Note: Descriptions are shown in the official language in which they were submitted.
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APPARATUS AND METHOD FOR RETRIEVING
INFORMATION FROM A VIDEODISC
_ CHNICAL FIELD
The present invention relates to reading of
stored in~ormation from an information storage member,
and more particularly to reading video information from
a videodisc.
EACKGROUND OF THE PRIOR ART
Sy~tems have heretofore been developed for
recordlng and reproducing signals at video frequencies
upon disc~, tapes, or other media. Such systems have
utilized, among other things~ optical recording upon
photo~ensltive media, electron beam recording on thermo-
plastic ~urfaces, and still other systems provide an
instantaneously reproducible record of video information.
The prior art can generally be divided lnto
system~ utilizing photographic surfaces, systems util-
izing electron beam sensitive surfaces, ~agnetic record-
ing systems, and as in the present invention, systems
in which a radiant energy beam causes an irreversible
change to a surface, thereby "writing" information
thereon to be read out by a compatible reading apparatus.
Photographic systems have been described in the
patents to P. C. Goldmark, et al, No. 3,234,326, which
teaches recording on a continuous web such as a tape or
~ilm, or the patent to W. R. Johnson, No. 3,361,873,
which teaches the photographic recordation of video
information on a rotating disc in a spiral path.
, The patent to W. C. Hughes, et al, No. 3,283,310
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is illustrative of recordation of information on a
thermo-plastic film surface, which utilizes an elec-
tronic beam writing apparatus such as was disclosed in
U. S. Patent No. 33120,991.
Yet other systems have employed an electron
beam to record information on a special storage medium.
One such system has been disclosed in the patent to
D. P. Gregg, No. 3,350,503. An alternative scheme
utilizing an electron beam on a photosensitive medium
10 such as photographic film has been taught in the patent
to R. ~. Dubbe, et al, No. 3,444,317.
In recent years, alternative methods have been
disclosed for high density recording which are based
upon either removal of material or vaporization of
material by laser beam bombardment. These methods
have been discussed briefly in the magazine "Elec-
tronics", of March 3, 1969, at page 110. Further, a
"laser thermal mlcro image recorder" was described in
some detail by C. O. Carlson and H. D. Ives in a paper
given at the 1968 WESCON meetin~ estern Electronic
Show and Convention), which paper wa~ published in
;Volume 12 of WESCON Technical Papers for 1968, at page
1 of Section 16/1. The authors have referred to artlcles
in the December 23; 1966, issue of "Science", Volume
54, No. 3756, at pages 1550 and 1551, the Proceedings
of the Fall Joint Computer Conference of 1966, pages
711-716, and an article in the "Bell Systems Technical
Journal", of March 1968, pages 385, 405.
These publications disciose a recording tech-
;3o nique which utilizes a thin metalllc film coating upon
a substrate. The thin metal film, under applied heat,
melts rapidly and forms small globules within a recorded
spot. A highly concentrated spot of laser illumination
can apply sufficient heat in a short enough time so
that a suitably modulated laser beam impinging upon a
moving surface can produce a pattern of holes in the
metallic surface which, when "read back', can reproduce
the information recorded.
As pointed out in the Carlson and Ives paper,
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supra, khe slze of the recorded spot or hole can be
much smaller than the diameter of the imaged laser beam.
By an appropriate choice of metal film materialg film
thickness, laser divergence and spot power, an appro-
priate system can be designed to record video frequencieswith reasonably high reso].utior.. Howeverg the quality
of the video signal reproduced from such recordings
has not been good due to a low signal-to-noise ratio
resulting from the direct recording of the video signal
onto the recording medium. Additionally, the use of a
modulated laser beam to select~vely produce a pattern of
holes on the metallic surface of a videodisc is not
conducive to faithfully represent an amplitude varying
analog video signal. Likewise, reading back the re-
corded information in the form of a track of holes orspots on the surface of the disc is ineffective to
faithfully recreate an analog video signal from the
recorded track on the disc. The lack of fidelity in
the reproduced video signal can be atkributed primarily
to the 'Lna~ility of the laser beatn modulator to "write
precise analog signals in the coating of the disc.
Consequently, the rcproduced signal developed by the
reader suffers ln the same manner.
BRIEF SUMMARY OF THE INVENTION
The present lnvention provides an improved
apparatus and method for reading stored information on
a videodisc, the s~tored information having the form of a
lineal series of regions representing a frequency modu-
lated information signa]
3 In accordance w-lth the invention, there is
provided an apparatus and method for reading stored
information on videodiscs, the information being in
the form of a lineal serles of regions of alternately
high and low reflect'lv-lty normal to the surface of the
disc. That is, when vlewed from a point above the sur-
face of the discg an lmpinging light beam directed nor-
mal to the surface would reflect alternately with high
and low levels of reflected lighk in a direction oppo-
site that of the impingi.ng l-lght beam.
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The reading system includes a polarized colli-
mated reading beam of light, and an optical system
between the light source and the videodisc for focusing
the reading beam upon the videodisc. Relative motion is
provided between the reading beam and the alternate
regions for generating re~lections ~rom the high re-
flective regions on the videodisc. A light sensor is
responsive to the reflections for generating a fre-
quency modulated electrical signal. The ~requency
modulated electrical signal so developed has its informa-
tional content in the form of a carrier frequency which
has a frequency varying with time.
In a preferred embodiment, the disc-shaped
videodisc has uniform rotational motion imparted to it -~
by atrotational driver synchronized with a transla-
tional driver. The translational driver provides rela-
tive motion between the reading beam and the disc for
causing the focused light beam to move radially across
the videodisc sur~ace as it rotates. An electrical
synchronizer provides a constant relatlonship between
the two drivers
The videodisc has a lineal series of regions
positioned in track-like ~ashion upon its upper surface.
The sequence of alternate regions of high and low re-
flectivity normal to the disc surface represents afrequency modulated signal.
; The reading system preferably includes a low
power laser for producing a reading light beam, a
polarizing beam splitting cube, and a quarter wave
plate. Light passes from the laser source through the
polarizing beam splitter and to the quarter wave plate.
The quarter wave plate is used to rotate the incident
reading light beam forty-five degrees on passage through
to the videodisc. At the videodisc the optical system
includes an objective lensJ and a hydrodynamic air
bearing for supporting the lens above the surface of
the videodisc. The objective lens has an aperture larger
in diameter than the diameter of the reading beam. The
- optical system makes use o~ mirrors and lenses for
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1057399
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diverging the reading beam from the laser source to at
least fill the entrance aperture of the objective lens.
The reflected light from the videodisc returns
by means of mirrors and lenses through the quarter wave
plate where it i8 once again rotated forty-five degrees
to impart a total of ninety degrees rotation relative to
the beam from the source. The emerging beam passes on
to ~he polarizing beam splitting cube which is adapted
to direct the reflected reading beam to a sensor and
away from the laser source.
The reflected light from the videodi~c direc-
ted toward the sensor represents a frequency modulated
signal having its informational content in the form of a
carrler frequency having a frequency varying with time.
The frequency modulated output signal of the sensor
may be changed into a time dependent voltage signal for
display on a standard television monitor.
The novel eatures which are believed to be
characteristic of the invention~ both as to organization
and method of operation, together with further ob~ects
and advantages thereo~, will be better understood from
the following description considered in connection with
the accompanying drawings in which several preferred
embodiments of the invention are illustrated by way of
, . 25 example. It is to be expressly understood, however,
that the drawings are for the purpose of illustration
and description only and are not intended as a defini-
tion of the limits of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a generalized diagram of the appara-
tus of the present invention;
` FIG. 2 is a view of the optical path through
the objective lens of FIG. l;
FIG. 3 is a representational indication of the
, 35 relative spacing between a point of impingement of the
; writing beam and of the reading beam; and
FIG. 4 is a diagram of a novel Pockels cell
stabilizing circuit.
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DETAILED DESCRIPTION OF THE INVENTION
Turning first to FIG. 1, the writing apparatus
~ includes a writing head 12 which is, in the pre~erred
A embodiment, a dry microscope objective lens 14 mounted
5 upon an air bearing support member 16. A 40X lens has
been ~ound to be satisfactory. A disc 18 is specially
prepared and may be constructed according to the
teachlngs of the prior art, in which a substrate has
coated thereon a very thin film o~ a metal with a reason-
10 ably low melting point and a high surface tension.
A crystal oscillator 20 controls the drive
elements. The disc 18 is rotated by a first, rotational
drive element 22 which is coupled to a spindle 24. A
second, translational drive element 26 controls the
15 position of the writing head 12.
A translating carriage 28, which is driven by
the translational drive element 26 through a lead screw
and travelling nut, moves the writing head 12 in the
radial direction relative to the rotating disc 18. The
20 carriage 28 is provided with appropriate mirrors and
len~es so that the remainder o~ the optics and elec-
tronics necessary to the writing device may be perman-
ently mounted.
In the preferred embodiment of the present
25 lnvention, the beam o~ a polarized cutting laser 30,
which is an argon ion laser, is passed through a
Pockels cell 32 which is driven by the Pockels cell
driver 34. An FM modulator 36 receives the video signal
that is to be recorded and applies the appropriate
30 control signals to the Pockels cell driver 34.
As described hereinafter, the video in signal
is o~ the type displayable on a TV monitor. Accord-
ingly, it is a voltage varying with time signal. The
FM modulator 36 is of standard design and converts the
35 voltage varying with time signal to a frequency modu-
lated signal having its informational content in the
form of a carrier frequency having frequency changes with
time corresponding to said voltage variations with time.
As is known3 the PoGkels cell 32 responds to
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applied signal voltage by rotating the plane of polariza-
tion of the light beam. Since a linear polarizer transmits
light only ln a predetermined polarization plane, a
polarizer, such as a Glan prism 38 in the preferred
embodiment, is included in the writing beam path to provide
a modulated writing beam 40. The modulated writing beam
effectively follows the output of the FM modulator 36.
The modulated writing beam 40 emerging from
the Pockels cell-Glan prism combination 32, 38 is applied
to a first mirror 42 which directs the writing beam 40
to the translating carriage 28. The first mirror 42
transmits a portion of the writing beam 40 to a Pockels
cell stabilizing circuit 44 which responds to the average
intensity of the writing beam to maintain the energy level
f of the beam.
A lens 46 is inserted in the path of the writing
i beam 40 to diverge the substantially parallel beam so that
it will spread to fill the entrance aperture of the
ob~ective lens 14 for optimum resolution. A dichroic
mirror 48 is included in the path oriented to substantially
transmit all of the writing beam 40 to a second, articulated
mirror 50. The articulated mirror 50 then directs the beam
through the lens 14 and is capable of shifting the point
of impingement of the beam 40 on the surface of the disc 18,
A series of holes is formed in the metal coating
by the writing beam. One hole is formed for each cycle
of the FM modulated signal represented by the modulated
writing beam 40. ~ince the modulated writing beam tracks
the output of the FM modulator 36, the holes formed in the
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coati.ng also track the output of the FM modulator.
0bviously, slnce the informational content in the OUtptlt
signal of the FM modulator 36 is ln the form fff frequency
changes ln time about a carrier frequency, and since
the "hole", "no hole" sequence represents the stored
lnf~rmatlon ~ d slnce the disc
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18 is rotating at a uniform speed, the "hole", "no
hole! sequence changes to represent the stored video
in~ormation by the holes being formed closer or farther
apart and the size of the hole becomes larger or smaller
as the writing beam 40 changes under the control of the
FM modulated output signal from the FM modulator 36.
The ob~ective lens 14 and the associated air
bearing 16 e~fectively fly on a cushion o~ air at a
~ubstantially fixed distance from the surface of the
di~c 18. That distance is determined by the geometry
of the bearing 16, the linear velocity of the disc 18,
and the force used to load the head against the disc 18.
The fixed spacing is required because the focal toler-
ance of a lens capable of resolving a l~m spot is also
of the order of 1 ~m.
A second, relatlvely low-power laser 52 pro-:
vides a monitoring beam 54. In the preferred embodi-
ment, the reading laser 52 is a helium-neo device which
enables the reading beam 5L~ to be distinguished ~rom the
writing beam 40 by wavelength. A polarlzing, beam
3plitter cube 56 tran~mit~ the reading beam 54 to a
mirror 58 that directs the beam 54 through a second
diverging lens 60 that spreads the reading beam 54 to
fill the entrance aperture of the objective lens 14.
A quarterwave plate 62 is placed in the opti-
cal path and, in con~unction with the plane polarizing
beam splitter 56, prevents light retlected from the
disc 18 ~rom re-entering the laser 52 and upsetting its
mode of oscillation. The quarterwave plate 62 rotates
the plane o~ polariæation of the beam by 45 degrees on
each pass so that the reflected beam is rotated 90
degrees with respect to the polarizing beam splitter
56 and is therefore not passed by it.
A second mirror 64 in the reading beam 54 path
directs the beam into the dichroic mirror 4~ and is
capable of l-imited ad~u~tment so that the paths of the
writing and reading beams are substantially identical,
except that the reading beam "spot" impinges on the disc
18 downstream from the writing beam spot as explained in
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~057399
greater detail below.
A filter 66 that is opa~ue to the argon ion
beam is interposed in the path of light reflected from
the beam splitter 56. The He.Ne reading beam 54 that
is return~d from the disc surface ls able to pass
through the filter 66 and through a lens 68 onto a photo-
detector 70.
The reflected light of the reading beam im-
pinges upon the photodetector 70. The photodetector 70
operates in its standard manner and generates an elec-
trical current representative of the light impinging
; thereupon. In this case~ the photodetector generates
the signal represented by the "hole", "no hole" con-
figuration formed in the coating. The "hole", "no
hole" configuratlon is representative of the output of
the FM modulator 36. The output of t;ie FM modulator 36
~ is a carrier frequency having frequency changes with
; time representing the video signal to be recorded. The
"hole"~ "no hole" configuration is representative o~ a
carrier fre~uenc~J having frequency changes with time
representlng the stored video signal. The output of
the photodetector 70 is an electrical signal repre-
senting the stored carrier frequency having frequency
changes with time representing the stored video signal.
The output of the photodetector 70 is applied
to a preamplifier 72 whlch provides a signal of suffi-
cient amplitude and signal strength for subsequent
; utilization. A video discriminator 74 then provides a
video output signal which can be utilized in several
3 ways, two of which are shown~ as examples only
The discriminator 74 is of standard design and
function. It takes the frequency modulated signal from
the photodetector 70 and changes it to a time dependent
voltage signal having its informational content in the
form of a voltage varying with time format suitable for
display in the TV monîtor 76.
In a first application, the vicleo output is
applied to a TV monitor 76 and an oscilloscope 78. As
` is well known, the TV monitor ls responsive to a voltage
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1~)57399
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varying with time signal. The information to be dis-
played on the TV monitor is represented by a voltage
change with time.
The TV monitor 76 shows the picture fidelty
of the recording, and the oscilloscope 78 indicates the
signal-to-noise ratio of the record and the quality of
the cutting, whether it is light or heavy. Not shown,
an appropriate feedback loop could be provided through
the Pockels cell stabilizing circuit 44 to assure an
adequate dlscrimination on the disc between a "hole" or
"black" area and "no hole or "white" area.
As an alternative utilization, the video out-
put of the discriminator 74 is applied to a comparator
80. The other input of the comparator 80 is taken from
the video input signal which is directed through a
delay line 81. A delay that is equal the accumulated
delays of the writing system and the time elapsed
between the instant of writing of the information and
the time required for that incremental area of the disc
to reach the reading point must be imparted to the input
vldeo signal.
Ideally, the video output signal of the dis-
~iminator 74 should be identical in all respects to the
video input signal, after the proper delay.
As previously mentioned, the output from the
discriminator 74 is a voltage varying with time signal.
The video in signal is also a voltage varying with time
signal. Any differences noted represent errors which
might be caused by imperfections in the disc's surface
or malfunc~ions of the writing circuits. This applica-
tion, while essential if recording digital information,
is less critical when other information is recorded.
The output of the comparator circuit 80 can be
quantized and counted~ so that an acceptable number of
errors can be established for any disc. When the errors
counted exceed the standard, the writing operation can
be terminated. If necessary, a new disc can be written.
Any disc with excessive errors can then be reprocessed
- to serve as a "new" disc for a subsequent recording.
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~057399
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Well-known techniques are available to trans-
late the write head assembly 12 in the radial direction
with respect to the rotating disc 18. While in FIG. 1
the rotational and translational drives 20, 22 are in-
5 dicated as independent, the drives are synchronized to :
enable the writing assembly 12 to translate a prede-
termlned increment for each revolution of the disc 18,
by means of the common crystal osclllator 20.
Turning next to FIG. 2 there is shown, in
somewhat exaggerated form, the slightly differing opti-
cal paths of beam 40 from the writing laser 30 and the
beam 54 from reading laser 52. The writing beam 40
coincides with the optical axis of the microscope objec- -
tive lens 14. The reading beam 54, in contrast, makes
15 an angle with the axis so that it falls some distance
X, equal to ~ kimes the focal length of the ob~ective,
"downstream" from where the writing beam 40 is "cutting". -~
The resulting delay between reading and writing allows
the molten metal to solidify so that the recording is
read in its final ~tate. If lt ~re read too soon
while the metal was still molten, it would not provide
pertinent information for ad~usting the recording par-
ameters.
This is best indicated in FIG. 3 where two
points in the same information channel are shown as dis-
placed. The point A, which is the point of impingement
~ of the writing beam 40, is shown as being on the optical
t axis ~ the ob~ective lens 14. Separated from point A,
in the direction of medium motion, as indicated by the
30 arrow, is the reading point B, which is at an angle
from the axis of the microscope ob~ective lens 14.
A distance between points A and B of two m~ has provided
a satisfactory monitoring of the writing operation.
; Turning finally to FIG. 4J there is shown an
35 idealized diagram of a Pockels cell stabilizing circuit
44, suitable for use in the apparatus of FIG. 1. As is
known, a Pockels cell rotates the plane of polarization
of the applied light as a function of an applied volt-
~ age. Therefore; the Pockels cell is used to rotate
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~oS7399
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plane polarized light, and the rotated light is passed
through a plane polarizer, such as a Glan prism. The
light iSSUillg from the polarizer will be amplitude
modulated in accordance with the applied voltage.
Stated another way, the standard operating mode
of a Pockels cell 32 and Glan prism 38 is for use as a
light intensity modulating means. Each cycle from the
FM modulator drives the Pockels cell through its full
operating range of ninety degrees. Within this oper-
ating range of ninety degrees, one operating point
passes all light applled thereto and identified as a
full light transmitting state. A second operating
point passes no light and is identi~ied as a full light
blocking state. The Pockels cell itself only rotates the
plane of polarization. The Glan prism passes light in
one plane of polarization and no light in the plane
displaced ninety degrees from that plane in which all
the light passes.
Depending upon the individual Pockels cell, a
voltage change o~ approximately 100 volts will cause
the cell to rotate the plane of polarization through
360 degrees. However, the transfer characteristic of
an individual cell may drift spontaneously, correspond-
ing to a voltage change of - 50 volts, and accordingly,
a feedback loop is desirable to maintain the cell with-
in a useful, reasonably linear, operatlng range.
The stabilizing circuit 44 includes a photo-
sensitive silicon diode 82, which is positioned to
receive a portion of the writing beam 40 reflected from
the mirror 42 of FIG. 1. The silicon diode 82 functions
in much the same fashion 8S a solar cell and is a source
; of electrical energy when illuminated by incident radia-
tion. One terminal of the silicon diode 82 is connected
to common reference potential 84, indicated by the
conventional ground symbol, and the other terminal is
connected to one input of a differential amplifier 86.
The silicon cell 82 is shunted by a load 88 which
enables a linear response.
The other input to the differentlal amplifier
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86 is connected through an appropriate potentiometer 90
to the common reference 84. A source of power 92 is
coupled to the potentiometer 90, which enables the
setting of the differential amplifier 86 to establish
the average light level transmitted by the Pockels cell
32 .
Accordingly, a pair of output terminals of the
dlfferential amplifier 86 are respectively connected
through resistive elements 94, 96 to the input terminals
of the Pockels cell 32 of FIG. 1. It is noted that the
Pockels cell driver 34 is a.c. coupled to the Pockels
cell 32, while the dlfferential amplifier 86 is d.c.
coupled to the Pockels cell 32.
In operation, the system is energized. The
light from the writing beam impinging on the silicon
diode 82 generates a differential voltage at the input
to the differential emplified 86. Initially, the poten-
tiometer 90 is adjusted to produce light at a predeter-
mined average level of intensity. mereafter, if the
~0 average level of intensity impinging on the silicon
cell 82 either increases or decreases, a correcting
voltage will be generated in the differential amplifier
86. The correcting voltage applied to the Pockels cell
32 is of a polarity and magnitude adequate to restore
25 the average level of intensity to the predetermined
level.
Thus there has been shown an improved video
disc recording assembly. A microscope objective lens
mounted on an air bearing "flies" at a predetermined
distance from the surface of a metallized disc. me
metallized coating is such that a laser beam can, under
suitable modulation, deliver sufficient energy to melt
localized areas of the surface. Under surface tension,
the molten metal retracts leaving a clear area of appro~
35 imately one micron in diameter.
A second, low-energy laser utilizing substan-
tially the same optical path is directed through the
, same microscope objective lensg but is brought to the
surface of the disc at a slight distance "downstream"
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from the point of writing. The reading beam is returned
through an appropriate o~tical system that excludes the
reflected energy of the writing beam and enables an
analysis ~ the information that has been written on
the disc.
The playback information can, among other
things, control the intensity of the writing beam to
assure adequate "recording levels"g determine whether
an unacceptable number of errors have been made in the
10 recording process.
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