Pulse or digital communications – Equalizers – Automatic
Reexamination Certificate
2000-07-20
2004-12-28
Ha, Dac V. (Department: 2634)
Pulse or digital communications
Equalizers
Automatic
C375S263000, C375S350000, C708S323000
Reexamination Certificate
active
06836511
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a digital reproduced signal processing apparatus and particularly, to a digital reproduced signal processing apparatus which employs partial response suited for use in a digital reproduced signal processing circuit provided in a playback system for reproduction of digital records based on code words where the minimum code length is relatively extensive.
BACKGROUND OF THE INVENTION
The attention has recently been focused on digital recording media including compact disks (referred to as CDs hereinafter) and digital versatile disks (referred to as DVDs hereinafter) which can save records almost semipermanently.
For playing back signals recorded on CDs and DVDs, a variety of analog reproduced signal processing apparatuses are commonly used because their circuit size is relatively simple and small and thus can be fabricated at less cost.
As a first conventional example, an analog DVD reproduced signal processing apparatus is now explained referring to FIG.
7
.
FIG. 7
is a block diagram
2
showing an arrangement of a conventional analog reproduced signal processing apparatus Z, where a reproduced signal runs as is explained below.
As shown in
FIG. 7
, the analog reproduced signal is read out from a recording medium
101
with a read head
102
and fed to an analog filter
103
. The filter
103
removes high frequency noise components in the signal and also emphasizes a particular frequency range of the signal to minimize jitters.
The signal filtered by the filter
103
is then transferred to a DC level control circuit
104
and a level comparator
105
.
The DC level control circuit
104
extracts a DC component from the filtered analog reproduced signal and releases it to the comparator
105
where it is used to determine a slice level.
The comparator
105
compares the filtered analog reproduced signal with the slice level to judge whether it is greater or smaller than the level and its binary judgment is released as a binary data.
The binary data is fed to a phase comparator
106
where it is compared in phase with a clock signal delivered from a voltage-controlled oscillator
108
. A phase error signal resulting from an error in the phase comparison is transmitted via a loop filter
107
to the voltage-controlled oscillator
108
where it is used for controlling the oscillation on the clock signal.
While a procedure of signal processing in the analog reproduced signal processing apparatus Z is carried out as described above, the reproduced signal read out from CD or DVD records has a fair but not satisfied level. It is hence desired to develop improved reproduced signal processing apparatuses for processing the signal at a higher level.
The DVD system uses signals based on “eight to fourteen modulation” (referred to as EFM hereinafter) codes or EFM-Plus codes where the minimum code length of a code word is 3T. The frequency response in an optical section of the DVD system is as shown in FIG.
6
.
In general, DVD records are at high density and the channel rate or reproduction rate of their reproduced signals is set to a higher frequency than that of the frequency response in the optical system shown in FIG.
6
. With the frequency response in the optical system, any 1T signal in the code word may hence be reproduced at a significantly attenuated level. More specifically, the setting of the minimum code length to 3T, for example, in FEM permits the minimum pit length to be identified of as large as 3T. If a pit having a length of 1T is contained in a code length of the reproduced rate, its signal size is too small to be identified or picked up, i.e. the 1T signal component is attenuated to a hardly reproduced level.
It is therefore said that the S/N (signal to noise) ratio in the reproduction section of the DVD system is considerably high. Using such a high S/N ratio and the EFM signals of which the minimum code length is limited, the DVD system provides a capability of high density recording.
For magnetic disk systems, a unique reproduced signal processing method called “partial response maximum likelihood” (referred to as PRML hereinafter) has been employed to increase the recording density.
The PRML method is a combination of a partial response technique in the communications technology and a maximum likelihood decoding technique in the code technology. It is hence necessary to select an optimum type of the partial response of which the frequency response matches that of a reproduced signal of RPML.
For example, the reproduced signal from a magnetic disk has a band-pass characteristic such as the magnetic recording characteristic shown in FIG.
8
. The partial response of which the frequency response matches that of the reproduced signal of the magnetic recording representing the band-pass characteristic may be characterized by (1, 0, −1). As shown in
FIG. 8
, the frequency response of the reproduced signal of the magnetic recording is very similar to that of the partial response (1, 0, −1) and they may easily be equalized without emphasizing a higher frequency range. In particular, the frequency characteristic of the partial response is closely related to the code words.
The application of the PRML method to the DVD signals has been attempted for increasing the recording density on a DVD medium with the help of the advantage of the PRML method. It is hence desired for processing the DVD signals of the PRML method to develop an improved digital reproduced signal processing apparatus which has a higher capability of reading the signals.
FIG. 9
is a block diagram showing the arrangement of a digital reproduced signal processing apparatus, as a second prior art, for magnet optical disks on which signals of the PRML method are recorded. A procedure of processing a reproduced signal in the signal processing apparatus Y is explained below.
As shown in
FIG. 9
, the analog reproduced signal read by a read head
202
is fed to an analog filter
203
where its high frequency noises are cut off and its signal components at a specific range are controlled to be close to the frequency characteristic of the partial response (1, 1). The analog reproduced signal filtered is then transmitted to a DC level control circuit
204
, a level comparator
205
, and an analog/digital converter
209
.
The DC level control circuit
204
extracts a DC component from the analog reproduced signal and transfers it to the level comparator
205
where its is used as a slice level. The comparator
205
compares the filtered analog reproduced signal with the slice level and releases its comparison signal to a phase comparator
206
.
The phase comparator
206
compares the comparison signal with a clock signal supplied from a voltage-controller oscillator
208
and transmits its phase error signal via a loop filter
207
to the voltage-controlled oscillator
208
. The clock signal is also fed to the analog/digital converter
209
and a Viterbi decoder
210
.
The filtered analog reproduced signal is converted by the analog/digital converter
209
to a digital signal which is then sent to the Viterbi decoder
210
of the partial response (1, 1) type where it is converted to a binary data
FIG. 10
is a frequency response diagram showing a frequency response of the reproduced signal from an magnet optical disk and a partial response (1, 1). As apparent from that figure, the two characteristic curves are similar in shape indicating that the frequency response is good without emphasizing a high frequency range in the equalization.
The digital reproduced signal processing apparatus Y for magnet optical disks carrying the PRML signals allows the signals to be reproduced from their recording medium at a higher fidelity than the analog reproduced signal processing apparatus Z of the first prior art.
However, the digital reproduced signal processing apparatus Y using the partial response (1, 1) still has some disadvantageous features which are hardly compatible with the DVD or CD system.
As described previously, the DVD or CD system employs commonly
Ha Dac V.
Matsushita Electric - Industrial Co., Ltd.
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