Dynamic information storage or retrieval – With servo positioning of transducer assembly over track... – Optical servo system
Reexamination Certificate
1999-01-19
2002-01-15
Tran, Thang V. (Department: 2651)
Dynamic information storage or retrieval
With servo positioning of transducer assembly over track...
Optical servo system
C369S044410, C369S044290
Reexamination Certificate
active
06339567
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a method of and an apparatus for optical information reproduction in which a light spot traces an information track consisting of a sequence of pits by which information is recorded on an information recording medium, to read the recorded information. More particularly, the present invention relates to tracking control for guiding the light beam converged onto the medium, along the center of the track.
The invention also relates to an offset removing circuit used in such an optical information reproduction apparatus.
In recent years, optical disks such as DVDs which can be used for optical recording and reproduction of information are drawing attention as information medium capable of storing a large amount of video information and computer data. The optical disks are provided with concentric or spiral tracks at a pitch of about 1 &mgr;m, and the information is recorded along the track by means of variations in a local optical constant or physical shape.
In order to reproduce information from the optical disk of this form of recording with a high quality, the optical information reproducing apparatus controls the position of convergence of the light spot for reading the information, so that the light spot keeps tracing the track. The position control of the light spot is effected in two dimensions. The control in the direction of the optical axis is effected by a focus control means, while the control in the radial direction of the disk is effected by a tracking control means. These controls are effected by feedback control in which the position of the light spot is controlled so as to eliminate the error which is the difference between the target position of the light spot and the current position.
Various methods have been devised for producing the tracking error signal necessary for the tracking control by an optical means. Among these various methods is a phase difference method which uses a signal obtained from a main light spot for reproducing the information recorded on the information medium. The principle of the phase difference method is disclosed in Japanese Patent Kokal Publication No. 52-93222, and its counterpart, U.S. Pat. No. 4,057,833 to Braat.
FIG. 5A
to
FIG. 5E
are drawings for explaining the principle of detection of the tracking error information in the phase difference method.
FIG. 5A
shows the relative positions of the information pits and the light spot. It shows how the light spot moves in the direction of from time t0 to t4. The running position, point (&bgr;), of the light spot is the center of the track from which information is to be reproduced. The point (&agr;) is on the left side of the center of the track from which information is to be reproduced. The point (&ggr;) is on the right side of the center of the track from which information is to be reproduced.
FIG. 5B
shows the photo-electric conversion means which detects the light reflected from the information medium, and converts it into electrical signals. The illustrated photoelectric conversion means is divided into first to fourth optical detectors by a division line extending in the direction corresponding to a track tangential direction, and a division line in the direction corresponding to a direction perpendicular to the track tangential direction. Ideally, the optical system is so designed that the center of the far-field pattern of the light reflected from the recording medium is formed at the center of the four optical detectors.
In other words, the first to fourth optical detectors are situated in the far-field of the information pits in separate quadrants of an imaginary X-Y coordinate system, whose origin is disposed on an optical axis of the optical system and whose X-axis effectively extends in the track tangential direction TT and whose Y-axis effectively extends transversely to the track tangential direction TT.
The first and second optical detectors are disposed on one side of the Y-axis. The third optical detector is disposed on the other side of the Y-axis, and disposed diagonally with respect to the first optical detector. The fourth optical detector is disposed on the other side of the Y-axis, and disposed diagonally with respect to the second optical detector.
The phrase “the optical detectors are situated in the far field of the information pits” is to be understood to mean that these detectors are located in a plane in which the different orders of the light beam reflected from the information medium are sufficiently distinct, i.e., in a plane which is sufficiently far from the image of the information pits.
The phrase “the X-axis effectively extends in the track tangential direction and the Y-axis effectively extends transversely to the track tangential direction,” is to be understood to mean that the imaginary projections of these axes on the information pits extend in the track tangential direction, and transversely to the track tangential direction.
Further explanation is given in U.S. Pat. No. 4,057,833, which is hereby incorporated by reference.
A phase difference is present between the two detection signals (A+C) and (B+D) obtained by adding the outputs of the optical detectors disposed diagonally to each other, and the phase difference is proportional to the off-track amount, i.e., the amount by which the light spot is deviated from the center of the track. This is shown in
FIG. 5C
, FIG.
5
D and FIG.
5
E.
FIG. 5C
shows how the phase relationship between the two detection signals varies with the scanning position of the light spot. The waveforms on the left are the detection signal waveforms obtained when the light spot scans a point (&agr;), i.e., on the left side of the center of the pit. As will be seen, the detection signal (A+C) is leading the detection signal (B+D) in phase. The waveforms in the center are the detection signal waveforms obtained when the light spot scans a point (&bgr;), i.e., the center of the pit. As will be seen, the detection signal (A+C) and the detection signal (B+D) are in phase. The waveforms on the right are the detection signal waveforms obtained when the light spot scans a point (&ggr;), i.e., on the right side of the center of the pit. As will be seen, the detection signal (A+C) is lagging behind the detection signal (B+D) in phase.
FIG. 5D
shows the phase difference between the detection signal (A+C) and the detection signal (B+D) with respect to the scanning position of the light spot. The phase difference is represented by the pulse width. The pulse on the “+” side (above 0 level) indicates that the detection signal (A+C) is leading the detection signal (B+D), while the pulse on the “−” side (below 0 level) indicates that the detection signal (A+C) is lagging behind the detection signal (B+D). When the detection signal (A+C) and the detection signal (B+D) are in phase, no pulse occurs in the “+” side and the “−” side.
FIG. 5E
shows the pulse width, i.e., the phase difference with respect to the scanning position of the light spot, and how it is in proportion with the off-track amount from the track center. This phase difference is converted into electrical signals, and a tracking error signal necessary for tracking is thus obtained.
It is known that the tracking error signal is associated with an offset (hereinafter referred to as “first offset”) which is dependent on the pit depth. The details is described on pp. 33-38 of Technical Paper of the Institute of Electronics and Communication Engineers of Japan, OPE 96-150, “Development of a High-Precision Learning Control Method in a DVD-ROM Drive.”
FIG. 6A
to
FIG. 6D
illustrates the principle of the offset generation. The drawing shows the waveforms of the outputs (A to D) of the four detectors obtained when the light beam is positioned at the center of the track. The pit depth and the presence or absence of the lens shift are taken as parameters.
When the pit depth is &lgr;/4 (&lgr; being the wavelen
Inoue Yoshiji
Shimamoto Masayoshi
Umeyama Takehiko
Mitsubishi Denki & Kabushiki Kaisha
Tran Thang V.
LandOfFree
Method of and apparatus for optical information reproduction does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Method of and apparatus for optical information reproduction, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Method of and apparatus for optical information reproduction will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2832142