface of the disc and
provided a transducer that was responsive to reflected energy.
One of the major problems to be encountered in the recording and
reproduction of video information, arises directly from a con-
sideration of the energy levels involved in such a process and
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`the restraints imposed by the considerations of size, weight
and operating conditions.
To be commercially desirable as a home instrument,
the system should be able to store and reproduce a 'Iprogram''
of at least 15 to 30 minutes in length. The record disc
should be of an easily handled size, comparable to the phono-
graph records currently in use. If the playback turntable
was operated at 1800 rpm, some 54,000 revolutions would provide
30 minutes of playback. Assuming a 1 micron track width and
1 micron spacing between adjacent tracks, a circular band
approximately 4.25 inches wide is required. Assuming that
the smallest radius at which information can be stored is
approximately three inches, the resultant disc is about 15
inches in diameter. The duration of the program or the speed
of the turntable can change the dimensions of the recorded
area, as can the width of the individual track and the spacing
between adjacent tracks.
Assuming that the video information has been recorded
in some digital fashion, the presence or absence of a signal
can be detected at an appropriate information rate. If the
width of the track is approximately one micron, and that the
space between adjacent tracks is also one micron, the quantity
of energy necessary to impart information from the disc can
be determined. It is necessary to provide sufficient radiant
energy to "illuminate" a "spot" of approximately one micron
in diameter and, at the same time, provide sufficient radiant
energy at the detector, so that the 'Ipresence" or "absence" of
a signal can be distinguished.
It has been discovered, in attempting to utilize the
ransmitted radiation techniques of the prior art, that the
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provision of an inordinately large amount of radiation into
the system is required in order to "transmit" a sufficiently
useful increment of energy for detection through the record.
It has also been determined that a substant:ial magnification
S is
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re~u.ired to enable a state-of-the-art transducer to respond
to a one micron diameter radiant spot.
If a light 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 systemJ
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 an information track
on the surface of the video disc and a light sensor for
xeceiving light refl.ected from the disc, -the reader and disc
arranged for relative lateral movement radially of the disc.
The apparatus has an objective lens mounted in the disc
reader adjacent the disc for focusing the light beam on the
information track of the disc and passing the reflected light
to the sensor. Means is provided for moving the objective
lens relative to the surface of the disc along the path of
the impinging light beam to maintain focus of the beam on
the information track as the rotating disc moves laterally
relative to the reader.
In a specific embodiment of the invention, the
read head is yieldably mounted on a head support and is
adapted to be maintained in close proximity to the video
disc surface, the objective lens being moun-ted on the read
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.
head for movement therewith. The means for moving the
objective lens may include adjustable bias means for applyiny
a varying force to the read head in a direction normal to
the disc to achieve and maintain a predetermined spacing
between the head and the disc substantially independent of
the radial position of the head.
According to another aspect of the present invention,
there is provided a method for readiny information recorded
on the surface of a video disc, the method including steps ::
of rotating the video disc, impinging a light beam upon an
information track on the surface of the disc, and sensing
reflected liyht received from the disc while the reader and
disc move relative to one another radially of the disc. The
liyht beam is focused on the information track of the disc
through an objective lens adjacent the disc, and the reflected
light passes throuyh the objective lens to the sensor. The
objective lens is moved relative to the surface of the disc : .
alony the path of the impinginy light beam to maintain focus
of the beam on the information track as the rotating disc
moves relative to the reader.
More specifically, the method of the present
invention .includes the step of providiny a read head yield-
ably mounted on a head support and adapted to be maintained
in close proximity to the video disc surface, the objective
lens being adapted for movement with movement of the read
head. A varying force may be applied to the read head in a
direction normal to the disc to achieve and maintain a pre-
determined spacing between the head and the disc subs-tantially
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independent of radial position of the head.
An important aspect oE the present inven-tion is
the abili-ty to direct the illuminating radia;tion to a
particular spot and to return the information from the spot
thus illuminated to a detector system. T;he prior art has
suggested the use of a pair of transducers in conjunction
with a summing amplifier to provide signal information and
a differential amplifier to provide feedback servo information
for error correction. However, given the limitations of the
extremely low radiation levels, the diffrac-tlon limited
characteristics of the image and the extreme sensitivity
of the system to noise and vibration, such an approach is
not entirely satisfac-tory. A difference "curve following"
technique described in the patent to W.D. Munro, ~.S.
Patent No. 2,838,683, issued June 10, 1958, has suggested
an alternative solution. .
Accordingly, it is an object of the present
invention to provide an improved playback assembly for a
disc upon which video information has been recorded.
It is yet another object of the present invention
to provide an impxoved tracking circuit for optically
scanning a video disc.
It is yet another object of the invention to
provide an improved scanning assembly for video disc which
includes an optical system for directing a radiant energy
spot to the disc and to detect reflected radiant energy
therefrom and to direc-t this reflected energy to a photo-
sensitive transducer.
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It is yet an additional object of the invention
to provide an improved articula-ted mirror assembly in the
optical path between a light source and the surface of the
video disc, which mirror assembly can be used to direct
the location of the spot relative to the disc surface within
certain limits.
The novel features which are believed to be
characteristic of the invention, both as to organization
and method of operation, together with further objects and
advantages thereof will be better understood from the
following description considered in connection wlth the
accompanying drawings in which several pre~erred embodiments
of the invention are illustrated by way o:E example. It is
to be expressly understood, however, that the drawings are
for the purpose of illustration only and are not intended
as a definition of the limits of the invention.
BRIEF DESCRIPTION OF TIIE DRAWINGS
FIG. 1 is an idealized side view of a playback
assembly according to the present invention;
FIG. 2 is a more detailed block diagram of the
elements in the optical playback system;
FIG. 3 is an idealized v.iew of an alternative
articulated mirror assembly;
FIG. ~ is a block diagram of a suitable detector
and tracking circuit;
FIG. 5 is a block diagram of an op-tical detector
of the prior art suitable for use in the present invention;
FIG. 6 is an enlarged side view of -the opti.cal
head and air bearing assembly;
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FIG. 7 is a top idealized view of a cam and
follower assembly for controlling the bias on the air
bearing assembly; and
FIG. 8 is a side view vf another alternative
articulated mirror arrangement useful in the system of the
presen-t invention.
DETAILED DESCRIPTION OF THE INVE'NTIO_
Turning first to FIG. 1, there is shown, in side
view, a playback assembly 10 suitable for use in the present
invention. The playback assembly 10 includes a laser
element 12 which moves wi-th the playback assembly 10. It
is, however, within the state-of-the-art to provide a
stationary laser which is coupled optically to the movable
assembly 10. Preferably, the laser 12 provides coherent,
polarized light. A read head 14 is mounted in arm 16 of the
playback assembly 10.
A video disc 2Q, which has video information
recorded upon it is mounted on a turntable 22, which ls
adapted to rotate the disc 20 at a relatively high speed.
In the preferred embodiment, the turntable
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speed is set a-t 1800 rpm.
Suitable video discs have been described and
claimed in the patents to Gregg, Johnson, supra.
The playback assembly 10 is mounted on a rotatable
element 24 which, in the view of FIG. 1, translates the readîng
head in the radial direction relative to the disc 20 and in
an arc that is generally orthogonal to the plane of the drawing.
The laser 12 generates a reading beam 26 which
generally passes from the laser 12 -through an optical sys~em
to the playback head 14. The beam is then directed to the
surface of the disc 20 and returns 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 heam
26 to the surface of the disc 20 through the optical sy~tem.
The information recorded upon the disc interacts with the
impinging beam and a reflectçd beam is produced which contains
the recorded information. The reflected light beam is returned
to the optical system which "analyzes" the returned beam to
determine whether the beam is properly tracking the signal
channel.
If the electronics determine that the laser spot is
not being directed to a predetermined area of the information
channel, appropriate servo signals are derived which, when
applied to the read head 14, cause the point of impingement
of the laser beam to shift in the radial direction to retain
alignment with the track that is being read.
In an alternative embodiment, the driver for the
rotatable element 24 for the playback assembly 10 can also be
3Q controlled by the servo signals which changes the position of
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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 each revolution of the turntable
22. In any case, the playback head D can be made to track
the information channel recorded on the disc ?0 with a
"coarse" adjustment being applied to the d:river of the rotatable
element 24 and a "fine" adjustment being applied to an
articulated mirror, described in ~reater detail below.
Turning next to FIG. 2, there is shown a diagram of
the ~lements of the reading system. The reading laser beam
26 is applied to a beam splitting prism 30. The prism 30
is rotated slightly with 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 transmitted
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.
`!i'` ' `
A returning keam 38 containing the information from
the disc 20 follows substantially the identical path. At the
quarter wave plate 36, the returning beam is now given an
additional quarter wave shift 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. Light from the laser 12 that is initially reflected
in the prism 30 and re-reflected ~rom the base of the prism
will, 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 ~ 2 substantially attenuates any trans-
mitted component. What is transmi.tted is cross polarized with
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respect to the laser 12.
The read head 14 .includes a fluid-bearing member 50
which is adjacent to and supportive of a microscope objective
lens 52. A limited amount of vertical adjustment is available
in the objective l~ns 52. Directing the illumination to the
objective lens 52 is an articulated mirror 54 which is mounted
adjacent to and cooperates with a second or fixed mirror 56
that is substantially paxallel with the articulated mirror 54.
The fixed mirror receives the reading beam 26 and directs it
to the articulated mirror 54.
The reading beam 26 undergoes at least one reflection
from the articulated mirror 54 before the beam is applied to
the objective lens 52. Two such reflections are illustrated
in the embodiment of FIG. 2. Similarly, the beam path is
such that a reflected beam 38 returning from the surface of the
disc 20 would also undergo two reflections from the articulated.
mirror 54 and two reflections from the fixed mirror 56 before
proceeding into the optical path including an additional fixed
: mirror 57 which ultimately leads to the read assembly 28. .
In the embodiment illustrated, the articulated mirror
54 is 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 orthogonal to the plane of th~ drawing. As
shown, a mirror driver 60 is connected to one end of the machine
54 and is operable to impart motion about the central pivot
58.
.If the driver 60 rotates the mirror 54 in the clock~
wise direction, as viewed in FIG. 2, the point of impingement
of the read beam 26 will be shifted to the left. This would
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represent a deflection of the beam in a first radial direction.
If the driver 58 rotates the mirror 54 in the counter-clockwise
direction, then the point of impingement of the transmitted
beam 26 will be shifted to the right, 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 identical paths between the surface
of the disc 20 and the beam splitter 30. The articulated
mirror 54 serves to "steer" the reading spot to a desired
location and then "reads" only the illuminated area, trans~
mitting that information back to the read assembly 28.
In alternative 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 di~c
surface 20. In such an arrangement, the magnitude of mirror
deflection required to steer the reading spot appropriately can
be greatly reduced~ The driver 60 therefore, need only impa~t
small, incremental motions to the articulated 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 ortho-
gonal 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 of a third
mirrox 54" for imparting rotational motion to the third mirror
54" about the long axis that is in the plane of the FIGURE,
In operation, the first driver 60' permits translation
of the beams in the "radial" direction to permit "fine" tracking
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of the inormation channel. The second driver 60" is used
to translate the beam in the circumferential direction, to
provide time synchronization, if d~sired, and to compensate
for eccentricity.
In other embodiments, the problem of time synchron-
ization can be handled mathematically, as a ~tep in the process
of electronically compensa-ting for eccentricity of the disc 20
and in such embodiments~ only the single articulated mirror is
used.
Turning next to FIG. 4, there is sho~n 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 image 38 is directed to impinge
upon a photocell 70 when a channel is being tracked properly,
with the spot on the outer half of the track, a predetermined
output signal is generated. The output of the photocell 70
is applied to a comparator 72. An adjustable bias 74 is applied
to -the other input of the comparator 72 and is adjusted to
provide a null when the predetermined output signal is being
applied. The error signals resul-ting from drift can be inte-
grated, and the outpu-t of the integrator can be applied to an
appropriate circuit to urge the movable playback assembly 10
relative to the center of the disc 20. The error signal is
also used to apply a signal directly to the mirror driver 60
of FIG. 2 to urge the beam to follow the track.
If, however, the track is not being followed properly,
depending, of course, upon the characteristics of the disc
surface, a condition will be presented in which the energy
impinging upon the photocell 70 will be different than the
bias provided by bias circuit 74, and accordingly, the error -~
-12-
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signal of appropriate polarity will be provided to correct
the position oE the light spot relative to the information
channel. The integrator output then is applied to the movable
playback assembly 10, and if the bias signal i9 greater,
a forcing function is generated tending to send the SPot
toward the periphery of the disc. If the received signal is
greater, the spot is directed to the center of the disc. As
the spot follows the spiral track properly, the differential
output tends toward the null.
In FIG. 5, there is illustrated the prior art optical
detector eIectronics utilized and shown as FIG. 10 in the
previously issued Gregg, et al., U.S. Patent No. 3,530,2S8,
assigned to the assignee of the present invention. For
convenience, the same reference numbers are used in Gregg,
et al and herein. A pair of photo detectors 96, 98 are
employed which, in combination, provide an additive information
signal and, when diferenced, an error signal which controls
servo elements that redirect the reading elements. As applied
to the present invention, the radial error signal could be
applied to either of the drivers 60, 60' of the articulated
mirror assemblies of FIGS. 2 and 3, respec-tively.
As shown in FIG. 5, a dual photo detector has two
sections 96, 98 whose outputs are applied to respective
amplifiers 100, lOl. The out~uts of the amplifiers lO0, 101
are summed in a summing network 106. The output from the
summing network represents the sum signal from the two photo
detector sections 96, 98 and constitutes the modulated signal
output of the transducer~
The signal amplitude from the first photo detector
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section is applied to a detector 102, and this detector
produces a negative unidirec-tional 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, th~ resulting error signal
is amplified in an ampli~ier 104, and it is applied to the
circuits of FIG. 3 and.driver 60'. The error signal applied
to the driver 60' causes the mirror 54' to shift the beams in
a radial direction with respect to the disc 20, as explained
above. The direction and amount o the shift depends on the
polarity and amplitude o the error signal, so as to maintain
the spot in perfect registry with the recording track on the
record 20.
The output signal from the summing network 106 is
applied to appropriate video detection and reproducing
circuitry such as is illustrated in FIGS. 17 and 18 o-f 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 FIG. l,across the disc 20 at very nearly
the rate which makes the signal approach zero. One method is
to integrate this component over short intervals until it
reaches a predetermined value, at which it triggers a solenoid.
This solenoid, in turn, actuates a light-duty friction ratchet -r~
which then turns the pick-up arm through a very small angle as
is taught in Gregg et al, supra.
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: Another method also suggested in Gregg et al,
supra, is to use an inexpensive elec-tric clock movement with
a reduction year to drive the arm continuously across the disc
at a rate just slightly above 2 microns for each 1/30 second
or revolution of the disc. In this case, the integrated signal
of the first method is used to interrupt the motor voltase
occasionally. To assist the proce$s, the arm 16 of FIG. 1 may
be biased slightly towards the center of the disc 20.
In FIG. 6, 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 of parallel leaf springs 120, 122. The spring force of
the leaf springs 120, 122 is generally insufficient to maintain
the springs in the horizontal position ~ith the playback head
14 unsupported by the fluid bearing that is generated by the
rotating disc 20. Within the read head 14 is the fluid bearing
member 50 and the microscope type objective lens 52. Also
contained in the read head 14 are the fixed and articula-ted
mirrors 54, 56, 57 necessary to direct the beam of light from
the source to the lens 52 and back from the surface of the disc
20.
A support post 124 extends outward of the read head
14 toward the inner end of the arm 16. Mounted to this support
post 124 is a bias spring 126, the other end of which is
fastened to a lever 128. The lever 128 is coupled to the arm
16 and, through a flexible cable 130, connects to a cam and
follower assembly 132, to be described in connection with FIG.
7, below.
Also included, but not described in detail, are
.
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appropriate interlocking solenoid assemblies opexating in
conjunction with the cam and follower assembly to maintain
the read head 14 out of contact with the clisc 20 as the arm
16 swings out of engagement with the disc 20, and whi.ch act
to prevent damage if, for any reason, the disc 20 should slow
appreciably while being tracked by the read head 14.
The bias spring 126, ~hen compressed, acts like a
solid rod~ enabling the lever 128 to directly cam the read head
14 upward and away from the disc 20, if this configuration is
- 10 desired. Alternatively, when the read head 14 is in position
over the disc, the lever 128 rotates in the opposite direction,
relieving the compression on the spring 126. ~nder normal cir-
cumstances, the weight of the read head 14 is supported by the
fluid bearing member 50 on the disc, thereby enabling the leaf
springs 120, 122 to be substantially parallel and horizontal.
According to the present invention, an additi~nal
bias is provided through t-he use of 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 relative surface velocity changes as the moving
arm 16 progresses toward the center of the disc and the fluid
bearing is less able to support the read head. Therefore, at
the outset, the lever 128 is rotated in the downward direction,
applying a stretch to the spring 126 which, in turn, imparts 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 of the disc 20 and the
surface velocity is reduced, a cam follower arrangement
-16-
bm.
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gradually rotates the lever 128 in the upward direction,
reducing the tension of the spring 126, thereby lessening the
bias on the read head 14. By selecting an appropriate cam
contour, the bias on the fluid bearing 50 can be maintained at
an optimum value for constant separation from the disc 20 for
the surface velocity of the disc at any radial location.
Turning now to FIG. 7, there is shown one form of
cam and follower assembly 132 that can drlve the lever 128
through the flexible cable 130 (also shown:in FIG. l)q A cam
140 is cut so that at the outermost position of the arm 16, a
follower 142 rests on a high lobe which maintains the head 14
in an "up" position, safely out of contact with the edge of the
rotating disc 20.
As the arm 16 tracks inwardly., the follower 142
immediately proceeds to the innermost point on the cam 140
surface, applying maximum bias to the read head 14. As the
arm then continues inwardly in the radial direction, the
follower 142 gradually rides outwardly from the center of the
cam 140, thereby reducing the bias forces on the read head 14.
. It is clear that tecnniques are readily available
fo~ transmitting simple mechanical motion from the cam follower
assembly 132 to the arm 16, and the specific details are
unnecessary in the present applicationr
In FIG. 8, there is shown an alternative configuration
- for the articulated mirror assembly that is mounted on the read
head 14. In this alternative embodiment, a fixed mirror 150
and an articulated mirror 152 are arranged on converging planes.
An incoming beam in the horizontal direction impinges upon the
articulated mirror 152, and through multiple reflection between
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the fixed mirror 150 and the articulated mirror 152, the
beam is ultimately rotated through 90 and is directed down-
ward into the reading assembly. Similarly, 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 of incidence of the mirror 150 and the
angle of convergence between the mirrors 150 and 152 are
controlled so that the incoming beam makes a plurality of
reflections off of the two mirrors before being directed into
the disc. Moreover, since the pair of mirrors, in addition to
providing a "folded" light path, also rotates the beam through
90, a separate 45 mirror can be omitted, 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 in 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 wi-th less light loss in the mirror
system.
Thus, there has been shown an improved video disc
reading assembly which steers the illuminating 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 detector
provides both the information signal and the servo signals
.
-18-
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necessary to track the information channel.
A novel air bearing assembly has also been dis-
closed, which enables a microscope lens to tra~el 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 of relative velocity between the disc
7 and the bearing member.
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