Dynamic information storage or retrieval – With servo positioning of transducer assembly over track... – Optical servo system
Reexamination Certificate
1999-10-27
2002-03-26
Hudspeth, David (Department: 2651)
Dynamic information storage or retrieval
With servo positioning of transducer assembly over track...
Optical servo system
C369S044280, C369S053140
Reexamination Certificate
active
06363039
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical information recording and playback apparatus which performs recording and playback using a light beam on an optical information recording medium having a transparent protective layer.
2. Description of the Related Art
Heretofore, it is known to provide an optical information recording and playback apparatus which performs recording and playback using a light beam on an optical information recording medium having a transparent protective layer, wherein the recording and playback is performed by scanning a light spot with its center displaced from the centerline of the information track on the optical information recording medium. An example of this type of optical information recording and playback apparatus is disclosed in Japanese Unexamined Patent Publication with a laid-open No. 8-77583. The configuration and features of this apparatus will be described below.
FIG. 7
is a diagram showing the configuration of a rewritable magneto-optical disk apparatus. In
FIG. 7
, reference numeral
41
is a magneto-optical disk, the information recording medium, formed from a transparent substrate of glass or plastic material with a magnetic film
42
formed thereon. The magneto-optical disk
41
is mounted on the rotational shaft of a spindle motor
43
and rotated at a prescribed speed by the rotation of the spindle motor
43
. Below the optical information recording medium
41
is disposed an optical head
44
, while a bias magnet
53
is disposed above the medium at a position opposite the optical head
44
.
The optical head
44
contains a semiconductor laser
45
as a light source for recording and playback; when recording information, the light beam of the semiconductor laser
45
is modulated by a laser driving circuit not shown in accordance with the information signal to be written. The light beam emitted from the semiconductor laser
45
is first made parallel by a collimator lens
46
, and then passes through a polarization beam splitter
47
and enters an objective lens
48
. The incident light beam is converged by the objective lens
48
and focused as a microscopic spot onto the magnetic film
42
of the magneto-optical disk
41
. In the meantime, a magnetic field oriented in a particular direction is applied from the bias magnet
43
to the magneto-optical disk
41
, and a series of information bits is recorded by the application of this magnetic field and the projection of the modulated light beam.
The light beam projected on the magneto-optical disk
41
is reflected at the medium surface thereof. The reflected light again passes through the objective lens
48
and enters the polarization beam splitter
47
at whose polarization plane the light is reflected toward a beam splitter
49
, thus being separated from the light coming from the semiconductor laser
45
. The light beam reflected into the beam splitter
49
is separated into two beams, one of which is passed through a sensor lens
50
and received by a photosensor
51
. The received light signal of the photosensor
51
is fed to an AT/AF circuit (autotracking, autofocus control circuit)
52
via an AT offset circuit
59
described later. Based on this signal, the AT/AF circuit generates a tracking error signal and a focus error signal. Using the thus generated tracking error signal and focus error signal, tracking control and focus control are performed by driving an objective lens actuator
54
and thereby moving the objective lens
48
in the tracking and focusing directions.
On the other hand, when playing back information recorded on the magneto-optical disk
41
, the light beam of the semiconductor laser
45
is set at a reading power not high enough to affect the recorded information, and this reading light beam is directed to the intended track to read the recorded information. More specifically, the reading light beam reflected from the surface of the optical information recording medium is passed through the objective lens
48
, the polarization beam splitter
47
, the beam splitter
49
, and a sensor lens
55
, and received by a photosensor
56
.
Next, off-tracking control will be described. Reference numeral
57
is an amplitude detection circuit for detecting the amplitude of a playback signal based on the received signal of the photosensor
56
. The amplitude detection circuit
57
detects cross talk and the amplitude of the playback signal when determining the off-track direction of the reading light beam, and also when determining the amount of off-tracking. The detected values are fed into a CPU
58
via an A/D converter contained therein. The CPU
58
is a processor circuit for controlling various parts of the apparatus. When the magneto-optical disk
41
is loaded into the apparatus, the CPU
58
controls the various parts to control the off-track direction and off-track amount of the reading light beam. Based on instructions from the CPU
58
, the AT offset adding circuit
59
applies an offset to the AT/AF circuit
52
to displace the reading light beam off-track in the desired direction by the desired amount.
Means for determining the off-track direction will be described. When the magneto-optical disk
41
is tilted toward its outer circumference relative to the objective lens
48
, as shown in FIG.
8
(
a
), a crescent-shaped secondary spot
801
b
is formed on the disk surface at a position outside the main spot
801
a
and displaced toward the outer circumference, resulting in increased cross talk from the outer track. On the other hand, when the magneto-optical disk
41
is tilted toward its inner circumference relative to the objective lens
48
, as shown in FIG.
8
(
b
), a crescent-shaped secondary spot
802
b
is formed on the disk surface at a position outside the main spot
802
a
and displaced toward the inner circumference, resulting in increased cross talk from the inner track.
Accordingly, by measuring the amount of cross talk from each of the adjacent tracks, the tilting direction of the magneto-optical disk
41
relative to the objective lens
48
can be identified, and thus the off-track direction of the light spot to reduce the amount of cross talk can be determined. More specifically, in the case of FIG.
8
(
a
), the outer circumferential direction is determined as the off-track direction, and in the case of FIG.
8
(
b
), the inner circumferential direction is determined as the off-track direction.
A prior art example of determining the off-track direction will be described below. A longest pattern signal is recorded in a test area provided on a magneto-optical disk. Then, its adjacent tracks are played back, and the amplitudes of the playback signals are compared. Based on the result of the comparison, the relative tilting direction of the optical information recording medium can be identified, and thus-the off-track direction of the light spot to reduce the amount of cross talk can be determined.
Next, means for determining the off-track amount will be described. While varying the off-track amount, a reading light beam is projected a predetermined number of times onto the track of the test area where the longest pattern signal is recorded. The off-track amount is obtained from the playback signal detected at this time or from the amount of cross talk leaking from specific tracks obtained by playing back the adjacent tracks.
However, the off-track amount that maximizes the amplitude of the playback signal does not necessarily coincide with the off-track amount that minimizes the cross talk; therefore, it is desirable that the off-track amount that maximizes the difference between the two quantities, as shown in
FIG. 9
, be determined as the optimum value.
Operation will be described when playing back information recorded on the magneto-optical disk
41
by using the thus determined off-track direction and off-track amount.
First, the CPU
58
reads the off-track direction and off-track amount from its internal memory and, based on the direction and amount, controls the AT
Fujikawa Yasuo
Hayashi Hideki
Horibe Ryusuke
Kadowaki Shin-ichi
Chu Kim-Kwok
Hudspeth David
Matsushita Electric - Industrial Co., Ltd.
Ratner & Prestia
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