U.S. patent number RE32,431 [Application Number 06/732,033] was granted by the patent office on 1987-06-02 for system for rotating an information storage disc at a variable angular velocity to recover information therefrom at a prescribed constant rate.
This patent grant is currently assigned to Discovision Associates. Invention is credited to Ludwig Ceshkovsky, Wayne R. Dakin.
United States Patent |
RE32,431 |
Dakin , et al. |
June 2, 1987 |
System for rotating an information storage disc at a variable
angular velocity to recover information therefrom at a prescribed
constant rate
Abstract
Method and apparatus for recovering information at a
substantially constant rate from a rotatable information storage
disc. The information is stored on the disc in a plurality of
substantially circular and concentrically arranged information
tracks, with a substantially uniform recording density, and it is
recovered by controllably rotating the disc, relative to a
transducer, at an angular velocity substantially inversely
proportional to the radius of the corresponding track. The
apparatus includes a coarse speed control potentiometer for
producing a measure of the radius of the particular information
track from which information is being recovered, and fine speed
control means for comparing the relative phase angles of a periodic
signal in the recovered information and a periodic reference signal
and for producing a fine speed control signal proportional to the
phase difference. The measure of radius and the fine speed control
signal are summed together and coupled to a voltage-controlled
oscillator to produce a composite speed control signal, which is
coupled to a servo to rotate the disc at a corresponding angular
velocity.
Inventors: |
Dakin; Wayne R. (Redondo Beach,
CA), Ceshkovsky; Ludwig (Fountain Valley, CA) |
Assignee: |
Discovision Associates (Costa
Mesa, CA)
|
Family
ID: |
27112343 |
Appl.
No.: |
06/732,033 |
Filed: |
May 8, 1985 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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Reissue of: |
961404 |
Nov 16, 1978 |
04223349 |
Sep 16, 1980 |
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Current U.S.
Class: |
386/204; 369/111;
369/47.4; 386/222; 386/353 |
Current CPC
Class: |
G11B
19/24 (20130101) |
Current International
Class: |
G11B
19/24 (20060101); H04N 005/785 () |
Field of
Search: |
;358/322,323,338,339,342
;369/44,45,46,47,48,50,56,111,43 ;360/73 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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2257817 |
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May 1974 |
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DE |
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49-16409 |
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Feb 1974 |
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JP |
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52-40308 |
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Mar 1977 |
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JP |
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1550915 |
|
Aug 1979 |
|
GB |
|
1602893 |
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Nov 1981 |
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GB |
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Primary Examiner: McElheny, Jr.; Donald
Attorney, Agent or Firm: Clark; Ronald J.
Claims
We claim:
1. Apparatus for recovering a video signal from a rotatable
information storage disc, wherein the video signal is stored on the
disc in a plurality of substantially circular and concentrically
arranged information tracks, each track having a substantially
uniform recording density, said video signal being recovered from
the disc at a substantially constant rate by a transducer that is
movable radially relative to the disc to be in a prescribed
relationship with a selected information track, said apparatus
comprising:
a potentiometer coupled to the transducer for producing a coarse
speed control signal that varies substantially linearly with the
radius of the selected information track from which the video
signal is being recovered by the transducer;
oscillator means for producing a periodic reference signal having a
prescribed constant frequency that corresponds to the rate at which
the video signal is to be recovered from the disc;
fine speed control means for comparing the phase angle of a
periodic pilot signal included with the video signal recovered from
the disc, with the phase angle of the periodic reference signal,
and for producing a fine speed control signal having an average
value that corresponds to the difference in the respective phase
angles, said fine speed control means including a phase-locked loop
for detecting the periodic pilot signal;
means for summing together the coarse speed control signal and the
fine speed control signal, to produce a composite voltage
signal;
voltage-controlled oscillator means for producing a composite speed
control signal having a frequency that corresponds to the composite
voltage signal; and
means for rotating the disc at an angular velocity corresponding to
the frequency of the composite speed control signal;
said .[.coarse speed control means.]. .Iadd.potentiometer
.Iaddend.and said voltage-controlled oscillator means being
constructed with sufficient linearity that the composite speed
control signal will always have a frequency that causes the disc to
be rotated at an angular velocity that results in the recovered
pilot signal having a frequency within the pull-in range of said
phase-locked loop, whereby said fine speed control means is always
operable to produce the fine speed control signal for effecting a
fine adjustment of the angular velocity of the disc, and whereby
the video signal is recovered from the disc substantially at the
prescribed constant rate.
2. Apparatus for recovering a video signal from a rotatable
information storage disc, wherein the video signal is stored on the
disc in a plurality of substantially circular and concentrically
arranged information tracks, each track having a substantially
uniform recording density, said video signal being recovered from
the disc at a substantially constant rate by a transducer that is
movable radially relative to the disc to be in a prescribed
relationship with a selected information track, said apparatus
comprising:
a potentiometer coupled to the transducer for producing a coarse
speed control signal substantially proportional to the radius of
the selected information track from which the video signal is being
recovered by the transducer;
oscillator means for producing a periodic reference signal having a
prescribed constant frequency that corresponds to the rate at which
the video signal is to be recovered from the disc;
fine speed control means for comparing the phase angle of a
periodic pilot signal recovered from the disc along with the video
signal, with the phase angle of the periodic reference signal, and
for producing a fine speed control signal having an average value
that corresponds to the difference in the respective phase angles,
said fine speed control means including a phase-locked loop for
detecting the relative phase angle of the periodic pilot
signal;
means for summing together the coarse speed control signal and the
fine speed control signal, to produce a composite voltage
signal;
voltage-controlled oscillator means for producing a composite speed
control signal having a frequency that corresponds to the composite
voltage signal; and
means for rotating the disc at an angular velocity corresponding to
the frequency of the composite speed control signal;
said .[.coarse speed control means.]. .Iadd.potentiometer
.Iaddend.and said voltage-controlled oscillator means being
constructed with sufficient linearity that the composite speed
control signal will always have a frequency that causes the disc to
be rotated at an angular velocity that results in the recovered
pilot signal having a frequency within the pull-in range of said
phase-locked loop, whereby said fine speed control means is always
operable to produce the fine speed control signal for effecting a
fine adjustment of the angular velocity of the disc, and whereby
the video signal is recovered from the disc substantially at the
prescribed constant rate.
3. Apparatus for recovering a video signal from a rotatable
information storage disc, wherein the video signal is stored on the
disc in a plurality of substantially circular and concentrically
arranged information tracks, each track having a substantially
uniform recording density, said video signal being recovered from
the disc at a substantially constant rate by a transducer that is
movable radially relative to the disc to be in a prescribed
relationship with a selected information track, said apparatus
comprising:
coarse speed control means for producing a coarse speed control
signal that varies substantially linearly with the radius of the
selected information track from which the video signal is being
recovered by the transducer;
oscillator means for producing a periodic reference signal having a
prescribed constant frequency that corresponds to the rate at which
the video signal is to be recovered from the disc;
fine speed control means for comparing the phase angle of a
periodic signal included in the video signal recovered from the
disc with the phase angle of the periodic reference signal, and for
producing a fine speed control signal having an average value that
corresponds to the difference in the respective phase angles;
means for summing together the coarse speed control signal and the
fine speed control signal, to produce a composite voltage
signal;
voltage-controlled oscillator means for producing a composite speed
control signal having a frequency that corresponds to the composite
voltage signal; and
means for rotating the disc at an angular velocity corresponding to
the frequency of the composite speed control signal, whereby the
video signal is recovered from the disc substantially at the
prescribed constant rate.Iadd.,
wherein said fine speed control means includes a phase-locked loop
for detecting the periodic signal recovered from the disc; and
wherein said coarse speed control means and said voltage-controlled
oscillator means are constructed with sufficient linearity that the
composite speed control signal will always have a frequency that
causes the disc to be rotated at an angular velocity that results
in the horizontal synchronization pulses in the periodic signal
recovered from the disc having a frequency within the pull-in range
of said phase-locked loop, whereby said fine speed control means is
always operable to produce the fine speed control signal for
effecting a fine adjustment of the angular velocity of the
disc.Iaddend..
4. Apparatus as defined in claim 3, wherein said coarse speed
control means .[.included.]. .Iadd.includes .Iaddend.a
potentiometer coupled to the radially movable transducer.
5. Apparatus as defined in claim 3, wherein the transducer is moved
radially relative to the disc in a continuous fashion, and the
video signal is recovered from the successive information tracks,
seriatum. .[.6. Apparatus as defined in claim 3, wherein said fine
speed control means includes a phase-locked loop for detecting the
relative phase angle of the periodic signal recovered from the
disc..]. .[.7. Apparatus as defined in claim 6, wherein said coarse
speed control means and said voltage-controlled oscillator means
are constructed with sufficient linearity that the composite speed
control signal will always have a frequency that causes the disc to
be rotated at an angular velocity that results in the horizontal
synchronization pulses in the periodic signal recovered from the
disc having a frequency within the pull-in range of said
phase-locked loop, whereby said fine speed control means is always
operable to produce the fine speed control signal for effecting a
fine
adjustment of the angular velocity of the disc..]. 8. Apparatus as
defined in claim .[.7.]. .Iadd.5.Iaddend., wherein the periodic
signal in the video signal recovered from the disc is a pilot
signal that is summed with the video signal, said pilot signal
having a predetermined constant
frequency. 9. A method for controllably rotating an information
storage disc relative to a transducer, to recover information that
is stored on the disc in a plurality of substantially circular and
concentrically arranged information tracks, wherein the information
is recovered from the disc at a substantially constant rate, said
method comprising the steps of:
determining the radius of the particular information track from
which information is being recovered by the transducer, and
producing a coarse speed control signal in accordance
therewith;
producing a periodic reference signal having a predetermined
constant frequency;
comparing a periodic signal included in the information recovered
from the disc with the periodic reference signal, to produce a fine
speed control signal representative of the comparison;
producing a composite speed control signal in accordance with the
coarse speed control signal and the fine speed control signal, said
composite signal being representative of the prescribed angular
velocity at which the disc is to be rotated; and
rotating the disc at the prescribed angular velocity, in accordance
with the composite speed control signal, whereby the information is
recovered by the transducer at the prescribed constant
rate.Iadd.,
wherein said step of comparing includes the step of detecting the
periodic signal recovered from the disc in a phase-locked loop;
and
wherein said steps of producing the coarse speed control signal and
producing the composite speed control signal are performed with
sufficient precision that the resulting composite speed control
signal will cause the disc to be rotated at an angular velocity
that results in the periodic signal in the recovered information
having a frequency within the pull-in range of the phase-locked
loop, whereby said step of comparing and producing the fine speed
control signal will always effect a fine
adjustment of the angular velocity of the disc.Iaddend.. 10. A
method as defined in claim 9, wherein the coarse speed control
signal is substantially directly proportional to the radius of the
information track
from which information is being recovered. 11. A method as defined
in claim 9, wherein:
said step of producing the composite speed control signal includes
the steps of
summing together the coarse speed control signal and the fine speed
control signal, to produce a composite voltage signal, and
producing the composite speed control signal in a
voltage-controlled oscillator, said composite speed control signal
having a frequency substantially inversely proportional to the
voltage of the composite voltage signal; and
the disc is rotated in said step of rotating at an angular velocity
substantially directly proportional to the frequency of said
composite
speed control signal. 12. A method as defined in claim 9, wherein
the composite speed control signal is substantially inversely
proportional to the radius of the selected information track from
which information is being recovered, whereby the information
storage disc is controllably rotated at an angular velocity
substantially inversely proportional to the radius of the selected
track and the disc is moved relative to the
transducer at a substantially constant linear velocity. 13. A
method as defined in claim 9, wherein:
said step of comparing includes the step of comparing the phase
angle of the periodic signal recovered from the disc with the phase
angle of the periodic reference signal; and
said fine speed control signal has an average value representative
of the
difference in the relative phase angles of the two periodic
signals. 14. A method as defined in claim 13, wherein:
the periodic signal recovered from the disc is a separate periodic
pilot signal that is summed with a video signal, said pilot signal
having a predetermined constant frequency. .[.15. A method as
defined in claim 13, wherein said step of comparing includes the
step of detecting the periodic signal recovered from the disc in a
phase-locked loop..]. .[.16. A method as defined in claim 15,
wherein said steps of producing the coarse speed control signal and
producing the composite speed control signal are performed with
sufficient precision that the resulting composite speed control
signal will cause the disc to be rotated at an angular velocity
that results in the periodic signal in the recovered information
having a frequency within the pull-in range of the phase-locked
loop, whereby said step of comparing and producing the fine speed
control signal will always
effect a fine adjustment of the angular velocity of the disc..].
17. Apparatus for controllably rotating an information storage disc
relative to a transducer, to recover information that is stored on
the disc in a plurality of substantially circular and
concentrically arranged information tracks, said apparatus
operating to recover the information at a substantially constant
rate, said apparatus comprising:
.Iadd.coarse speed control .Iaddend.means for producing a coarse
speed control signal that varies according to the radius of the
particular information track from which the information is being
recovered;
means for producing a periodic reference signal having a
predetermined constant frequency;
fine speed control means for comparing a periodic signal included
in the information recovered from the disc with the periodic
reference signal, and for producing a corresponding fine speed
control signal representative of the comparison;
means, responsive to the coarse speed control signal and the fine
speed control signal, for producing a composite speed control
signal representative of the prescribed angular velocity at which
the disc is to be rotated; and
means, responsive to the composite speed control signal, for
rotating the disc at the prescribed angular velocity, whereby the
information stored thereon is recovered by the transducer at the
prescribed constant rate.Iadd.,
wherein the fine speed control means includes phase detector means
for comparing the relative phase angles of the periodic signal
recovered from the disc and the periodic reference signal;
said fine speed control signal has an average value representative
of the difference in the relative phase angles of the two periodic
signals;
said fine speed control means includes a phase-locked loop for
detecting the periodic signal recovered from the disc; and
said coarse speed control means and said means for producing the
composite speed control signal are constructed with sufficient
precision that the composite speed control signal produced thereby
will cause the disc to be rotated at an angular velocity that
results in the periodic signal in the recovered information having
a frequency with the pull-in range of said phase-locked loop,
whereby said fine speed control means is always operable to produce
the fine speed control signal for effecting a fine
adjustment of the angular velocity of the disc.Iaddend.. 18.
Apparatus as defined in claim 17, wherein:
said means for producing a composite speed control signal
includes
means for summing together the coarse speed control signal and the
fine speed control signal, to produce a composite voltage signal,
and
voltage-controlled oscillator means, responsive to the composite
voltage signal, for producing the composite speed control signal,
said composite speed control signal having a frequency
substantially inversely proportional to the composite voltage
signal; and
said means for rotating the disc is responsive to the frequency of
the
composite speed control signal. 19. Apparatus as defined in claim
17, wherein the coarse speed control signal is substantially
directly proportional to the radius of the information track from
which information
is being recovered. 20. Apparatus as defined in claim 19,
wherein:
said transducer is movable radially relative to the disc, to be
positioned in a prescribed relationship relative to the information
track from which information is being recovered; and
said coarse speed control means includes a potentiometer that is
coupled to said transducer and that is adapted to produce the
coarse speed control
signal. 21. Apparatus as defined in claim 17, wherein the composite
speed control signal is substantially inversely proportional to the
radius of the selected information track from which information is
being recovered, whereby the apparatus operates to controllably
rotate the storage disc at an angular velocity substantially
inversely proportional to the radius of the selected track and the
disc is moved relative to the transducer at a
substantially constant linear velocity. 22. Apparatus as defined in
claim 21, wherein:
said transducer is movable radially relative to the disc, to be
positioned in a prescribed relationship relative to the selected
information track; and
said coarse speed control means includes a potentiometer coupled to
said transducer and adapted to produce the coarse speed control
signal. .[.23. Apparatus as defined in claim 17, wherein:
said fine speed control means includes phase detector means for
comparing the relative phase angles of the periodic signal
recovered from the disc and the periodic reference signal; and
said fine speed control signal has an average value representative
of the difference in the relative phase angles of the two periodic
signals..].
Apparatus as defined in claim .[.23.]. .Iadd.17.Iaddend.,
wherein:
the periodic signal recovered from the disc is a separate pilot
signal having a predetermined constant frequency. .[.25. Apparatus
as defined in claim 23, wherein said fine speed control means
includes a phase-locked
loop for detecting the periodic signal recovered from the disc..].
.[.26. Apparatus as defined in claim 25, wherein said coarse speed
control means and said means for producing the composite speed
control signal are constructed with sufficient precision that the
composite speed control signal produced thereby will cause the disc
to be rotated at an angular velocity that results in the periodic
signal in the recovered information having a frequency within the
pull-in range of said phase-locked loop, whereby said fine speed
control means is always operable to produce the fine speed control
signal for effecting a fine adjustment of the angular velocity of
the disc..].
Description
BACKGROUND OF THE INVENTION
The present invention relates generally to systems for recovering
information from a record medium, and, more particularly, to
systems for recovering information from an information storage disc
that is rotatable at a variable angular velocity.
Information storage discs having a plurality of substantially
circular and concentrically arranged information tracks are
becoming increasingly popular, particularly when used for storing
video signals. Each information track on the disc typically stores
one complete frame of the video signal, so recovery of the video
signal at a prescribed constant rate necessary for proper reception
of the signal by a conventional television receiver ordinarily
necessitates a rotation of the disc relative to a transducer at a
substantially constant angular velocity.
An example of a servo apparatus for recovering a video signal from
such a disc is described in a co-pending and commonly assigned
application for U.S. patent Ser. No. 890,670, filed in the name of
W. R. Dakin et al and entitled "Video Disc Player". The servo
apparatus described in the application includes a spindle motor for
rotating the disc, a tachometer coupled to the motor for producing
a signal having a frequency proportional to its angular velocity,
and a phase detector for comparing the phase angle of the
tachometer signal with that of a periodic reference signal. The
output of the phase detector, which is proportional to the phase
difference of its two input signals, is suitably amplified and
coupled to the motor to rotate the disc at the appropriate constant
angular velocity.
An example of another servo apparatus for recovering a video signal
from such a disc is described in a co-pending and commonly assigned
application for U.S. patent, Ser. No. 920,701, filed in the name of
J. S. Winslow et al and entitled System for Recovering Information
from a Movable Information Storage Medium. The servo apparatus
described in the application is similar to the apparatus described
previously, except that substituted for the tachometer signal is a
pilot signal, which has been extracted from the signal recovered
from the disc. This system operates more effectively to recover the
signal from the disc at a prescribed constant rate, however, since
it is not as susceptible to errors caused by relative slippage of
the disc and eccentricities of the disc.
Since information is recovered from a disc of the type described
above by rotating it at a constant angular velocity, and since each
information track extends over a full circumference on the disc, a
video signal stored in tracks at the periphery of the disc is less
densely recorded than a signal stored nearer the center of the
disc. The entire information recording capability of the disc is
thus not utilized to its fullest extent, and, as a result, the
playing time of the disc is substantially lower than it otherwise
could be.
One technique for increasing the playing time of an information
storage disc is disclosed in a co-pending and commonly assigned
application for U.S. patent Ser. No. 961,405, filed simultaneously
herewith by the same inventors and entitled System for Recording
Information on a Rotatable Storage Disc, In a Substantially Uniform
Recording Density. In that application, a technique is described
for recording information onto a storage disc of the aforedescribed
type, wherein the the disc is rotated relative to a radially
movable transducer at an angular velocity that is varied according
to the radius of the particular information track in question. In
particular, a substantially uniform recording density over the
entire disc can be achieved by rotating the disc at an angular
velocity inversely proportional to the radius of the selected
track. The amount of information recorded on each information track
is thus generally proportional to the length of the track. Since
known prior apparatus for recovering information from rotatable
information storage discs operate to rotate the disc only at a
constant angular velocity relative to a transducer, they are
generally incapable of recovering information from discs of the
type having such a uniform recording density.
It will thus be appreciated that there is a need for a method and
apparatus for rotating an information storage disc of the type
described above at a variable angular velocity relative to a
transducer, to recover information from the disc at a prescribed
constant rate. Moreover, there is a particular need or a method and
apparatus for recovering information from a disc having a uniform
recording density. The present invention fulfills these needs.
SUMMARY OF THE INVENTION
The present invention is embodied in apparatus and a related method
for controllably rotating an information storage disc relative to
transducer means for recovering information stored thereon. The
information is stored in a plurality of substantially circular and
concentrically arranged information tracks, and the transducer
means is radially movable relative to the disc to recover the
information from a selected one of the tracks. The recording
density of the stored information is such that, to recover the
information at a prescribed constant rate, the disc must be rotated
at an angular velocity that varies according to the radius of the
selected track.
The apparatus includes means for producing a coarse speed control
signal that varies according to the radius of the selected
information track, means responsive to the coarse signal for
producing a composite speed control signal representative of the
prescribed speed at which the disc is to be rotated, and means
responsive to the composite speed control signal for rotating the
disc, accordingly. In accordance with the invention, the apparatus
further includes means for comparing a periodic signal included in
the information recovered from the disc with a periodic reference
signal, and for producing a corresponding fine speed control signal
that is coupled to the means for producing the composite speed
control signal, to adjust such composite signal in a prescribed
fashion, accordingly. As a result, the angular velocity of the disc
is controllably adjusted, such that the information is recovered
therefrom by the transducer means at the prescribed constant
rate.
More particularly, an apparatus constructed in accordance with the
present invention is especially adapted for use in recovering a
video signal from an information storage disc having a
substantially uniform recording density over its entire
information-bearing surface. To properly recover the video signal,
the angular velocity of the disc relative to the transducer means
must be maintained substantially inversely proportional to the
radius of the information track from which information is being
recovered by the transducer means.
The coarse speed control means preferably comprises a potentiometer
mechanically coupled to the transducer means, which is movable
radially relative to the disc. An electrical signal produced by the
potentiometer is substantially proportional to such radius and is
coupled to the means for producing the composite speed control
signal, which, in the preferred embodiment comprises a
voltage-controlled oscillator (VCO). The composite speed control
signal produced by the VCO has a frequency substantially inversely
proportional to the radius of the selected track, and is coupled to
a conventional servomechanism for rotating the disc at a
corresponding angular velocity. Thus, as a result of the coarse
speed control means, alone, the angular velocity of the disc will
be controllably adjusted as the transducer means moves from track
to track, and, depending on the linearity of the potentiometer and
the VCO, the resultant velocity will be within a prescribed
tolerance of the precise velocity necessary for recovering the
information at the prescribed constant rate.
A fine adjustment of the angular velocity of the disc is
accomplished by the fine speed control means, which operates to
compare the timing of the video signal recovered from the disc with
a periodic reference signal. The fine speed control means
preferably includes a phase-locked loop for detecting a pilot
signal included in the video signal recovered from the disc and
producing a periodic signal having a corresponding frequency, along
with a stable reference oscillator for generating the periodic
reference signal. The respective periodic signals are suitably
frequency divided and compared to each other in a phase detector to
produce the fine speed control signal, which has an average voltage
proportional to the difference in the phase angles of the
respective signals.
The fine speed control signal is transmitted to a summing
amplifier, where it is combined with the coarse speed control
signal, for coupling to the VCO. As a result, the frequency of the
composite speed control signal produced by the VCO will be adjusted
automatically, to cause the servo to rotate the disc at an
appropriate angular velocity such that the video signal is
recovered from the disc at the prescribed constant rate. By way of
example, if the coarse speed control means, by itself, operated to
rotate the disc at too low an angular velocity, the fine speed
control means would detect a relative phase lag in the recovered
video signal and would adjust the average voltage level of the fine
speed control signal it produces, accordingly. The voltage applied
to the VCO would thus be controllably adjusted, whereby the
frequency of the composite speed control signal would be increased
and the angular velocity of the disc increased, accordingly, until
the proper speed were achieved. The apparatus would operate in a
similar fashion to decrease the angular velocity of the disc if a
phase lead in the recovered video signal were detected.
Other aspects and advantages of the present invention will become
apparent from the following description of the preferred embodiment
taken in conjunction with the accompanying drawings, which
disclose, by way of example, the principles of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a plan view of a prior video disc, showing, in schematic
form, two information tracks, each extending over one complete
circumference and containing one frame of a video signal;
FIG. 2 is a plan view of the video disc of FIG. 1, showing, in
enlarged form, the successive light-reflective and light-scattering
regions forming the two information tracks;
FIG. 3 is a simplified block diagram of a prior art apparatus for
rotating the video disc of FIG. 1 at a constant angular velocity,
to recover the video signal therefrom;
FIG. 4 is a plan view of a video disc for use with apparatus of the
present invention, showing, in schematic form, a plurality of
frames of a video signal recorded on the disc, each of the frames
having substantially the same length;
FIG. 5 is a plan view of the video disc of FIG. 4, showing, in
enlarged form, the successive light-reflective and light-scattering
regions for two information tracks, one located near the periphery
of the disc and the other near the center of the disc; and
FIG. 6 is a simplified block diagram of apparatus of the present
invention for rotating the video disc of FIG. 4 at a precisely
controllable angular velocity, such that the video signal is
recovered from the disc at a prescribed rate.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring now to the drawings, and particularly to FIGS. 1 and 2,
there is shown a typical prior art record disc 10 for storing
information, such as a video signal. As shown in FIG. 1, the signal
is recorded in a plurality of substantially circular information
tracks forming a spiral pattern on the disc, with each track
containing one frame of video information. The even fields for all
of the tracks form a sector indicated by the reference numeral 12,
while the odd fields form a sector indicated by the numeral 14 and
the vertical blanking intervals form sectors indicated at 16 and
18. It will be apparent that an information track located at the
periphery of the video disc 10, such as the track indicated by the
numeral 19, is of considerably longer length than is an information
track located nearer the center of the disc, such as the track
indicated by the numeral 20.
FIG. 2 is another schematic view of the disc shown in FIG. 1,
wherein indicia of the recorded video signal are shown in enlarged
form. The form of this video disc is described in more detail in a
co-pending and commonly assigned application for U.S. patent Ser.
No. 890,407, filed in the name of John S. Winslow and entitled
"Mastering Machine". The video signal is ordinarily stored on the
disc 10 in the form of a frequency modulated carrier and it
comprises an alternating sequence of light-reflective and
light-scattering regions, 24 and 26, respectively. The
light-scattering regions 26 normally have a uniform width in the
radial direction, and the lengths of adjacent light-scattering and
light-reflective regions are essentially equal, because, together,
they represent a single cycle of the frequency modulated signal.
Since the same amount of information is recorded on each track,
i.e., one video frame, the size of the successive light-scattering
regions changes as a function of its radial position. This can be
observed in FIG. 2 by noting the relative sizes of the
light-reflective and light-scattering regions, 24 and 26,
respectively, for a track near the periphery of the disc 10, and
the light-reflective and light-scattering regions, 24' and 26',
respectively, for a track near the center of the disc.
FIG. 3 is a simplified block diagram of a prior art system utilized
in rotating the disc 10 of FIGS. 1 and 2 at a constant angular
velocity of 30 revolutions per second. The system includes an
oscillator 30 for generating a reference frequency that is
frequency divided by a divider circuit 32 to produce a speed
reference signal, a spindle motor 28, and an AC tachometer 34
coupled to the spindle motor for producing a signal having a
frequency proportional to the actual angular velocity of the disc
10. The system further includes a phase detector circuit 36 for
comparing the respective phase angles of the tachometer signal and
the speed reference signal and for producing a corresponding
control signal. The control signal is, in turn, processed in a
lead/lag compensator 38 and an amplifier 40 for coupling to the
motor 28 to controllably adjust its angular velocity to properly
rotate the disc 10 at 30 revolutions per second.
Referring now to FIGS. 4 and 5, there is shown a video disc 110 for
use with apparatus of the present invention. FIG. 4 shows, in
schematic form, one frame 112 of a video signal, extending over one
complete information track near the center of the disc, along with
a plurality of consecutive video signal frames 114, 116, 118, and
120 extending over a pair of tracks located near the periphery of
the disc, each of these latter frames extending over substantially
less than a complete circumference of the disc. The length of each
frame on the disc is substantially the same.
FIG. 5 shows, in enlarged form, the successive light-reflective and
light-scattering regions 122 and 124 for a portion of the video
signal frame designated 118, along with corresponding regions 122'
and 124' for a portion of the frame designated 112. It should be
noted that, since each video signal frame recorded on the disc is
substantially of equal length, the nominal lengths of the
respective light-scattering regions 124 and 124' are, likewise,
substantially equal.
Referring now to FIG. 6, there is shown a simplified block diagram
of apparatus for controllably rotating the disc 110 of FIGS. 4 and
5 at a variable angular velocity relative to an optical transducer
128, such that the video signal is recovered from the disc at a
prescribed constant rate. The optical transducer is movable
radially relative to the disc to recover the video signal from a
selected one of the information tracks, and the apparatus operates
to rotate the disc such that the selected track is moved at a
substantially constant linear velocity relative to the transducer.
The prescribed constant rate at which the video signal is recovered
from the disc corresponds to conventional broadcast format
standards, whereby the signal can be properly received and
displayed by a conventional television receiver (not shown).
The apparatus includes a coarse speed control potentiometer 130
mechanically coupled to the transducer 128 for producing a coarse
speed control signal, which is generally proportional to the radius
of the particular information track from which information is being
recovered, a voltage-controlled oscillator (VCO) 132, responsive to
the coarse speed control signal, for producing a composite speed
control signal having a frequency that corresponds thereto, along
with a spindle motor servo 134, responsive to the composite speed
control signal, for rotating the disc 110 at the corresponding
angular velocity. The coarse speed control signal, being generally
proportional to the radius of the selected track, thus yields a
coarse approximation of the particular angular velocity required
for recovery of the video signal at the prescribed constant
rate.
In accordance with the invention, the apparatus further includes
fine speed control circuitry 136 for comparing the phase angle of a
periodic signal included in the information recovered from the disc
110 with that of a periodic reference signal derived from a crystal
oscillator 138. The circuitry 136 produces a fine speed control
signal for combination with the coarse speed control signal in a
summing amplifier 140 and coupling to the VCO 132. The frequency of
the composite speed control signal and, in turn, the angular
velocity of the disc 110 are thus properly adjusted such that the
periodic signal from the disc and the periodic reference signal
from the oscillator have synchronous frequencies, and the video
signal is therefore recovered from the disc at the prescribed
constant rate.
The periodic signal that is recovered from the disc 110 and
detected by the fine speed control circuitry 136, preferably
comprises a pilot signal, having a predetermined constant
frequency, that is additively combined with the video signal.
Alternatively, many other periodic signals can be utilized, such
as, for example, the successive vertical or horizontal
synchronization pulses or the successive chrominance bursts in the
video signal, itself.
The coarse speed control potentiometer 130 is mechanically coupled
to the optical transducer 128 and is adapted to produce an
electrical current signal substantially directly proportional to
the radius of the information track from which the transducer is
recovering information. This current signal is transmitted over
line 140 to the summing amplifier 140, where it is combined with
the fine speed control signal supplied over line 142 from the fine
speed control circuitry 136 and converted to a composite voltage
signal for transmission over line 144 to the VCO 132. The VCO, in
turn, produces the composite speed control signal for transmission
over line 146 to the spindle motor servo 134, which operates to
rotate the information disc 110 at an angular velocity
corresponding to the frequency of the composite speed control
signal. For reasons that will become apparent subsequently, the
potentiometer 130 and the VCO 132 and preferably sufficiently
linear that, disregarding the effects of the fine speed control
signal, the instantaneous frequency of the composite speed control
signal will always be within a tolerance of approximately 5% of the
actual frequency necessary for recovery of information from the
disc at the prescribed constant rate.
The spindle motor servo 134 includes a spindle motor 148 for
rotating the disc 110, an AC tachometer 150 coupled to the spindle
motor for producing a tachometer signal having a frequency
proportional to its angular velocity, and a phase detector 152 for
comparing the phase angle of the tachometer signal with that of the
composite speed control signal. The phase detector produces a
control signal proportional to the detected phase difference, for
processing by a lead/lag compensator 154 and amplification by an
amplifier 156 and, in turn, coupling to the spindle motor 148 to
control its angular velocity. The servo 134 operates in a
conventional manner to synchronize the respective frequencies and
phase angles of the composite speed control signal and the
tachometer signal, whereby the angular velocity of the spindle
motor 148, and, therefore, the disc 110, are made to follow the
varying frequency of the composite speed control signal.
The fine speed control circuitry 136 receives on line 158 the pilot
signal included with the video signal recovered from the disc 110,
and produces the fine speed control signal for use by the apparatus
in precisely adjusting the angular velocity of the disc to result
in a recovery of the video signal at the proper rate. The circuitry
136 includes a phase-locked loop circuit 160 for detecting the
pilot signal, along with a phase detector 162 for comparing the
phase angle of the detected signal with that of the periodic
reference signal derived from the crystal oscillator 138.
The phase-locked loop 160 operates in a conventional manner to
detect the successive cycles of the pilot signal supplied on line
158 and to produce a periodic signal having the same frequency. Use
of a phase-locked loop insures that, under ordinary circumstances,
spurious pulses or signal dropouts will be eliminated and a
substantially noise-free periodic signal will be produced. The
"pull-in range" of the phase-locked loop must exceed the maximum
range of frequencies over which the frequency of the recovered
pilot signal is expected to vary. Since, as previously mentioned,
the tolerance on the linearity of the coarse speed control
potentiometer 130 and the VCO 132 is 5%, it follows that the
pull-in range of the phase-locked loop must exceed 5% of the
nominal pilot signal frequency. The periodic signal produced by the
phase-locked loop 160 is transmitted over line 164 to a
conventional edge detector 166, which produces a short duration
pulse for each transition of a particular polarity, for
transmission over line 168 to a first input terminal of the phase
detector 162.
The crystal oscillator 138 produces a periodic clock signal for
transmission over line 170 to a divide-by-N circuit 172, where it
is frequency divided to produce the periodic reference signal
having the same frequency as the nominal frequency of the periodic
signal produced by the phase-locked loop circuit 160. This periodic
reference signal from the divide-by-N circuit is transmitted over
line 174 to a conventional edge detector 176, which produces a
short duration pulse for each transition of a particular polarity,
for transmission over line 178 to a second input terminal of the
phase detector 162.
The phase detector 162 compares the respective phase angles of the
two input pulse sequences it receives from the two edge detectors
166 and 176, and produces the fine speed control signal, which has
an average voltage proportional to the phase difference. The fine
speed control signal is transmitted over line 142 to the summing
amplifier 140, where it is combined with the coarse speed control
signal for transmission to the VCO 132. When a phase lead in the
pilot signal is detected by the phase detector, indicating that the
video signal is being recovered from the disc 110 at a rate too
high, the average voltage of the fine speed control signal will
automatically increase in value, thereby causing a corresponding
decrease in the frequency of the composite speed control signal
produced by the VCO, and a corresponding decrease in the angular
velocity of the disc, to correct for the phase lead. Conversely,
when a phase lag in the pilot signal is detected, indicating that
the video signal is being recovered at a rate too low, the average
voltage of fine speed control signal will automatically decrease in
value, thereby causing the angular velocity of the disc to be
increased, accordingly, to correct for the phase lag.
The phase detector 162 preferably comprises a bidirectional counter
in the form of a 3-stage shift register in which the input from one
edge detector 166 or 176 is applied to the left stage, the input
from the other edge detector is applied to the right stage, and the
output (i.e., the fine speed control signal) is produced by the
center stage. During normal operation, a pulse received on one
input will cause a positive-going transition in the output signal,
and a pulse received on the other input will cause a negative-going
transition in the output signal. Thus, if the pulses in the two
input signals are received in an alternating fashion, indicating
that they have the same frequency, the output will comprise a pulse
sequence whose average voltage is proportional to the phase
difference of the two inputs. This bidirectional counter is
described in more detail in an article written by R. A. Millar,
entitled "Digital Control of Shaft Speed and Position", published
in IEEE Spectrum, January, 1968.
From the foregoing description, it should be apparent that the
present invention provides an effective apparatus and related
method for recovering information from an information storage disc
having a plurality of substantially concentrically arranged
information tracks, with the information being recorded on the
tracks at a substantially uniform recording density. The apparatus
includes both coarse and fine speed control circuitry for effecting
a precise control of the angular velocity of the disc, such that
the information is recovered from the disc at a prescribed constant
rate.
Although the invention has been described in detail with reference
to its presently preferred embodiment, it will be understood by one
of ordinary skill in the art that various modifications can be made
without departing from the spirit and scope of the invention.
Accordingly, it is not intended that the invention be limited,
except as by the appended claims.
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