Dynamic information storage or retrieval – With servo positioning of transducer assembly over track... – Optical servo system
Reexamination Certificate
1998-07-10
2001-03-20
Edun, Muhammad (Department: 2753)
Dynamic information storage or retrieval
With servo positioning of transducer assembly over track...
Optical servo system
C369S044410, C369S124010
Reexamination Certificate
active
06205094
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a tracking system for optical storage media e.g. for a laser disk, which insures a correct alignment of a laser beam on a track being read. The invention more specifically relates to a circuit for processing the signals provided by a photosensor.
FIG. 1
schematically shows a conventional tracking system for a laser disk. A laser beam L is directed perpendicularly to the surface of an optical disk
10
by a semi-reflecting transparent plate
12
. A servo mechanism
13
displaces the laser beam L so that is follows the desired track of the disk. The laser beam reflected by disk
10
is sensed by a photosensor
14
. A circuit
16
processes the signals provided by photosensor
14
to produce a tracking signal T indicative of the position of the laser beam on photosensor
14
.
Tracking signal T is provided to a servo-control circuit
18
which accordingly controls the servo mechanism
13
to make the tracking signal converge to a desired value.
FIG. 2
shows in more detail the structure of a photosensor
14
used for optical storage media, e.g. laser disks, compact disks (CD), Digital Video Disks (DVDs), and illustrates the signals produced by such a photosensor when the laser beam is correctly aligned.
Photosensor
14
is a square matrix of four independent sensor areas A, B, C and D.
When the laser beam is correctly aligned, it produces a spot centered on sensor
14
. The surface of the laser disk has a mirror layer and pits are used to store the digital information on the disk. Due to the depth of the pits, light from the laser will be reflected in-focus or out of focus to the photo-sensor. An in-focus reflection results in a clear spot on the sensor. To simplify, it can be said that the digital information on the laser disk results in a spot
o-spot signal on the sensor.
On the left of
FIG. 2
, it is schematically shown that the laser beam is passing from a non-reflective area to a reflective area, whereby circle S progressively fills with light. The unlit portion of circle S is shown in dotted lines. The interface I between the sensor pairs A/B and C/D is assumed to be parallel to the track being read. Then, the growing spot arriving on sensor
14
has a head edge E which moves along interface I, progressively scanning sensor
14
within circle S (from left to right in FIG.
2
). On the right of
FIG. 2
, the laser beam is passing from a reflective area to a non-reflective area of the disk. Then, the spot progressively fades out as a tail edge E scans circle S along interface I.
When the laser beam is correctly aligned, sensor areas A and D receive the same amount of light at any time, which is also true for sensor areas B and C. When the spot starts to appear on the left of
FIG. 2
, signals A and D, produced by sensor areas A and D, progressively rise until edge E reaches the center of sensor
14
. Then, signals A and D stay constant at a maximum value while signals B and C, produced by sensor areas B and C, start to rise progressively. Signals B and C reach their maximum value when the spot is full, i.e. when edge E reaches the right most limit of circle S.
When the spot starts to fade out, on the right of
FIG. 2
, signals A and D progressively decrease until edge E reaches the center of sensor
14
. At this point, signals A and D are at their minimum value while signals B and C start to decrease progressively until the spot has completely faded out.
Waveforms A+C and B+D will be described later.
In
FIG. 3
, the laser beam is incorrectly aligned and produces a spot S on sensor
14
which is not centered. The spot will be shifted upwards or downwards, depending on the direction of the alignment error.
In the example of
FIG. 3
, the spot is shifted downwards. In this case, the light intensity received by sensor area D starts increasing or decreasing earlier than the light intensity received by area A, while the intensity received by area C starts increasing or decreasing later than the intensity received by area B. This is illustrated by the waveforms of signals A to D. Moreover, the maximum amplitude of signals A and B is lower than that of signals C and D, because sensor areas A and B will always receive less light than areas C and D.
It is to be understood that the description given above is schematic and describes the reality in a somehow simplified way.
It will be remarked that the misalignment of the laser beam is indicated both by the amplitude differences and the phase shifts of the sensor signals.
SUMMARY OF THE INVENTION
An object of the invention is to provide a simple, yet reliable, circuit for producing a tracking signal indicative of the alignment of the laser beam from the sensor signals.
Such a tracking signal circuit uses the phase differences and not the amplitudes of the sensor signals.
The invention more specifically aims at a tracking system for an optical disk, comprising matrix of four sensors receiving a beam reflected by the disk; one processing channel for each sensor signal, including means for producing a binary signal from the sensor signal, and an adjustable edge delay circuit operating on the binary signal; one adder for each pair of channels corresponding to diagonal sensors of the matrix; and a phase detector comparing the outputs of the adders.
According to an embodiment of the invention, the means for producing the binary signal comprises a limiter, the output signal of which is input to a data slicer which outputs the binary signal.
According to an embodiment of the invention, each edge delay circuit comprises means for charging a capacitor at a constant current when the corresponding binary signal switches to a first logic state; means for discharging the capacitor at said constant current when the binary signal switches to the second logic state; means for stopping charging and discharging of the capacitor between the moment when the voltage across the capacitor reaches a high threshold or a low threshold and a subsequent switching of the binary signal; and a first comparator connected to switch the state of an output signal when the voltage across the capacitor crosses a third threshold comprised between the first and second thresholds.
According to an embodiment of the invention, said means for charging, discharging and stopping comprise first and second equal value constant current sources coupled to a first supply terminal; a first switch controlled by the binary signal for selectively coupling the first current source to the first or second terminal of the capacitor; a flip-flop set to a first state when the high threshold is reached and set to a second state when the low threshold is reached; a second switch controlled by the flip-flop for selectively coupling the second current source to the first or second terminal of the capacitor; and third and fourth constant current sources having values equal to those of the first and second current sources, each coupling a second supply terminal to a respective terminal of said capacitor.
According to an embodiment of the invention, the adjustable delay circuit comprises a second comparator for setting the flip-flop to the first state, receiving the voltage on the first terminal of the capacitor and a first reference voltage; and a third comparator for setting the flip-flop to the second state, receiving the voltage on the second terminal of the capacitor and said first reference voltage.
According to an embodiment of the invention, the system comprises a servo-control system for displacing the beam in one direction perpendicularly to the disk tracks when the phase detector indicates a phase lag, and for displacing the beam in the opposite direction when the phase detector indicates a phase lead.
The foregoing and other objects, features, aspects and advantages of the invention will become apparent from the following detailed description of embodiments, given by way of illustration and not of limitation with reference to the accompanying drawings.
REFERENCES:
patent: 4541082 (1985-09-01), Horikoshi et al.
patent: 4866
Edun Muhammad
Kiel Paul P.
Laks Joseph J.
Thomson Licensing S.A.
Tripoli Joseph S.
LandOfFree
Tracking system for optical storage media does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Tracking system for optical storage media, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Tracking system for optical storage media will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2447735