Dynamic information storage or retrieval – Control of storage or retrieval operation by a control... – Control of information signal processing channel
Reexamination Certificate
1999-10-26
2004-07-27
Huber, Paul W. (Department: 2653)
Dynamic information storage or retrieval
Control of storage or retrieval operation by a control...
Control of information signal processing channel
C369S047280
Reexamination Certificate
active
06768706
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a frequency control apparatus, and more particularly to a frequency control apparatus which is capable of performing the pull-in operation for a reproduced signal from an optical disk.
Furthermore, the present invention relates to a digital signal playback or reproducing apparatus, and more particularly to a digital signal playback or reproducing apparatus which decodes a digital signal reproduced from a recording medium.
A recording medium, such as an optical disk, is capable of storing information in a highly densified manner. A reproducing or playback system for this kind of recording medium includes a phase-locked loop circuit (i.e., PLL circuit) to obtain the data from a reproduced signal at accurate time intervals in a phase-locked manner. However, the pull-in range of the PLL circuit is theoretically limited to a narrow range of ±5~6%. For actual signals, this range may be further reduced to ±3~5%. When the optical disk is used, the relative speed of the signal widely varies. Namely, the reproducing or playback speed is variable in a wide range from an ordinary speed to a high speed equivalent to 20 times the ordinary speed. Thus, to control the pull-in process, it is necessary to provide a frequency control apparatus in the PLL circuit.
Conventional frequency control apparatuses are roughly classified into three groups. A first conventional frequency control apparatus uses a sync signal interval. In this case, the sync signal includes a transitional length (i.e., length between adjacent transitions) pattern longer than a run length restriction of the data (as known in a DVD). The sync signal interval is detectable based on a detected longest transitional length. An EFM (eight to fourteen modulation) signal of a compact disk (CD) has a maximum transitional length appearing at a predetermined probability. Thus, it is possible to detect the maximum transitional length of the EFM signal. The interval of the sync signal or the maximum transitional length is countable by using a clock generated from a voltage-controlled oscillator. Based on the counted value, it is feasible to check whether or not the detected value is correct. However, the first conventional frequency control apparatus requires a relatively long time to complete the judgement as an error detecting cycle is equivalent to the interval of the sync signal.
A second conventional frequency control apparatus uses the maximum transitional length pattern itself to judge whether or not a correct zero-cross detection number is obtained at the maximum transitional period. The PLL circuit controls the pull-in process accurately up to the range of ±5~6%. However, the second conventional frequency control apparatus is the same as the first conventional frequency control apparatus in that a relatively long time is required to complete the judgement because of a long error detecting cycle equivalent to the sync signal interval.
Furthermore, a third conventional frequency control apparatus utilizes an average transitional length which is defined by a ratio of an average of transitional lengths to a master clock counter.
FIG. 10
is a block diagram showing an example of the third conventional frequency control apparatus. A counter
1
counts a master clock supplied from an oscillator. A comparator
2
compares the count value sent from the counter
1
with a predetermined reference value
1
. The reference value
1
is sufficiently longer than a maximum transitional length of a reproduced signal.
The comparator
2
produces a coincidence signal at predetermined intervals. The coincidence signal serves as a reset signal which is supplied to the counter
1
and a zero-cross detector
3
. The zero-cross detector detects and makes a count of zero-cross every time the reproduced signal crosses a zero-level (threshold level) which is determined considering the reproduced signal.
The zero-cross detection detector outputs an accumulated count value as a cross count value. The cross count value is supplied to a subtracter
4
. The subtracter
4
reduces a predetermined reference value
2
from the received cross count value. The reference value
2
represents the ideal number of transition calculated based on an average transitional length in relation to the length of the reference value
1
. The difference signal produced from the subtracter
4
is supplied to an error judging circuit
5
. The error judging circuit
5
produces an error signal corresponding to the inputted difference signal. The error signal is supplied to a loop filter in the PLL circuit which produces a clock for detecting the reproduced signal, thereby controlling its characteristics.
According to the third conventional frequency control apparatus, the zero-cross detector
3
is reset at predetermined intervals. The zero cross is detected at the predetermined intervals. This is effectively applied to a scrambled signal. The error judgement is feasible at the intervals shorter than the sync signal intervals. Thus, it becomes possible to realize a high-speed pull-in operation which is usable as a rough adjustment.
However, as the conventional frequency control apparatus shown in
FIG. 10
performs the error judgement at the predetermined intervals of the absolute time, there is a possibility that the error judgement is useless when the rate of the reproduced signal is changed from the ordinary reproduced speed to a high-speed reproduced speed, or from one high-speed reproduced speed to another high-speed reproduced speed.
Furthermore, each of the above-described conventional frequency control apparatuses requires the conditions that the zero-cross threshold level (i.e., setting of the zero level) is ideal and the transitional length (i.e., zero cross) is correctly judged. However, a rewritable optical disk, such as magneto-optical disk or a phase change disk, or a write once or a read only optical disk produces a reproduced signal which is characterized in that a signal level reduces with increasing frequency as shown in FIG.
11
. Thus, the reproduced signal has a waveform peak level varying in accordance with the signal frequency. Furthermore, the waveform becomes asymmetric in the up-and-down direction. On these media, the center level (i.e., zero level) varies largely. Especially, this tendency remarkably appears when the optical disk is used for high-density recording.
To solve this problem, an automatic threshold control (ATC) can be used to adjust the center level of the reproduced signal to an optimum zero level. However, the threshold level may be deviated during a converging process of the ATC or when the ATC is performed for a vertically asymmetric signal to equalize the zero level of the zero-cross detection to the center of a maximum amplitude of the signal.
For example,
FIG. 12A
shows a case where a reproduced signal al has a symmetric waveform in the up-and-down direction and the threshold level is set to an ideal level I. In this case, a correct decoding data a
3
is obtained in synchronism with a bit clock a
2
. However, during the converging process of ATC, the threshold level deviates from the ideal position to a level II with respect to a reproduced signal by having a symmetric waveform in the up-and-down direction as shown in FIG.
12
B. In this case, a decoding data b
3
obtained in synchronism with a bit clock b
2
differs from the correct decoding data a
3
.
Furthermore,
FIG. 12C
shows another case where a reproduced signal c
1
has an asymmetric waveform in the up-and-down direction but the threshold level is set to an ideal level III. In this case, a correct decoding data c
3
is obtained in synchronism with a bit clock c
2
. However, when the ATC is performed to equalize the threshold level to the center level IV of a maximum amplitude of an asymmetric reproduced signal d
1
, a decoding data d
3
obtained in synchronism with a bit clock d
2
differs from the correct data c
3
.
When the threshold level is deviated from the ideal position as shown in
FIG
Huber Paul W.
Lowe Hauptman & Gilman & Berner LLP
Victor Company of Japan, LTD
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