Note: Descriptions are shown in the official language in which they were submitted.
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la
METHOD AND APPARATVS FOR INFORMATION RETRIEVAL
_
FROM AN OPTICALLY READABLE STORAGE MEDIUM
TECHNICAL FIELD
This invention relates generally to informa-
tion retrieval from an optieally readable storage
medium and, more speeifieally, to a new and improved
method and apparatus for retrieving information from
video dises and the like in a rapid and reliable manner.
, .
BACKGROUND OF THE PRIOR ART
Video dise players and the like are known for
recovering information that has been stored, for instanee,
as a succession of light refleetive and nonreflective
regions along spirally-formed information tracks in a
dise-shaped reeord earrier. The reeord earrier ean be
rotated at a relatively high rate, while an optieal
system is employed for directing a radiant beam, such
as a laser beam, to impinge upon the information traeks
and for gathering a refleeted beam that has been modula-
ted by the reflective and non-reflective regions of the
information track. Such a player includes a carriage
for translating the video disc relative to the radiant
beam at a rate equal to the pitch of the spirally-
recorded tracks and beam steering means for manipulating
the radiant beam to precisely follow the path defined
by such tracks.
260
-lb-
A frequency modulated electrical signal is
recovered from the reflected, light modulated beam and
is applied to appropriate signal processing circuitry
for deriving a video signal for display on a video
monitor. Various control signals that are utilized to
operate the player are also derived from the reflected
beam.
Heretofore, the capability has existed of
displaying a selected frame of video information by
prescribing the address of the frame or information
track wherein it is stored, and causing the carriage
to translate in a rapid manner to the vicinity of such
information track. The address information has been
specially encoded in both of the vertical blanking
intervals present in each revolution of an information
track comprising the two standard fields of a video
frame.
More particularly, in order to retrieve the
selected frame or information track, the carriage has
~een translated at a uniform rate, greater than the
normal play speed of the player, towards such informa-
tion track. During the course of translating the car-
riage, track addresses of at least some of the tracks
crossed were detected, although it is obviously not
possible to detect every track address in such a mode
of operation. As soon as it was detected that the
selected information track had been reached or crossed,
the drive signal to the carriage was terminated. In-
variably, however, the carriage travelled past the
location of the selected track and it was at least
necessary to drive the carriage once in the opposite
direction to return to it.
By this prior technique, it was determined
- to always approach the selected information track from
one direction when preparing to actually stop the car-
riage at the track. In other words, if the carriage
,
1~4~6~
were driven initially in the normal forward direction
for retrieval, after overshooting the selected track
the carriage would be driven in the reverse direction
past the selected track again. Upon passing the track,
the reverse drive signal would be terminated and the
carriage would again overshoot. Finally, the normal
play mode of the video disc player would be utilized
to simply play into and stop at the selected informa-
tion track.
If the selected track was reached in such a
search mode by rapidly translating the carriage in the
reverse direction, the carriage drive signal would be
terminated upon reaching or passing the selected track,
with the accompanying overshoot. From this position
the video disc player utilized the normal play mode to
play into and stop at the selected track, since the
carriage was then located on the proper side for ap-
proaching the selected track.
It will be apparent that the aforedescribed
techniques for recovering the information stored in
selected tracks were prone to deficiencies. For in-
stance, overshoot when approaching the selected track
from either the forward or the reverse direction resul-
ted in delays in retrieving the information stored on
the track. The requirement of always approaching the
selected track from one direction compounded the delay.
Hence, there has been a need for an improved,
rapid information retrieval technique for use with
video disc players and the like, wherein information
is optically recovered by means of a radiant beam
being impinged upon an information storage medium, that
solves the aforedescribed problems. The present inven-
tion fulfills this need.
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BRIEF SUMMARY OF THE INVENTION
Briefly, and in general terms, the present
invention provides a new and improved method and appar-
atus capable of rapidly retrieving information stored
in a selected one of a plurality of optically readable
information tracks by an information recovery system
employing radiant beam information recovery means.
Basically, the present invention includes an
improved electronic method and apparatus for control-
ling movement of an information storage medium, such
as a disc-shaped record carrier, relative to a radiant
beam that is utilized to recover information by scan-
ning along information tracks formed therein. A partic-
ular information track may be targeted for recovery in a
retrieval mode of operation, for instance by specifying
an address included with the information in the track.The location of this target track relative to the loca-
tion of the information track currently being scanned
by the radiant beam is monitored and, as a function of
the difference in locations, signals are prescribed
for controlling relative movement between the target
track and the radiant beam in order to rapidly re-
position the storage medium relative to the radiant
beam to enable scanning of the targeted information
track. ~lore specifically, the rate of relative move-
ment is progressively decreased in a prescribed manner,from an initial translation rate determined by the
distance separating the radiant beam and the targeted
information track. The manner of decreasing the rate
of relative movement is selected to intermittently re-
determine the rate at predetermined intervals as theradiant beam approaches the target track, and to ensure
that the rate is such that the radiant beam wlll not be
caused to overshoot. The separating distance between
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the target track and the radiant beam may be determined
by deriving addresses stored in the information tracks
as the radiant beam scans in the retrieval mode and,
by comparing these addresses with the address of the
target trac]c.
In a presently preferred embodiment, by way
of example and not necessarily by way of limitation, a
video disc player embodying features of the present
invention miyht include a carriage controller for con-
trolling the position of a carriage on which a video
disc is mounted for translation relative to a beam of
radiation employed for scanning information tracks on
the video disc. A comparison of the address of an
information track selected for retrieval with the
address the information track being scanned by the beam
; of radiation is made and a determination is reached
whether the carriage should be driven in a forward or
in a reverse direction to retrieve the target track.
Beam steering means for manipulating the radiant beam
~to precisely follow the path defined by the tracks in
a p~ay mode of operation, may be disabled in the re-
trieval mode.
As a function of the distance separating the
beam of radiation and the target track, the carriage
controller prescribes a sequence of drive signals to
be applied to a carriage motor for varying the rate of
movement of the carriage. For example, the carriage
controller may prescribe one of four drive signals,
resulting in a particular initial rate of movement,
upon determining that the beam of radiation is separ-
ated from the target track by at least a particularthreshold distance. As the carriage is moved at this
initial rate, the separating distance between the beam
of radiation and the target track decreases, and another
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distance threshold may be crossed. In such event, the
carriage controller switches a different drive signal
of lower magnitude to the carriage motor to effect a
slowing of the carriage. Progressive switching of
drive signals of lower magnitude is effected upon cross-
ing each prescribed distance threshold until the target
track is reached and all drive signals are switched off,
thereby stopping the carriage at the target track.
In a further aspect of the present invention,
the drive signal applied to the carriage motor immedi-
ately prior to stopping at the target track is selected
to be the normal play speed of the video disc player.
Preferahly, switching of this drive signal to the car-
riage motor also re-enables the beam steering means so
that the radiant beam accurately follows the informa-
tion and therefore reliably recovers track addresses,
as the carriage plays into the target track in the play
mode and stops.
In an alternative embodiment of the present
invention, monitoring of the separating distance be-
tween the target track and the radiant beam is accom-
plished during an initial portion of the retrieval mode
by counting the number of tracks crossed by the radiant
beam. Such a technique may be preferable when the
carriage is being translated at such a high rate as to
make detection of track addresses relatively unreliable.
Due to eccentricities inherent in the information
tracks, and the need for beam steering means to accur-
ately follow the tracks, monitoring of track addresses
is again relied upon as the carriage shifts into the
normal play mode prior to stopping at the target track.
The method and apparatus for information re-
trieval of the present invention satisfies a need for
1~4~Z60
. --6
rapid and reliable information retrievable from optically
readable tracks by information recovery systems utili-
zing radiant beam information recovery means.
The above and other objects and advantages of
this invention will be apparent from the following more
detailed description when taken in conjunction with the
accompanying drawings of illustrative embodiments.
BRIEF DESCRIPTION OF THE DRAWINGS
In the accompanying drawings:
FIG. 1 is a generalized block diagram of a
video disc player in which some of the basic concepts
; of the present invention are illustrated;
FIG. 2 is a flow chart illustrating an algo-
rithm applicable to the carriage controller shown in
FIG. 1, for varying the carriage speed and direction of
movement in the search mode;
FIG. 3 is a waveform showing the response of
the carriage motor as it is driven to a target track
in either the forward or the reverse direction;
FIG. 4 is an electrical schematic of one em-
bodiment of a carriage driver as shown in FIG. 1 suit-
able for utilizing the results of the algorithm of
FIG. 2;
FIG. 5 is a generalized diagram of a signal
recovery subsystem, suitable for use with the video
disc player of FIG. l; and
FIG. 6a is a fragmentary cross-sectional view
of three information tracks of a video disc, while
FIG. 6b is a waveform applicable to the signal recovery
subsystem of FIG. 5.
DETAILED DESCRIPTION OF THE INVENTION
Referring now to FIG. 1 of the drawings for
purposes of illustration, there is shown a new and
improved system for information retrieval embodying
features of the present invention. In the drawings,
114~26~
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the same numeral will be used in the several views to
represent the same element.
In FIG. 1, a generalized video disc player is
shown for purposes of illustrating the principles of
the invention and includes a spindle on which a video
disc 10 is mounted for rotation by a spindle motor 12
at a preselected angular rate. Information is recorded
on the video disc 10 in a frequency modulated format
as successive light reflective and non-reflective
regions along tracks that are formed either as a con-
tinuous spiral or in discrete concentric rings. Thevideo disc player also includes an optical system 14
which produces a laser read beam 16 and directs it
- through an optical read head 18 having an objective lens
for focusing the read beam to a precise spot on the
, video disc 10.
A reflected beam 20, modulated by the infor-
mation recorded in the tracks, is gathered by the readhead 18 and returned through the optical system 14 to
a signal recovery subsystem 22. The spindle motor 12
is mounted on a carriage 24 for translation of the
video disc 10 in the direction indicated by a double-
headed arrow 26 by means of a carriage motor 28.
Coarse steering of the read beam 16 along the informa-
tion tracks is accomplished by translating the carriage
24 either at a uniform rate equal to the pitch of
spirally-formed tracks or stepwise if the tracks are
formed as discrete concentric rings.
A tracking subsystem 30 is included in the
video disc player to enable the read beam to follow
eccentricities that are inherent in the information
tracks with present technology. Also, since the des-
cription that follows will assume that the information
tracks are formed in a spiral manner, wherein one com-
plete revolution of the video disc comprises one infor-
mation track, a stop motion subsystem 32 is shown in
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FIG. 1 to enable the video disc player to stop orfreeze on a particular track. Both the tracking and
stop motion subsyst.ems receive a control signal on
line 34 from the signal recovery subsystems will be
described below only as necessary for a full understan-
ding of the present invention.
The tracking subsystem 30 is employed for
maintaining radial tracking of the focused read beam 16
on one information track, and is responsive to the
control signal on line 34 to develop an error signal on
line 31 to the optical system 14 to bring the light
spot back onto the center-of-track position.
The tracking subsystem 30 normally operates
in a closed loop mode of operation when the player is
operating at a play speed. However, the tracking sub-
system 30 is disabled in a retrieval mode, such that
the differential tracking error is temporarily removed
from controlling the operation of the radial tracking
mirror. The tracking subsystem 30 can also be tempor-
arily disabled by the stop motion subsystem 32, whichthen generates various combinations of signals on a
line 33 to control the movement of the radial tracking
, mirror for directing the point of impingement of the
focused spot from the preferred center of track position
on a first track to a center of track position on an
adjacent track in order to effect stop motion.
The signal recovery subsystem 22 develops an
FM signal that includes the video information and all
other information stored in the information tracks, and
applies that signal on a line 36 to a signal processing
subsystem 38. The latter subsystem includes a conven-
tional FM detector for demodulating the FM signal into
a standard format video signal, which is then applied
on a line 40 to a video monitor 42 for display and to
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g
the stop motion subsystem 32. Means are also included
in the signal processing subsystem 38 for separating
the vertical sync signal from the video signal so that
the sync signal can be applied on a line 44 to the stop
motion subsystem 32 to be used in a manner described
below.
Each information track or frame of video in-
formation that is recorded on the video disc 10 is iden-
tified by a unique address encoded once in each of the
pair of vertical intervals between the two fields comp-
rising a frame. For purposes of the present invention,the video signal from the signal processing subsystem 38
is also applied on a line 40 to an address recovery sub-
system 46, wherein the address associated with each in-
formation track is decoded in any manner familiar to
those of ordinary skill in the art. These addresses,
for instance may be encoded in a selected digital format
on a selected horizontal line in each vertical interval.
A signal representing the address information
is then directed on a line 48 to a function generator
50 and to a carriage controller 52. The function gener-
ator 50, which may be embodied in a remote control, dis-
plays the address of the information track currently
being read, and includes means for selecting both a
retrieval mode of operation and the address of the
information track targeted for retrieval, in accordance
with the principles of the present invention.
In essence, when the retrieval mode is selec-
ted, the function generator 50 applies both an enabling
signal on a line 54 and the address of the target track
on a line 56 to the carriage controller 52~ wherein acomparison of the address of the current track being
read with the address of the target track is made and a
; determination is reached whether the carriage 24 should
be driven in a forward or reverse direction to retrieve
114~Z60
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the target track. Also, the carriage controller 52
prescribes a sequence of drive signals to be applied to
a carriage driver 60 in order to move the carriage motor
28, and hence the carriage 24 and the video disc 10, to
retrieve the target track in a rapid manner without
overshooting it. The algorithm by which these deter-
minations are made is illustrated in FIG. 2, and will
be described in detail below. The function generator
50 also applies a signal on a line 58 to the tracking
subsystem 30 to disable it in the search mode as des-
cribed above.
The determination of the direction in whichthe carriage 24 should be driven to reach the target
track results in either a forward (FWD) signal being
applied from the carriage controller 52 to carriage
driver 60 over line 62 or a reverse signal (REV) being
applied to the carriage driver on a line 64. In addi-
tion, the sequence of signals prescribed by the carriage
controller 52, so that the carriage 24 rapidly homes in
on the target track, results in one of four carriage
drive signals (Sl, S2, S3 or S4) being applied to
the carriage driver 60 from the carriage controller on
one of the lines 66, 68, 70 or 72, respectively. The
output of the carriage driver 60 is directed over a
line 74 to drive the carriage motor 28, and a tachometer
' 76 is shown as being mechanically interconnected to the
carriage motor to provide an indication of its actual
speed and direction by means of a feedback signal on a
; line 78 to the carriage driver.
The carriage drive signals Sl - S4 represent
four possible speeds at which the carriage motor 28 can
be driven, and hence four rates at which the carriage
24 and the video disc 10 can be translated relative to
, the read beam 16. One of these drive signals, S4 cor-
responds to the normal play speed of the video disc
6~
player which results in translation of the carriage 24
at a rate equal to the recorded pitch of the informa-
tion tracks. The other drive signals S3, S2 and Sl
correspond to successively greater carriage translation
rates.
In accordance with the present invention, the
carriage controller 52 prescribes a preferred sequence
for applying the drive signals to the carriage motor 28,
as a function of the distance between the track current-
ly being read by the player and the track targeted for
retrieval in the function generator 50, for retrievalof the information stored in the target track. In this
regard, as the carriage moves and the distance to the
target track decreases, the drive signal from the car-
riage driver 60 on line 74 to the carriage motor 28 is
sequentially stepped downward as a sequence of distance
thresholds, Dl, D2 and D3 are crossed. This results ina prescribed deceleration of the carriage motor 28 and
hence the carriage 24, as the target track is approached
(see FIG. 3). When the target track is within a pre-
scribed distance, represented by distance threshold D3,
the tracking subsystem 30 is re-enabled by a signal on
line 72 from the carriage controller 52. Then, as the
target track is reached, all drive signals to the car-
riage driver 60 are set to zero, and the stop motionsubsystem 32 is enabled by a signal on line 79 from
the carriage controller 52.
Referring now to FIG. 2, the manner in which
the carriage controller 52 determines the direction in
which to drive the carriage 24 to retrieve the target
;~ track and prescribes an optimum sequence of drive sig-
nals is diagrammed. It will be apparent that, in addi-
tion to the possibility of implementing the algorithm
by means of suitable hardware, such as digital logic
elements, all or part of the algorithm may be performed
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by conventional programming on a digital computer or a
microprocessor.
Initiation of a search mode by the function
generator 50 commences with a conventional start step
80, followed bya step 82 in which the question is asked
whether the address A2 of the target track (target
address) is greater than the address Al of the track
currently being read by the video disc player (current
address). An affirmative answer to this question indi-
cates that the proper direction to drive the carriage24 to retrieve the target track is forward and leads to
- step 84, resulting in the forward signal on line 62
being set to a true state (FWD = 1), while the reverse
signal on line 64 is set to a false signal (Rev = 0).
If the answer is no, then the forward signal on line 62
and the reverse signal on line 64 are set to the false
(FWD = 0) and the true (REV = 1) states, respectively,
by step 86.
Once the direction that the carriage is to be
driven has been fixed either by step 84 or by step 86,
a sequence of steps are utilized to determine the dis-
tance D to be traversed to the target track, as represen-
ted by the absolute magnitude of the difference between
; the target address A2 and the current address Al.
More specifically, with reference to FIG. 2,
; the question is asked at step 88 whether the distance D
is greater than the first distance threshold Dl from the
target track. If the answer is yes, then the drive sig-
nal Sl on line 66 is set to a true state (Sl = 1) in
step 90, resulting in the carriage motor 28 being driven
at a particular speed. Since the distance threshold Dl
represents the greatest distance to the target track,
the drive signal Sl is selected to cause the carriage
motor 28 to operate at its fastest available speed
3Z6~
until the next distance threshold D2 is reached. On
the other hand, if the answer to the question posed in
step 88 is no, theI~ the carriage motor 28 will be driven
at a prescribed speed less than the maximum in order that
S the carriage motor not be driven at such a rate that it
could not be stopped without overshooting the target
track. Initially the answer to the question posed in
step 88 may be no, of course, if the current track being
: read when the search mode commences is closer to the
target track than the distance threshold D1.
As mentioned previously, eccentricities in
the video disc 10, which are unavoidable with present
technology, require utilization of a tracking subsystem
30 such as that shown in FIG. 1 to accomplish fine steer-
ing of the read beam to accurately follow the path of theinformation tracks in a play mode of operation. It was
also noted, however, that in a retrieval mode of opera-
tion as described herein, the tracking subsystem 30 is
- disabled. Notwithstanding this disablement of the track-
ing subsystem 30, a certain amount of FM information is
recovered from the video disc 10 and provided to the
signal recovery subsystem 22 as the read beam 16 rapidly
crosses tracks in the search mode.
Although the FM information recovered from the
disc is such that the video monitor 42 cannot provide a
stable display, sufficient information will be recovered
at various intervals to derive the addresses of some
tracks as they are crossed. In this regard, identical
address information is diametrically encoded for each
track, in the vertical interval associated with each
field comprising a frame, so that there will be an
opportunity to recover address information every half
revolution of the disc. If the video disc is rotating
at a typical 1800 r.p.m., this will occur once approxi-
mately every 16 milliseconds. It is contemplated,
therefore, that the algorithms of FIG. 2 be repeated
1~4~Z60
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upon each updating of the current address Al, typically
at intervals of 16 msec. Thus, after step 90, as well
as all other steps wherein a drive signal is set, the
algorithm returns to the start step 80 in anticipation
of updated information respecting the current address Al.
When the answer to step 88 is no, either
because the target track is initially closer to the
current track being read than the distance threshold
Dl, or because the carriage 24 has been driven closer
in the search mode at the maximum speed represented by
the drive signal Sl, the question is posed by step 92
whether the distance D is greater than the next distance
threshold D2. If the answer is yes, then the drive
signal Sl on line 66 is set to a false state (Sl = 0)
; 15 and the drive signal S2 on line 68 is set to a true
state (S2 =1) by step 94. Consequently, the carriage
motor 28 will be driven at a speed corresponding to the
drive signal S2, and the carriage controller will
remain in this state until the next distance threshold
; 20 D3 is reached.
From the foregoing, it should now be apparent
that a primary purpose of sequentially down stepping
the drive signal applied to the carriage drive 60 is to
decelerate the carriage motor 28 in a relatively predict-
able fashion and to intermittently redetermine the
; speed of the motor as the carriage approaches the
target track. In this manner, the effects of variabil-
ity in the dynamic characteristics of particular carriage
motors and carriages is minimized by selecting the
drive signals and distance thresholds to allow intermit-
tent redetermination of carriage position and speed
during the course of homing in on the target track. As
a resul-t, the carriage is driven towards the target as
rapidly as practicable without overexciting the carriage
motor and possible experiencing overshoot.
1~4(3Z6~)
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. .
To complete the algorithm, the question is
posed at step 96 whether the distance D is greater than
the third distance threshold D3 to the target track and,
if the answer is yes, the preceding drive signals Sl and
S2 are set to a false state (Sl = 0, S2 = 0), and the
drive signal S3 that is applied on line 70 from the
carriage controller 52 to the carriage driver 60 is set
to a true state (S3 = 1) by step 98. When the distance
D finally becomes less than the third distance threshold
D3, but is still greater than zero, as determined by the
next question posed in step 100, the fourth drive signal
S4 is set to a true state (S4 = 1), and all previous
drive signals S1, S2 and S3 are set to a false state
(Sl =0, S2 = 0, S3 =0) by step 102.
; 15 It will be recalled that in the presently pre-
ferred embodiment, the carriage motor speed represented
; by the drive signal S4 is chosen to be the normal play
speed of the video disc player in which the carriage
24 is translated at a rate equal to the pitch of the
spiral tracks formed on the video disc 10. Therefore,
the drive signal S4 is also applied on line 72 to the
tracking subsystem 30 in order to re-enable it upon
re-establishing play speed, since the tracking subsystem
30 was disabled by a signal on line 58 from the function
generator 50 upon initiation of the search mode. This
was because the various sequential drive signals Sl, S2
and S3 all cause the carriage 24 to translate at rates
greater than the normal play speed represented by the
drive signal S4. At these higher translation rates, it
is neither practical nor desirable to attempt to fine
steer the read beam 16 as tracks are rapidly crossed.
Of course, when the drive signal S4 is applied, the
tracking subsystem 30 preferably is re-enabled so that
the read beam 16 will most accurately follow the informa-
tion tracks and track addresses can be reliably retrieved.
114~26~
-16
Finally, when the difference D becomes zero,
making the answer to the question posed in step 100 yes,
all previous drive signals Sl, S2, S3 and S4 are set to
a false state (Sl = 0, S2 = 0, S3 = 0 and S4 = 0) by
step 104. Of course, with all the drive signals Sl - S4
set to a false state, the carriage 24 will stop. At the
same time, a stop signal on a line 79 to the stop motion
subsystem 32 is set to a true sta~e (STOP = 1) by step
- 104. The purpose of this top signal is to enable the
stop motion subsystem 32 such that the target track will
be frozen on the display of the video monitor 42. Since
the information tracks are recorded in spiral fashion,
it is required that read beam 16 be jumped back once
~ each revolution of the video disc 10 so that the read
:, 15 beam repeatedly retraces the same frame of video infor-
mation. A particular manner of generating an approp-
riate jump-back signal and controlling the tracking
subsystem is described in detail in the prior art.
Briefly, the stop motion subsystem 32 is em-
ployed as a means for generating a plurality of control
signals for application to the tracking subsystem 30 on
` line 33 to achieve the movement of the focused spot
tracking the center of a first information track to a
separate and spaced location in which the spot begins
tracking the center of the next adjacent information
track. The stop motion subsystem 32 performs its func-
tion by detecting a predetermined signal recovered from
the frequency modulated video signal which indicates the
proper position within the recovered frequency modulated
video signal for initiating the jumping operation. This
detection function is achieved, inpart by internally
generating a gating circuit conditioned by the vertical
sync signal received on line 44 to indicate that portion
of the recovered video signal received on line 40, within
2~0
:'
-17-
which the predetermined signal should be located.
In response to the predetermined signal, which
has been termed a "white flag" in the aforementioned
related application, the stop motion subsystem 32 gener-
`5 ates a first control signal for application to the
tracking subsystem 30 for temporarily interrupting the
application of the differential tracking error to the
radial tracking mirrors in the optical system 14. The
stop motion subsystem 32 generates a second control
10 signal for application to the radial tracking mirrorsfor causing the radial tracking mirrors to leave the
center of tracking position on a first information
track and jump to an adjacent information track. The
~stop motion subsystem terminates the second control
; 15 signal prior to the focused spot reaching the center of
focus position on the next adjacent information track.
A third control signal may be generated by the
stop motion subsystem 32 at a time spaced from the ter-
mination of the second control pulse. The third control
20 pulse is applied directly to the radial tracking mirrors
;for compensating for the effects on it which were added
by the second control pulse. While the second control
pulse is necessary to have the reading beam jump from a
first information track to an adjacent information track,
the spaces involved are so small that the jumping opera-
tion cannot always reliably be achieved using the second
control signal alone. Therefore, the third control sig-
nal may be employed for compensating for the effects of
the second control jump pulse on the radial tracking
30 mirror at a point in time when it is assured that the
focus spot has, in fact, left the first information
track and has yet to be properly positioned in the cen-
ter of the next adjacent information track. Finally,
the differential error signal may be gated through to
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-18-
the radial tracking mirror at a time calculated for the
gated portion of the differential tracking error to
assist the compensation pulse in bringing the focused
; spot under control upon the center of track position of
5 the next adjacent information track.
Referring now specifically to FIG. 3, control
of the carriage motor drive signal, and hence carriage
motor speed, as a function to the distance D to the
'.
G target track, ii'3i illustrated by waveforms, including
~ 10 approaches from both the forward and the reverse direc-
. .
- tions.
Assuming for the moment that the carriage 24
is positioned further from the target track than the
distance threshold D3 and should be driven in the for-
15 ward direction to reach the target track, it can be seen
that the maximum drive signal Sl is initially applied to
the carriage driver 60 so that the carriage motor 28 will
assume its maximum available speed. As the carriage 24
, approaches the distance threshold D3, the drive signal
20 applied to the carriage driver 60 steps down to S2. Of
course, the carriage motor 28 and the carriage 24 have a
certain amount of inertia and the speed decays to a
speed dictated by the drive signal S2 over a period of
time. Preferably, the dynamic characteristics of the
25 carriage motor 28 and the carriage 24 are critically
damped so that a speed represented by the drive signal
S2 is reached as quickly as possible.
It should be noted that, in any event, the
dynamic characteristics of the carriage motor 28 and
30 carriage 24, as well as the distance thresholds such as
D3 and D2, should be chosen so that the carriage motor
speed does decay to the speed represented by the drive
signal S2 prior to the carriage reaching the next dis-
tance threshold D2. Otherwise, the purpose of stepping
1146~Z6V
-19-
.:.
- down the drive signal to the carriage driver in order to
redetermine the carriage motor speed at particular inter-
vals along route to the target track will be defeated.
Those skilled in the art will recognize that particular
carriage motors and carriages will display a certain
amount of variability in their dynamic response charac-
teristics and that the various drive signals and dis-
tance thresholds should be selected with this variabi]-
ity in mind, i.e. sufficient distance between thresholds
should be allowed so that even a carriage motor and
assembly with a relatively slow time response will de-
celerate to the speed represented by the next drive sig-
nal prior to the next distance threshold being reached.
This process of stepping down the drive signal
and allowing the speed of the carriage motor 28 to sub-
stantially completely decay before again stepping down
the drive signal at the next distance threshold is
: repeated until the normal play speed represented by
drive signal S4 is reached and the tracking subsystem 30
is re-enabled. Then as ~he carriage 24 reaches the tar-
get track, the carriage 24 is stopped and the stop motion
subsystem 32 is enabled as described above.
For purposes of convenience, the identical
sequence of drive signals and distance thresholds is
shown for implementation when the target track must be
approached in the reverse direction. For particular
systems, it may be that the carriage motor and the
carriage will display differing response characteristics
in the forward and the reverse directions, in which case
a different sequence of drive signals and distance thresh-
olds would be selected. It will also be appreciated that
the particular number of drive signals and distance
thresholds in the sequence i5 not critical to the present
invention.
,
114~26~
-20-
- A particular electrical circuit for implemen-
; ting the carriage driver is shown in FIG. 4. The cir-
guit includes a first quad analog switch 108 having four
IN, OUT and CONTROL connections. The drive signals
Sl-S4 are applied individually to the four CONTROL
connections on the lines 66, 68, 70 and 72, while the
four IN connections are tied to a positive supply vol-
tage V on a line 110. The four OUT connections are each
connected individually through a resistor Rl-R4, respec-
tively, to the first two IN connections of a second quadanalog switch 112 on a line 114. The second switch 112
receives its corresponding two CONTROL inputs individ-
ually from the forward and reverse signals on lines 62,
64 from the carriage controller 52. Only two connections
are utilized on the second analog switch 112. A suitable
commerclal device for these two quad analog switches is a
Motorola type MC14016.
The first OUT connection of the second analog
switch 112 is applied through a resistor R5 on line 116
to the inverting input of a first operational amplifier
118 having a feedback resistor R6. The second OUT con-
nection of the second analog switch 112 is likewise
applied through an equivalent resistor R5 on line 120 to
the inverting input of a second operational amplifier
122, having an identical feedback resistor R6. The out-
put of the second operational amplifier 122 is then
applied to the inverting input of a third operational
amplifier 124 through a resistor R7 on a line 128, the
operational amplifier having an identical feedback resis-
tor R7 to provide unity gain.
The operation of this circuit as thus far des-
cribed will now be explained. Depending on which, if
any, of the drive signals Sl - S4 is in a true state,
the corresponding OUT connection of the first analog
switch will have the supply voltage V applied to it.
Thus, a current will be supplied through a particular
,
114tDZ~;~
-21-
~,:
resistor, such as resistor Rl when drive signal Sl is
i
set to a true state (Sl = 1), to the two IN connections
on the second analog switch 112 on line 114. This cur-
rent will be directed through either one of the two OUT
connections of the second switch 112 depending on which
of its two control signals, the forward or reverse sig-
nals from the carriage controller 52, are in a true
state. Hence, the current will cause an inverted vol-
tage to appear at the output of the first operational
` 10 amplifier 118 if the forward signal is true (FWD =1).
On the other hand, a noninverted voltage will appear at
the output of the third operational amplifier 124 if the
reverse signal is true (REV =1).
- One of these two voltages are then applied
through identical resistors R8 on lines 130 or 132 for
summing with the feedback signal from the carriage tach-
ometer 76 through a resistor R9 on line 78, at the invert-
ing input of a fourth operational amplifier 134, having
a feedback resistor R10. The output of this fourth oper-
ational amplifier 134 is then applied to a power ampli-
fier 136 on a line 138 for appropriately energizing the
carriage motor 28 over line 74 in the forward or the
reverse direction. In accordance with well-known princi-
ples, the carriage tachometer 76 is intended to generate
an equal and opposite signal to the drive signal from the
first operational amplifier 124, as the case may be, tonull the input at the fourth operational amplifier 134,
i.e., negative feedback control.
An alternative approach for determining car-
riage location or distance D from the target track canbe described with reference to FIGS. 5 and 6.
A suitable subsystem for implementing the sig-
nal recovery subsystem shown in FIG. 1, is disclosed in
FIG. 5. A diode detector array 140 includes a central
photodetector 142 for deriving the informational content
~14~326~
-22-
of the modulated light beam and has a pair of diametri-
cally opposed tracking diodes 144, 146 on either side.
An electrical signal proportional to the intensity of
light received on the central detector 142 is provided
on lines 148, 150 to a summing junction and then to a
wide band amplifier 152, having an output directed on
line 36 to the signal processing subsystem 38 as des-
cribed above. Each tracking diode 144, 146 is disposed
to detect the portion of the modulated light beam corres-
ponding to individual tracking spots, which are producedby splitting the read beam 16 into three separate beams
by means of a diffraction grating in the optical system
14, and each diode generates an electrical signal on
lines 154 and 156, respectively, to tracking preampli-
; 15 fiers 158, 160. One preamplifier 158 had an output dir-
ected on a line 162 to the inverting output of an ampli-
fier 164 and the output of the other preamplifier 160 is
directed on a line 166 to the noninverting input of the
amplifier. The output of the amplifier 164 then provides
a tracking error signal on line 34 to the tracking sub-
system 30 and to the stop motion subsystem 32 as des-
cribed in connection with FIG. 1.
A fragmentary radial cross section of three
tracks of the video disc are illustrated in FIG. 6A;
while in FIG. 6B, the open loop differential tracting
error signal is illustrated which appears on line 34
at the output of differential amplifier 164 when the
tracking subsystem 30 is disabled and the carriage
translates rapidly in a search mode of operation.
It will be apparent that the waveform of FIG.
6B can be utilized as an indicator of trac~k crossings
and to provide a count of the number of tracks crossed
in the search mode. Considering this information in
combination with the target address A2 and the address
114~26~
;
-23-
of the particular track from which the search is initi-
ated, a continuous monitoring of carriage position rela-
tive to the target track could be derived by counting
track crossings. It should be noted, however, that such
a technique would be preferable only at relatively high
speeds of translation of the carriage. This is because
the eccentricities inherent in a video disc tend to
create false "track crossings" even when the carriage is
standing still, if the tracking subsystem is disabled.
At high rates of translation of the carriage, the effects
of these false "track crossings" due to eccentricities
will not be significant, but as the speed of the carriage
slows towards a play speed their effects may prevent an
accurate track count. Hence, in an alternative embodi-
ment, it is contemplated that track crossings may be
counted by means of the open loop differential tracking
error signal at relatively high rates of speed, while
dependence will be shirted to detection of addresses as
the carriage speed approaches play speed.
The aforedescribed information retrieval system
of the present invention satisfies a need for improved
systems capable of rapidly accessing information tracks
in apparatus of the type utilizing a radiant beam to
optically read the information stored in such tracks.
It will be apparent from the foregoing that,
while particular forms of the invention have been illus-
trated and described, 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.
-
