Note: Descriptions are shown in the official language in which they were submitted.
70b3~ ,
VIDEO DISC PLAYER
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 photosensitive 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 respresent-
ative 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 trans-
ducer included a servo controlled pair of flexible, ~ibre 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 ener~y.
One of the major problems to be encountered in the recording and
reproduction of video informationj arises directly from a con-
sideration of the energy levels involved in such a process and
the restraints imposed by the considerations of size, weight and
operating conditions,
To be commerically desirable as a home instrument, the
system should be able to store and reproduce a "program" of at
least 15 to 30 minutes in length. The record disc should be of
0 an easily handled size, comparable to the phonograph records
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1070834
currently in use. If the playback turntable was operated at
1800 rpm, some 54,000 revolutions would provide 30 minutes of
playback, ~ssuming a 1 micron track width and 1 micron spacing
between adjacent tracks, a circular band approximately 4.25
inche~ 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 progxam 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 spa~e
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 "presence" or "absence" of a signal can be
distinguished.
It has been discovered, in attempting to utilize the
transmitted radiation techniques of the prior art, that the
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 substantial magnification is
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1070834
required 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 pro-
vided before the light transmitted through or reflected from the
disc will be of sufficient intensity to register upon the photo-
sensitive device.
In a preferred embodiment of the present invention, an
articulated mirror is utilized in conjunction with a second
mirror to provide multiple reflecting paths. With a plurality
of reflections, assuming the use of a highly collimated source,
small amounts of mirror motion are necessary to move the point
of impingement of the radiant spot upon the disc. Moreover, a
plurality of reflections provides a longer optical path which
.. :.
enable~ the use of longer focal-length lense9, for directing a
radiant spot to the disc and for focusing the image of the
reflected spot upon the photosensitive transducer.
An important aspect of the present invention is the
ability to direct the illuminating radiation to a particular spot
and to return the information from the spot thus illuminated 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 differential 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 satisfactory. A difference "curve
A
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~070834
following" technique described in the patent to W.D. Munro,
U.S. Patent No. 2,838,683, issued June 10, 1958, has suggested
an alternative solution.
According to the present invention there is provided
an apparatus for reading information recorded on an information
track on a surface of a disc, the apparatus including source
means for producing a reading beam of light radiation with beam
directing means being provided for directing the reading beam
from the source means along a folded, substantially U-shaped
optical path and then onto the information track. The reading
beam is modulated and reflected by the information track, and
sensing means is provided for receiving the modulated beam of
radiation reflected from the information track.
In a specific embodiment of the invention, the beam
directing means includes optical means for directing the reading
beam along the first and third path portions parallel to the
disc surface, and a second path portion joining one end of the
first path portion with one end of the third path portion, and
a fourth path portion joining the end of the third path portion
remote from the second path portion with the information track,
the end of tha first path portion being remote from the second
path portion joining with the source means. In a specific
embodiment of the invention, the first and third path portions
of the folded path are parallel to each other. The fourth path
portion may be perpendicular to the first and third path portions
and perpendicular to the disc surface.
In a specific embodiment of the invention, a pair of
mirrors is disposed in the fourth path portion, the mirrors being
arranged in opposing spaced relation to one other to cause the
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.
1070~334
reading beam to be reflected at least once from each of the
mirrors, and means is provided for articulating at least one
of the mirrors to steer the reading beam to a precisely selected
location on the surface of the disc.
The present invention also resides in a method of reading
information recorded in an information track on a reflective
surface of a disc, the method including the steps of producing
a reading beam from a source of light radiation, directing the
reading beam from the source along a folded, substantially U-
shaped optical path and then onto the information track, thereading beam being modulated and reflected by the information
track, and sensing the modulated beam of radiation reflected
from the radiation track.
In a specific embodiment of the invention, the beam
directing step includes optically directing the reading beam
..
along the path to produce first and third path portions parallel
to the disc surface, a second path portion joining one end of
the first path portion with one end of the third path portion,
and a fourth path portion joining the end of the third path
portion remote from the second path portion with the information
track, the end of the first path portion being remote from the
second path portion joining with the source means.
More specifically, the first and third path portions of
the folded path may be parallel to each other, and the fourth
path portion may be arranged perpendicular to the first and
third path portions and perpendicular to the disc surface.
The method of the present invention in a specific embodi-
ment may further include the steps of interposing a pair of
mirrors in the fourth path portion, arranging the mirrors in
-- 5 --
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. : - .- -
~070834
opposing spaced relation to one another to cause the reading
beam to be reflected at least once from each of the mirrors,
and articulating at least one of the mirrors to steer the reading
beam t:o a precisely selected location on the surface of the disc.
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 with the accompanying drawings in which several pre~-
ferred embodiments of the invention are illustrated by way ofexample. 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 THE 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 view of an alternative articulate~
mirror assembly;
FIG. 4 is a block diagram of a suitable detector and
tracking circuit;
FIG. 5 is a block diagram of an optical detector of the
prior art suitable for use in the present invention;
FIG. 6 is an enlarged side view of the optical head and
air bearing assembly;
FIG. 7 is a top idealized view of a cam and follower
assembly for controlling the bias on the air bearing assembly;
and
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FIG. 8 is a side view of another alternative
articulated mirror arrangement useful in the system of the
present invention.
DETAILED DESCRIPTION OF THE INVENTION
Turning first to FIG. 1, there is shown, in side view,
a playback assembly 10 suitable for use in the present in-
vention. The playback assembly 10 includes a laser element 12
which moves with 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 20, which has video information recorded
upon it is mounted on a turntable 22, which i9 adapted to
rotate the disc 20 at a relatively high speed. In the preferred
embodiment, the turntable speed is set at 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
reading 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 system 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.
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1070834
In operation, the laser directs a reading light beam
26 to the surface of the disc 20 through the optical system
including a mirror arrangement 29. The information recorded
upon the disc interacts with the impinging beam and a reflected
beam i8 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 lO can also be
controlled by the servo signals which changes the position of
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 o~ the turntable
22. In any case, the playback head lOcan be made to track the
information channel recorded on the disc 20 with a "coarse"
adjust~ent being applied to the driver of the rotatable element
24 and a "fine" adjustment 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 splitting prism 30. The prism 30 is rotated
slightly with respect to the optical path. ~ lens 32 is
-- 8 --
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~ 1070834
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.
A returning beam 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 splitter30 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 from 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 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 adjacent to and supportive of a microscope objective lens 52.
A limited amount of vertical adjustment is available in the
objective lens 52. Directing the illumination to the objective
lens 52 is an articulated mirror 54 which is part of mirror
arrangement 29 and mounted adjacent to and cooperates with a
second or fixed mirror 56 that is substantially parallel with the
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1070834
articulated mirror 54. The fixed mirror receive~ 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
embodi~lent 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 be~ore 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 the drawing. As shown, a mirror driver
60 is connected to one end of the mirror 54 and is operable to
impart 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 of impingement of the
read beam 26 will be shifted to the left. This would 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"
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only the illuminated area, transmitting 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 pro~ide 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 driver 60 therefore, need only impart
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 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 of a third
mirror 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
of the information channel. The second driver 60" is used to
translated the beam in the circumferential direction, to provide
time synchronization, if desired, and to compensate for
eccentricity.
In other embodiments, the problem o 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, only the single articulated mirror is used.
Turning next to FIG. 4, there is shown a preferred
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~070834
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 resulting from drift can be integrated, and the output 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 signal of appropriate
polarity will be provided to correct the position of 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 is 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. S, there is illustrated the prior art optical
dete~tor electronics utilized and shown as FIG. 10 in the previously
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~07()834
issued Gregg, et al., U. S. Patent No. 3,530,258, 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 differenced, 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 respectively.
As shown in FIG. 5, a dual photo detector has two
sections 96, 98 whose outputs are applied to respective amplifiers
100, 101. The outputs of the amplifiers 100, 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 trans-
ducer.
The signal amplitude from the first photo de~ector
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, the resulting error signal is
amplified in an amplifier 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. ~he direction
and amount of the shift depends on the polarity and amplitude of
~ -13-
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the error signal, so as to maintain the spot in perfect registry
with ~he 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 of 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. 1 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 is reaches a pre-
determined value, at which it triggers a solenoid. This solenoid,
in turn, actuates a light-duty friction ratchet which then turns
the pick-up arm through a very small angle as is taught in Gregg
et al, supra.
Another method also suggested in Gregg et al, supra,
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 revolution of
the disc. In this çase, the integrated signal of the first method
is used to interrupt the motor voltage occasionally. To assist
the process, 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 with the playback head 14 unsupported by
0 the fluid bearing that is generated by the rotating disc 20.
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107()834
Within the read head 14 is the fluid bearing mem~er 50 and the
microscope type objective lens 52. Also contained in the read
head 14 are the fixed and articulated mirrors 54, 56, 57 necessary
to direict 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
appropriate interlocking solenoid asse~bblies operating in con-
junction with the cam and follower assembly to maintain the
read head 14 out of contact with the disc 20 as the arm 16 swings
out of engagement with the disc 20, and which 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, when 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 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. Under normal circumstances,
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 additional bias
is provided through the use of the bias spring 126 to maintain
a substantially constant separation between the read head 14 and
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~o7v834
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 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 drive the lever 128 th~ough
the flexible cable 130 (also shown in Fig. 1). 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 techniques are readily available for
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transmitting simple mechanical motion from the cam follower
assembly 132 to the arm 16, and the specific details are un-
necessary in the present application.
In FIG. 8, there is shown an alternative configuration
for the articulated morror 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
the fixed mirror 150 and the articulated mirror 152, the beam is
ultimately rotated through 90 and is directed downward 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 o~ 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. ~oreover,
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 of
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 with 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
-17-
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107~834
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 necessary to track the
information 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
p,ovided to impart a variable bias to the fluid bearing as a
function of relative velocity between the disc and the bearing
member.
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