Dynamic information storage or retrieval – Control of storage or retrieval operation by a control... – By medium defect indicative control signal
Reexamination Certificate
1999-10-26
2004-09-28
Huber, Paul W. (Department: 2653)
Dynamic information storage or retrieval
Control of storage or retrieval operation by a control...
By medium defect indicative control signal
C369S047180, C369S053320
Reexamination Certificate
active
06798725
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a wave-shaping apparatus and a reproduction signal processing apparatus including the same, and specifically a wave-shaping apparatus for optimally performing digitization or A/D (analog/digital) conversion of a reproduction signal obtained by an optical pickup circuit, and a reproduction signal processing apparatus including the same.
2. Description of the Related Art
Recently, optical disks are being used in various conditions and stored in various manners. Under the circumstances, surfaces of optical disks are often scratched or exposed to dust. In order to read information from an optical disk having dust or scratches, a high precision signal determination is required.
Especially, an optical disk having information at a high density such as a DVD (digital versatile disk) has an inferior SNR (signal
oise ratio) at the shortest pit portion. Accordingly, it is required to minimize errors when a signal is digitized.
Information recording to a CD (compact disk) is performed by EFM (eight to fourteen modulation), and information recording to a DVD is performed by 8-16 modulation. These modulation systems make the spectrum of recording pattern to the disk substantially DC component-free. In order to make use of such characteristics of these modulation systems, a reproduction signal is usually digitized by a digitizing circuit for controlling the slicing level of the signal by performing negative feedback control, so that the duty ratio of the digitized signals is 50:50.
With reference to
FIG. 14A
, a digitizing circuit
1300
will be described.
An optical pickup circuit
2
outputs a reproduction signal RS obtained from an optical disk
1
. The reproduction signal RS is capacitance-coupled by a capacitor C and supplied with a prescribed bias voltage of, for example, VCC/2. The resultant signal is input to a negative (−) input terminal
110
A of a comparator
110
and compared with a slicing level which is input to a positive (+) input terminal
110
B of the comparator
110
. Thus, a digitized signal DG
1
is output from the comparator
110
.
A charge pump
101
is driven, i.e., the potential of a charge capacitor
102
is increased or decreased, in accordance with the polarity of the digitized signal DG
1
output from the comparator
110
. A charge voltage stored in the charge capacitor
102
is current-amplified by a buffer
103
and ripple-removed by a low pass filter
104
. Then, the resultant signal is input to the positive input terminal
110
B of the comparator
110
.
For example, when the potential of the negative input terminal
110
A is higher than the potential of the positive input terminal
110
B, the output level from the comparator
110
is 0. Then, a charge pump
101
A is turned on to raise the potential of the charge capacitor
102
. Thus, the potential of the positive input terminal
110
B of the comparator
110
is raised. In this manner, negative feedback control is performed so that the difference between the potential of the positive input terminal
110
B and the potential of the negative input terminal
110
A becomes 0.
When the potential of the negative input terminal
110
A is lower than the potential of the positive input terminal
110
B, a charge pump
101
B is turned on to reduce the potential of the charge capacitor
102
. Thus, the potential of the positive input terminal
110
B of the comparator
110
is lowered. In this manner, negative feedback control is performed so that the difference between the potential of the positive input terminal
110
B and the potential of the negative input terminal
110
A becomes 0.
FIG. 14B
is a waveform diagram of a high frequency reproduction signal RS. When such a high frequency reproduction signal RS in input to the digitizing circuit
1300
, a negative feedback digitizing slicing level SL is controlled so that the duty ratio of the “0” level L
0
and the “1” level of the digitized signal DG
1
output from the digitizing circuit
1300
becomes 50:50.
A response ability of the negative feedback control is determined by the driving current value of the charge pump
101
, the capacitance of the charge capacitor
102
, and the time constant of the low pass filter
104
.
Recently, electric circuits are more and more digitized. An analog reproduction signal from an optical disk is processed by an A/D converter with multiple bits, and then processed by a digital signal processing circuit for reproducing the signal entirely by digital processing. The digital signal processing circuit is generally referred to as a “digital read channel”. Use of a digital read channel realizes stable operations by suppressing dispersion among circuits and reduces an error rate by digital signal processing such as, for example, PRML (partial response/most likelihood).
When an optical disk has defects such as dust or scratches, laser light from an optical pickup circuit is blocked by the dust or scratches. Accordingly, the level of the reproduction signal is significantly fluctuated in terms of both direct current or alternate current. In order to accurately digitize the reproduction signal despite such changes, the digitizing circuit
1300
needs to have a higher response ability so as to follow the changes.
However, the spectrum of recording pattern to the optical disk is not completely DC component-free around a defect change frequency (several kilohertz or less). Thus, the cut-off frequency of the low pass filter
104
needs to be raised to improve the response ability of the digitizing circuit
1300
. In such a case, a DC fluctuation component of the reproduction signal is mixed into a negative feedback signal, i.e., the signal input to the positive input terminal
110
B as an external disturbance. This prevents accurate digitization, and a data slicing error occurs and generation of jitters is increased.
FIG. 15
shows a circuit designed for alleviating the data slicing error. The reproduction signal RS is passed through a high pass filter
105
, before being input to the digitizing circuit
1300
, to remove a low frequency band fluctuation component, so that the reproduction signal RS has a line of symmetry between the upper half and the lower half even when the signal level is fluctuated by defects. In this manner, the control load performed by the digitizing circuit
1300
is alleviated and thus the data slicing error is alleviated.
However, such a circuit removes the low frequency band fluctuation component in a steady state, i.e., even when there is no defect. Therefore, a data slicing error occurs by the lack of information provided by the low frequency bank fluctuation component and the generation of jitters is increased.
FIG. 16
shows a signal level when a light beam passes through a defect. Since light reflected by the optical disk is not obtained, the level of the reproduction signal RS is lowered to a black level BL. This phenomenon is conspicuous in the case of dual layer DVDs. When performing A/D conversion of the reproduction signal RS, the dynamic range of A/D conversion usually needs to be maximized with respect to an amplitude A of a signal of normal reproduction from the viewpoint of SNR. However, when the signal level is reduced to the black level BL by the dropout, the reproduction signal RS exceeds the dynamic range. This adversely influences the digital signal processing.
SUMMARY OF THE INVENTION
According to one aspect of the invention, a wave-shaping apparatus includes a first detection circuit for detecting a reproduction signal reproduced from an optical disk by an optical pickup circuit using a first detection time constant to detect an upper envelope of the reproduction signal; a second detection circuit for detecting the reproduction signal using a second detection time constant to detect a lower envelope of the reproduction signal; an averaging circuit for putting a weight represented by a weighting coefficient to each of the upper envelope and the lower envelope to calculate an average value of each of th
Aoki Kazuhiro
Horibe Ryusuke
Kanekami Youichi
Kawakami Shinichi
Huber Paul W.
Matsushita Electric - Industrial Co., Ltd.
Snell & Wilmer LLP
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