Dynamic information storage or retrieval – With servo positioning of transducer assembly over track... – Optical servo system
Reexamination Certificate
2002-04-24
2004-05-18
Hindi, Nabil (Department: 2655)
Dynamic information storage or retrieval
With servo positioning of transducer assembly over track...
Optical servo system
C369S044310, C369S044410
Reexamination Certificate
active
06738325
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention generally relates to optical disk units, and more particularly to an optical disk unit that can record data on an optical disk.
2. Description of the Related Art
Optical disks of a direct-read-after-write type can be categorized into two types: a write-once type and an erasable type. Guiding grooves are formed on a compact disk recordable (CD-R), which is a write-once optical disk, and a compact disk rewritable (CD-RW), which is an erasable optical disk. The grooves wobble slightly in a radial direction at a center frequency of 22.05 kHz. Address information at the time of recording called Absolute Time In Pregroove (ATIP) is multiplexed and recorded in the grooves by frequency shift keying (FSK) with a maximum deviation of ±1 kHz.
In an optical disk unit for data recording and reproduction of such a recordable optical disk, a light beam is emitted onto the optical disk, the reflected light is detected by a plurality of photodetectors, a tracking error signal is generated by performing a given calculation, and a tracking actuator is driven based on the tracking error signal by a tracking servo circuit.
Here, in an optical disk unit for data recording and reproduction of the CD-R that is a write-once optical disk, the power of a light beam is set to a read power at the time of reproduction, while, at the time of recording, the power of the light beam alternates between a write power and the read power (write power>read power) in accordance with the value 0 and the value 1 of a recording signal. Therefore, at the time of not only reproduction but also recording, a tracking error signal is generated by sampling and holding a reflected light of a time at which the power of the light beam is set to the read power.
Further, in an optical disk unit for data recording and reproduction of the CD-RW that is an erasable optical disk, the power of a light beam alternates between a write power and an erase power (write power>erase power>read power) in accordance with the value 0 and the value 1 of a recording signal at the time of recording.
Therefore, at the time of reproduction, a tacking error signal is generated by detecting a reflected light of a time at which the power of the light beam is set to the read power, while, at the time of recording, the tracking signal is generated by sample-holding a reflected light of a time at which the power of the light beam is set to the erase power.
This is because a reflected light of a time at which the power of the light beam is set to the write power is unstable due to the effect of formation of recording regions, which are pits in the CD-R and marks in the CD-RW.
The differential push-pull method is one of the known tracking control methods. According to the differential push-pull method, a spot
2
of a main light beam (a main light beam spot
2
) is formed on a groove
1
forming a track n and leading and trailing spots
3
and
4
of sub light beams (leading and trailing sub light beam spots
3
and
4
) are formed in positions separated from the main light beam spot
2
by a given length in directions opposite to each other along the width of the groove
1
as shown in FIG.
1
. The leading sub light beam spot
3
precedes the main light beam spot
2
in a scanning direction indicated by the arrow in FIG.
1
. The trailing sub light beam spot
4
follows the main light beam spot
2
in the scanning direction. A reflected beam from the main light beam spot
2
is detected by two photodetectors
10
A and
10
B shown in
FIG. 2
that are divided in the directions of the width of the groove
1
perpendicular to the scanning direction of
FIG. 1. A
reflected beam from the leading sub light beam spot
3
is detected by two photodetectors
12
A and
12
B that are divided in the width directions of the groove
1
. A reflected beam from the trailing sub light beam
4
is detected by two photodetectors
14
A and
14
B that are divided in the width directions of the groove
1
. The letters As and Bs of the light beam spots
2
through
4
of
FIG. 1
correspond to the detectors
10
A and
10
B,
12
A and
12
B, and
14
A and
14
B of
FIG. 2
, respectively.
FIG. 2
is a diagram showing a configuration of a conventional tracking error signal generation circuit employing the differential push-pull method. In
FIG. 2
, the detection signals of the photodetectors
10
A and
10
B are supplied via a sample-and-hold circuit
16
to the non-inverting input terminal and the inverting input terminal of a subtractor circuit
18
, respectively. A difference signal output from the subtractor circuit
18
is supplied to the non-inverting terminal of a subtractor circuit
20
. The sample-and-hold circuit
16
samples the detection signals of the photodetectors
10
A,
10
B,
12
A,
12
B,
14
A, and
14
B at read-power or erase-power timings and holds the sampled detections signals during a write-power time (period).
The detection signals of the photodetectors
12
A and
12
B are supplied via the sample-and-hold circuit
16
to one input terminal of an adder circuit
22
and one input terminal of an adder circuit
24
, respectively. The detection signals of the photodetectors
14
A and
14
B are supplied via the sample-and-hold circuit
16
to the other input terminal of the adder circuit
22
and the other input terminal of the adder circuit
24
, respectively. The adder circuit
22
adds the two supplied signals and supplies an output signal to the non-inverting input terminal of a subtractor circuit
26
. The adder circuit
24
adds the two supplied signals and supplies an output signal to the inverting input terminal of the subtractor circuit
26
. A difference signal output from the subtractor circuit
26
is amplified by a gain k in an amplifier
28
for the differential push-pull method and supplied to the inverting input terminal of the subtractor circuit
20
.
An error signal output from the subtractor circuit
20
is supplied to an adder
30
, which adds an offset at the time of reproduction or recording (a reproduction or recording offset) supplied from a switch
32
to the error signal. Thereby, the error signal is output from a terminal
34
as a tracking error signal (TES). By performing tracking control so that the tracking error signal is caused to be zero, tracking is performed so that the main light beam spot
2
follows the center of the groove
1
.
The read power and the write power, and further, the erase power and the write power of a laser light (a light beam) emitted from a laser diode are different so as to cause a shift of the optical axis of the laser light. As shown in
FIG. 3
, a laser light emitted from a laser diode
36
is as indicated by a solid line at a read-power or erase-power time and as indicated by a broken line at a write-power time, thus causing a shift of the optical axis of the laser light by an angle &thgr;. If the shift of the optical axis is caused in the width directions of the groove
1
, recording regions (pits in the case of the CD-R) formed by the write power of the laser light are deviated from the center of the groove
1
since the tracking error signal is also generated at the read-power or erase-power time at the time of recording. In order to correct this deviation of the recording regions, the switch
32
is provided to perform switching between the reproduction offset and the recording offset. During recording, the switch
32
remains switched to the recording-offset side all the time.
In the production process of the conventional optical disk unit, a signal is actually recorded on an optical disk and the recorded signal is reproduced. Thereby, the offset of the tracking error signal at the time of recording is detected and maintained. However, if a change in the characteristic of the laser diode
36
is caused by a change in temperature or the passage of time so as to effect a change in the amount of a shift of the optical axis from its position at the read-power time to its position at the write-power time, a trac
Anderson Kill & Olick P.C.
Lieberstein Eugene
Meller Michael N.
Teac Corporation
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