Optical disk apparatus and PLL circuit

Details Optical disk apparatus and PLL circuit Optical disk apparatus and PLL circuit

2001-05-10

2004-01-20

Tran, Thang V. (Department: 2651)

Dynamic information storage or retrieval

With servo positioning of transducer assembly over track...

Optical servo system

C369S059170, C369S053190, C369S059190

Reexamination Certificate

active

06680887

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical disk apparatus for optically reproducing a signal on an information carrier utilizing an optical beam from a light source, such as, for example, a semiconductor laser, and a PLL circuit for removing an off set in a data slice balance circuit.
2. Description of the Related Art
One type of optical disk apparatus reproduces a signal by directing an optical beam generated by a light source, such as a semiconductor laser on a circular information carrier (for example, an optical disk), in a converged state, rotating at a prescribed rotation rate. The information carrier has a very narrow spiral track including information pit arrays. An optical disk apparatus generally performs three types of controls, namely, rotation control for rotating an optical disk at a prescribed rotation rate, focusing control for directing an optical beam on the optical disk in a converged state, and tracking control for controlling the optical beam to correctly scan the track on the optical disk.
There are various types of optical disks including reproduction-only disks, such as CD-audio, CD-ROM and DVD-ROM, having information recorded in pit arrays, once-recordable disks, such as CD-R and DVD-R, and recording and reproduction disks, such as CD-RW, DVD-RAM and DVD-RW. The recording density, the reflectance of the disk and the type of reproduction signal vary in accordance with the type of disk. Accordingly, it is required that the optical disk apparatus optimizes the reproduction method so as to be suitable for the features of each type of disk. Specifically, in order to reproduce two or more types of disks or switch the reproduction rate, the optical disk apparatus conventionally optimizes the reproduction method by adapting the frequency characteristics of a waveform equalizer included in the optical disk apparatus.
FIG. 27
is a graph illustrating the relationship between the gain, noise and frequency of a waveform equalizer. A reproduction signal from the optical disk is a random signal, but a signal component having a higher frequency has a drastically reduced gain due to inter-code interference, as compared to a signal component having a lower frequency. Accordingly, as shown in
FIG. 27
, a prescribed frequency of the reproduction signal is boosted by the waveform equalizer so as to improve the gain of the signal component having a higher frequency, and further the gain is rapidly reduced by a higher-order equal ripple filter or the like so as to remove noise in a frequency band which is higher than the prescribed frequency FC.
FIG. 28
is a graph illustrating a jitter characteristic with respect to a focusing position. When the focusing position is shifted from the optimum position (indicated as the “just focusing position 2701” in FIG.
28
), the jitter is increased. This occurs since the gain of the reproduction signal is reduced as shown in FIG.
27
and thus the S/N ratio of the reproduction signal is lowered, resulting in deterioration of the quality of the reproduction signal. In order to detect the jitter and determine the optimum frequency of the waveform equalizer at which the jitter is minimum, the focusing position needs to be kept at the just focusing position
2701
.
As a technology to realize this, Japanese Laid-Open Publication No. 10-69657 discloses measuring the jitter of a waveform-equalized reproduction signal and two-dimensionally varying the focusing position and a cut-off frequency of the waveform equalizer so as to find the optimum focusing position and cut-off frequency at which the jitter is minimum.
In a conventional optical disk apparatus having the above-described function, the quality of the reproduction signal is kept high by executing all combinations of the values of two parameters, for example, the focusing position and the cut-off frequency, and measuring the jitter at each combination so as to find the optimum combination at which the jitter is minimum.
However, as it becomes necessary to reproduce more types of media or optical disks, the above conventional method is not always successful. Especially when reproducing recordable disks, such as CD-R, CD-RW, DVD-R, DVD-RW and DVD-RAM, the jitter is not reduced unless the focusing position is adjusted separately from the cut-off frequency in order to find the cut-off frequency and boost amount.
In addition, as the reproduction rate of the disks is varied, it is necessary to reproduce signals of a greater number of frequency bands in order to deal with, for example, CDs of the standard rate to 48× and DVDs of the standard rate to 16×. It is not appropriate to execute all of the combinations of the parameters of the reproduction rate and find the optimum combination at which the jitter is minimum each time an optical disk apparatus starts operation, since this results in the start-up time becoming unbearably long. It is also not appropriate to execute all of the combinations of the parameters of the reproduction rate and find the optimum combination at which the jitter is minimum each time the reproduction rate is changed, since this results in it being impossible to read the signal immediately.
SUMMARY OF THE INVENTION
According to one aspect of the invention, an optical disk apparatus, for optically reproducing information recorded on an information carrier by an optical beam emitted by a light source, includes a converging section for directing the optical beam to the information carrier in a converged state; a light detection section for generating a reproduction signal corresponding to the optical beam reflected by or transmitted through the information carrier; a waveform equalization section for varying a frequency characteristic of the reproduction signal generated by the light detection section; a jitter measuring section for measuring a jitter of a signal output from the waveform equalization section; and a minimum jitter value searching section for determining a minimum value of the jitter in an initial area of an X-Y plane defined by a variable x and a variable y which can vary the jitter measured by the jitter measuring section. The minimum jitter value searching section divides the initial area into a plurality of first divided areas having an identical shape to each other, each of which has a respective first jitter measuring point at the center of gravity thereof; and performs a search operation to find one of the plurality of first divided areas which has the first jitter measuring point at which a minimum jitter is measured among the first jitter measuring points. The minimum jitter value searching section divides the first divided area, having the first jitter measuring point at which the jitter is minimum, obtained by the search operation into a plurality of second divided areas having an identical shape to each other, each of which has a respective second jitter measuring point at the center of gravity thereof; performs a search operation to find one of the plurality of second divided areas which has the second jitter measuring point at which a minimum jitter is measured among the second jitter measuring points; and determines an optimum combination of the variable x and the variable y which corresponds to the second divided area, having the second jitter measuring point at which the jitter is minimum found by the search operation.
In one embodiment of the invention, the minimum jitter value searching section divides the initial area into four first divided areas having an identical shape to each other, each of which has a respective first jitter measuring point at the center of gravity thereof; and performs a search operation to find one of the four divided areas which has the first jitter measuring point at which a minimum jitter is measured among the first jitter measuring points. The minimum jitter value searching section divides the first divided area obtained by the search operation into four second divided areas having an identical shape to each other, each of which has a respecti

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