Dynamic information storage or retrieval – With servo positioning of transducer assembly over track... – Optical servo system
Reexamination Certificate
1998-08-14
2001-01-16
Edun, Muhammad (Department: 2753)
Dynamic information storage or retrieval
With servo positioning of transducer assembly over track...
Optical servo system
C369S044280, C369S044130
Reexamination Certificate
active
06175540
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to an optical information recording and reproducing apparatus, and more particularly to a tracking control apparatus and method wherein a light beam accurately trace signal tracks formed on a recording medium to include a wobbled area.
2. Description of the Prior Art
Recently, an optical recording medium, an optical magnetic recording medium and so on have been developed as a recording medium for recording various information such as video and audio information, etc. and is commercially available in the market. Such an optical recording medium includes a read-only type disc such as CD, CD-ROM, DVD-ROM, etc., a write-once-read-many type disc such as CD-R, DVD-R, etc., and a rewritable type disc such as CD-RW, DVD-RAM, etc.
In reproducing the optical disc, a light beam must be irradiated onto the center of a track recorded with an information to trace the track at a constant speed so that a photo detector may accurately detect a variation in a light quantity reflected by the track and further an accurate information may be reproduced from electrical signals from the photo detector. In this case, a converged state of a light irradiated onto the track is controlled in accordance with a distribution of a reflected light irradiated onto the photo detector. Also, a tracking state is controlled in accordance with a distribution of a reflected light irradiated onto the photo detector. Electrical signals converted by means of the photo detector are processed to thereby detect a rotation speed of the optical disc, which is controlled in accordance with the detected rotation speed. Further, since a reproducing position on the optical disc, that is, a track position onto which a laser beam is irradiated is seen from the reproduced information, the optical disc is accessed in a random basis.
However, since a recording disc has not been recorded with signals as described above, it must be manufactured to have such a signal track structure that is capable of recording an information. The signal track must be formed to have land and groove pattern in such a manner to indicate a tracking state, and support signals indicating a rotation speed of the disc are written into the signal track. Also, a physical position information is written into the signal track so that a random access of the recording optical disc is available at the time of recording.
Accordingly, in an optical disc such as CD-R, CD-RW and so on, as shown in
FIG. 1
, there are formed signal tracks
2
and
4
continued in a spiral shape and having land and groove patterns. In such an optical disc, groove signal tracks
4
only are usually used as tracks for recording an information. The groove track
4
includes an wobbled area
1
in which a certain period of wobbling signal is preformatted. The wobbling signal includes an address and the like indicating the physical position of a signal track. A rotation speed information of the optical disc and a recording position information of the optical disc, that is, an information about a position of the signal track onto which a laser beam is irradiated, are obtained from the wobbling signal on the wobbled area
1
.
Likewise, in a DVD-RAM, as shown in
FIG. 2
, signal tracks
6
and
8
having land and groove patterns are intermittently formed. These signal tracks
6
and
8
is divided into data sectors which each consists of a head field and a recording field. The head field is provided with prepit stream
12
and
14
, and the recording field is provided with land or groove racks
6
or
8
. Each prepit stream
12
and
14
include a sector identification code so as to indicate the physical position of the data sector. The first prepit stream
12
indicates the physical position of the land signal track
12
and the second prepit stream
14
indicates the physical position of the groove signal track
14
. Wobbled areas, in which a certain period of wobbling signals are preformatted in the same phase, are formed at each side of the land or groove signal track
6
or
8
.
In order to allow a light beam to trace the center of land and groove tracks defined in a recording medium, there has been widely used a tracking control apparatus which is classified into three beam system and one beam system called “pushpull system”. Most information recording and reproducing apparatus have employed the tracking control apparatus of one beam system because the tracking control apparatus of one beam system is capable of simplifying a configuration of optical pickup as well as enhancing a light efficiency at the time of recording in comparison to the tracking control apparatus of three beam system. advantages of the one beam system tracking control apparatus will he apparent from the following description with reference to
FIGS. 3A and 3B
.
Referring to
FIG. 3A
, the one beam system tracking control apparatus includes a spindle motor
18
for rotating an optical disc
16
, an optical pickup
19
for accessing the optical disc
16
, and a tracking error detector
20
and a tracking servo
26
that are connected, in series, to the optical pickup
19
. The optical pickup
19
includes a beam splitter BS for guiding a light beam from a laser diode LD to an objective lens OL and a light beam from the objective lens OL to a photo detector PD, and an actuator AC for moving the objective lens OL in the up, down, left, and right direction to perform a focusing and a tracking. The objective lens OL converges a light beam progressing from the beam splitter BS to the optical disc
16
. The beam splitter BS allows a light beam from the laser diode LD to be irradiated, via the objective lens OL, onto land signal tracks
2
and
6
or groove signal tracks
4
and
8
of the optical disc
16
, and allows a light beam reflected by the optical disc
16
to be progressed to a photo detector PD. The actuator AC moves the objective lens OL in the left and right direction in such a manner that a light beam trace the center lines of the land signal tracks
2
and
6
or the groove signal tracks
4
and
8
. The photo detector PD converts a reflective light quantity received, via the objective lens OL and the beam splitter BS, from the optical disc
16
into an electrical signal. As shown
FIG. 3B
, the photo detector PD consists of four photo detecting pieces PDa, PDb, PDc and PDd so as to detect a distribution of a light irradiated on the signal tracks
2
and
6
, or
4
and
8
. The tracking error detector
20
receiving electrical signals from the four photo detecting pieces PDa, PDb, PDc and PDd includes a differential amplifier
22
for detecting a difference signal between a sum signal of electrical signals from two photo detecting pieces PDa and PDd positioned at the left side on the basis of a track direction and a sum signal of electrical signals from two photo detecting pieces PDb and PDc at the right side, and a low pass filter(LPF)
24
for making a low pass filtering of the difference signal from the differential amplifier
22
and applying the filtered difference signal to the tracking servo
26
as a tracking error signal Te. The tracking servo
26
receiving the tracking error signal Te from the LPF
24
responds to the tracking error signal Te to control a current signal or a voltage signal applied to the actuator AC, thereby moving the objective lens OL in the left or right direction, that is, in the outer or inner circumference direction of the optical disc in such a manner that a light beam traces the center line of the track.
According to the tracking control apparatus of one beam system as described above, however, since a direct current(DC) offset is generated as the objective lens OL is moved in the left or right direction, it is difficult to allow a light beam to trace the center line of the signal track. This DC offset is generated when the center of the objective lens OL departs from the optical axis as shown in
FIGS. 4A and 4C
; while it is not generated when the center of the objective lens OL falls at the optical
Edun Muhammad
LG Electronics Inc.
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