Dynamic information storage or retrieval – With servo positioning of transducer assembly over track... – Optical servo system
Reexamination Certificate
1998-08-25
2001-07-17
Tran, Thang V. (Department: 2651)
Dynamic information storage or retrieval
With servo positioning of transducer assembly over track...
Optical servo system
C369S013010, C369S044260
Reexamination Certificate
active
06262955
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a track control system for tracking a laser light beam along the center of an optical recording medium, and more particularly to a tracking control method and apparatus that is adaptive to an optical recording medium having wobbled groove signal tracks. Also, this invention is directed to an optical recording medium suitable for the tracking control method.
2. Description of the Prior Art
Recently, optical recording media have been prevalent as recording media for recording information such as video and audio data. Such optical recording media include a read-only type disc such as CD-ROM, DVD-ROM, etc.; a write-once-read-many (WORM) type disc such as CD-R, DVD-R, etc.; and a rewritable type disc such as CD-RW, DVD-RAM, etc.
As shown in
FIG. 1
, the read-only type disc includes spiral-shaped tracks
2
extending from the inner circumference into the outer circumference thereof. Information pit trains
4
representing recorded information are defined on the track
2
. The information pit trains
4
are usually arranged in a line along a reference line and distances between adjacent tracks are constant.
Typically, a one-beam or three beam system tracking control is used to reproduce the information recorded on the pit trains
4
on the tracks of the read-only type disc. The one-beam or three-beam system tracking control allows a light beam to trace the track depending upon a symmetry of the information pit trains in the width direction of the tracks.
On the other hand, in the WORM type disc and the rewritable optical disc, guiding grooves for the tracking control are provided instead of the pit trains. An example of such a disc is a CD-R type disc shown in FIG.
2
.
The CD-R in
FIG. 2
has land and groove signal tracks
10
and
12
that are defined in parallel in a spiral shape. Each groove signal track
12
has a wobbled area in which both sides thereof are wobbled in the same phase; while each land signal tracks
10
has a wobbled area in which both sides thereof are wobbled in a different phase. Information is recorded on the same-phase wobbled area. In other words, information is recorded only on the groove signal tracks
12
. In a disc having the land and groove tracks
10
and
12
as described above, the tracking control is easy because the boundaries between adjacent tracks
10
and
12
are distinguished. However, the recording capacity is reduced because an information is recorded only on the groove signal tracks
12
.
To increase the recording capacity of the disc having the land and groove signal tracks, a so-called ‘land/groove recording system disc’ has been developed, wherein information can be recorded on both the land and groove signal tracks like a DVD-RAM shown in FIG.
3
.
The DVD-RAM of
FIG. 3
has land and groove signal tracks
10
and
12
that are divided into a number of sectors defining the size of an information to be recorded. Each sector includes an identification information area, hereinafter referred simply to as “ID area”, in which an address information and the like is recorded in prepit trains
14
, and a recording area in which information can be recorded on both land and groove tracks
10
and
12
.
Each prepit train
14
of the ID area is divided into a first prepit train
14
a
for the land track and a second prepit train
14
b
for the groove track. These first and second prepit trains
14
a
and
14
b
are positioned at an extending line of the land and groove boundary side, respectively. Since the first and second prepit trains
14
a
and
14
b
are positioned as mentioned above, an optical pickup reads all the first and second prepit trains
14
a
and
14
b
positioned at the extending line of the boundary side of a track being currently traced when a laser light beam traces the land track
10
as well as the groove track
12
.
Two identification information signals for two prepit trains
14
a
and
14
b
read in this manner are processed employing only one of the prepit trains, for example, only the first prepit train
14
a
when a light beam is positioned at the land track
10
. Accordingly, an information is recorded on both the land and groove tracks
10
and
12
.
To accurately track an optical disc, it is necessary to have a tracking control apparatus for controlling a position of light beam in such a manner that the light beam is traced along the center lines of the land and groove tracks
10
and
12
. A tracking control apparatus of push-pull system is usually used as such a tracking control apparatus. The push-pull system tracking control apparatus performs a tracking on a basis of a tracking error signal Te produced by subtracting two photo detecting signals from a two-divisional photo detector
20
as shown in FIG.
5
.
However, the push-pull system tracking control apparatus must invert the polarity of a tracking error signal Te for one of the land and groove tracks
10
and
12
when it is used for an optical disc of land/groove recording system as mentioned earlier. This is caused by a fact that the tracking error signal Te has positive(+) and negative(−) amplitudes as shown in
FIG. 4
as the light beam is moved in a radial direction.
Referring to
FIG. 4
, the tracking error signal Te has a positive(+) amplitude when the light beam is positioned between the center line of the land track
10
and the center line of a groove track
12
when moving towards the inner circumference; and has a negative(−) amplitude when the light beam is positioned between the center line of the land track
10
and the center line of a groove track
12
when moving towards the outer circumference. Accordingly, the tracking control apparatus must respond to the tracking error signal Te depending upon whether a light beam is traced along the land track
10
or the groove track
12
.
More specifically, if a light beam is traced along the land track
10
and the tracking error signal Te has a negative(−) polarity, then the tracking control apparatus must move the light beam toward the inner circumference. Otherwise, if a light beam is traced along the groove track
12
and the tracking error signal Te has a negative(−) polarity, then the tracking control apparatus must move the light beam toward the outer circumference. As a result, the push-pull system tracking control apparatus must identify the land and groove tracks and control the polarity of the tracking error signal based on the identified result when it is used for an optical disc of land/groove recording system.
For instance, as shown in
FIG. 5
, a conventional push-pull system tracking control apparatus used for an optical disc of land/groove recording system includes a subtractor
22
and a low pass filter(LPF)
24
that are connected, in cascade, to a two-divisional photo detector
20
. The two-divisional photo detector
20
converts the light beam reflected by the land or groove track
10
or
12
into an electrical signal to generate first and second photo detecting signals for representing a light distribution state. The subtractor
22
subtracts the first and second photo detecting signals from the two-divisional photo detector
20
to produce a tracking error signal Te. As shown in
FIG. 4
, the tracking error signal Te has a waveform that changes polarities as the light beam is moved in the radial direction of the optical disc. The LPF
24
eliminates high frequency noise signals included in the tracking error signal Te.
The push-pull system tracking control apparatus further includes a buffer
26
and an inverter
28
connected, in parallel, to the LPF
24
, a control switch
30
for selecting any one of output signals of the buffer
26
and the inverter
28
, and a tracking controller
32
and a tracking actuator
34
connected, in series, to the control switch
30
. The buffer
26
passes the tracking error signal Te from the LPF
24
and the inverter
28
inverts the tracking error signal Te from the LPF
24
, and both signals Te and Te′ are applied to
LG Electronics Inc.
Tran Thang V.
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