Digital recorded data reproducing device

Details Digital recorded data reproducing device Digital recorded data reproducing device

2001-11-13

2004-01-06

Edun, Muhammad (Department: 2655)

Dynamic information storage or retrieval

Binary pulse train information signal

Including sampling or a/d converting

C369S059100, C369S124010

Reexamination Certificate

active

06674707

ABSTRACT:

TECHNICAL FIELD
The present invention relates to a digital recorded data reproducing device which reproduces digital data recorded in a recording medium and, more particularly, to one having its phase-locked loop and offset correction improved, and to one which has reproduction digital data quality improved as well as playability enhanced under unfavorable conditions such as a deterioration in a reproduced waveform quality due to tilt, a reproduction on condition of bad signal-noise ratio, and a frequent occurrence of defect or the like.
As is generally known, a tilt represents a deviation of an angle made between a perpendicular standing on a signal surface of an optical disk and an optical axis of a laser beam, and a defect represents a disturbing factor to a reproduced waveform such as a scratch or a fingerprint adhesion on the signal surface of the optical disk.
BACKGROUND ART
A method of keeping a linear velocity constant to uniformize recording density on a recording medium is commonly employed as a method for recording digital data on an optical disk medium as seen in Compact Disk or DVD.
When digital data are reproduced based on a reproduction signal of an optical disk which is subjected to mark width modulation and to digital modulation recording so that linear recording density is constant, a phase-locked pull-in is conventionally performed by detecting a phase of a clock component held by the reproduction signal and constituting a phase-locked loop.
At the time, when the frequency of the clock component held by the reproduction signal and the frequency of a clock generated by the phase-locked loop differ vastly, it is likely that a phase-locked pull-in is not completed or that a pseudo pull-in to a different frequency is performed.
As a means for avoiding this problem, conventionally a reproduction linear velocity cycle is detected by a specified pulse length or pulse interval included in a reproduction signal and control of a disk rotation speed or of a free-running frequency of a phase-locked loop is performed, thereby enabling a normal phase-locked pull-in.
For example, there is a disk reproduction system as shown in
FIG. 23. A
digital recording code as shown in FIG.
24
(
a
) is recorded on an optical disk
55
so that linear recording density is constant. Recorded data are supposed to be data regulated to have three to fourteen pieces of consecutive “0” or “1” as in 8-16 modulation system, for example. A signal obtained by a reproduction at a reproducing means
56
such as an optical pickup has its amplitude attenuated in a higher frequency component according to an increase in the recording density of the recorded data in a linear direction as shown in FIG.
24
(
a
). This is because an effect of interference is significantly produced according to an increase in recording density, and thus, the signal obtained by reproduction is amplified by a preamplifier not shown and is subjected to correction which emphasizes a higher frequency component by a waveform equalizing means
2
thereafter.
A reproduction signal subjected to high frequency emphasis is sampled to a digital signal with multiple bits by an analog/digital converter
3
as a means for converting an analog signal to a digital signal as shown in FIG.
24
(
b
). At this time, a reproduction clock generated by a VCO (voltage Control Type Oscillator)
40
is employed as a sampling clock, and when a phase of the reproduction clock by the VCO
40
and a phase of a clock component held by the reproduction signal by the reproducing means
56
are synchronized with each other, sampling data as shown in FIG.
24
(
c
) are obtained.
FIG.
24
(
c
) illustrates sampling data suitable particularly for Partial Response Maximum Likelihood (hereinafter, abbreviated as “PRML”) signal processing system. The PRML signal processing system is one which requires no high frequency component in a reproduction signal by adding a waveform interference intentionally and enhances an error rate of reproduction data by using a maximum likelihood decoding method, which demodulates the most probable series by a probability calculation considering the waveform interference, together in a reproduction system in which an amplitude of a high frequency component of a signal deteriorates and a signal-noise ratio increases with an increase in recording density in a linear recording direction.
The sampled digital signal with multiple bits is inputted to an offset correction means
4
to correct an offset component included in the reproduction digital signal. The reproduction digital signal subjected to offset correction is inputted to a transversal filter
6
to perform a partial response equalization.
At this time, it is characteristic that an equalization output signal is multivalued by applying the partial response equalization as shown in FIG.
24
(
d
). A weighting factor of a tap of the transverse filter
6
is supplied by a tap weighting factor setting means
57
for setting weighting factors employing Least Mean Square (hereinafter, referred to as “LMS”) algorithm which minimizes a mean square value of an equalization error. The multivalued output signal of the transversal filter
6
is demodulated by a viterbi decoder
58
as a kind of maximum likelihood decoder to obtain binary digital data.
A phase-locked reproduction clock used in sampling by the analog/digital converter
3
is controlled as follows. That is, a position where an output signal of the offset correction means
4
crosses zero level is successively detected from the output signal, a synchronization pattern length in a specified period over
1
frame is detected employing an output of a zero-cross length detector
59
which counts the sampling number between neighboring zero-crosses, and further a detection cycle of a synchronization pattern is detected by a frequency error detector
13
, thereby deciding a frequency error amount to perform frequency control of the reproduction clock. Further, phase information of reproduction digital data is detected by a phase comparator
9
employing the output signal of the offset correction means
4
, and a phase error amount to perform phase synchronizaiton control of the reproduction clock and the reproduction digital data is decided.
The VCO
40
is controlled by a loop filter
14
for frequency control and a digital/analog converter
42
b
employing the frequency error amount outputted from the frequency error detector
13
so as to control frequency to an area where the reproduction clock can be synchronized with the reproduction digital signal. Meanwhile, the VCO
40
is controlled by a loop filter
60
for phase control and a digital/analog converter
42
a
employing the phase error amount outputted from the phase comparator
9
so that the reproduction clock is synchronized with the reproduction digital signal. That is, the VCO
40
has outputs of the digital/analog converter
42
a
and the digital/analog converter
42
b
, which are added by an adder
61
, inputted as its control input.
By a series of the operation, a phase of the reproduction clock and a phase of a clock component held by the reproduction digital data can be synchronized with each other, resulting in a stable and accurate reproduction of digital data recorded on an optical disk medium.
However, when a phase error detection as a part of the phase-locked loop is performed based on a signal before the partial response equalization processing as described above, phase error information is incorrect, and thus a jitter of the phase-locked loop increases, under such conditions as the deterioration in the reproduction signal due to tilt and insufficiencies of equalization characteristics of the waveform equalizing means. Thus, a signal sampled by the analog/digital converter is out of normal phase state, and therefore the performance cannot be sufficiently achieved in a partial response equalization by the transversal filter. Therefore, the quality of the reproduction signal is deteriorated and the deterioration in the error rate may be caused.
Further, though

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