Pulse or digital communications – Receivers – Interference or noise reduction
Reexamination Certificate
2000-03-27
2004-06-15
Tran, Khai (Department: 2631)
Pulse or digital communications
Receivers
Interference or noise reduction
C375S263000, C375S290000
Reexamination Certificate
active
06751276
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a method and an apparatus for decoding a digital signal, and, more particularly, to a digital signal recording/reproducing apparatus which performs a decoding process on recorded digital signals and a digital signal transmission apparatus which performs a decoding process on transmitted digital signals.
A digital signal recording/reproducing apparatus uses a PR (Partial Response) class IV algorithm that performs NRZI code conversion in order to improve the recording density of magnetic recording media and improve the S/N ratio of decoded digital signals. Because a magnetic recording/reproducing system has a differential characteristic, recording and reproduction of data spreads the signal waveform, which causes an intersymbol interference. The PR system uses the intersymbol interference positively to shape a signal such that the power spectrum of a code is adequate for the transmission characteristic of a transmission line.
FIG. 1
is a schematic block diagram of a conventional digital signal recording/reproducing apparatus
100
which uses the PR class IV algorithm. A precoder
1
has a modulo-2 digital adder
2
a
and a 1-bit delay operation element
3
a
and has the characteristic of 1/(1+D). The precoder
1
receives, for example, a digital input signal Din as shown in
FIG. 1
, and performs a process of 1/(1+D) on the input signal Din to convert the input signal Din to an NRZI code S
1
as shown in FIG.
2
. The polarity of the NRZI code is inverted every time the input signal Din rises from “0” to “1”. The polarity inversion means a transition of a signal value from “1” to “0” or from “0” to “1”.
A write buffer
4
whose structure is similar to that of the precoder
1
has the characteristic of 1/(1−D). The write buffer
4
produces an output signal S
2
by giving an intersymbol interference opposite to the one that is given by a recording/reproducing system to the NRZI code S
1
from the precoder
1
.
A recording/reproducing system
5
has a write head
6
, a recording medium
7
and a read head
8
. The output signal S
2
of the write buffer
4
is written on the recording medium
7
via the write head
6
and the written signal is read from the recording medium
7
via the read head
8
.
The writing operation to the recording medium
7
by the write head
6
has the differential characteristic of (1−D), and the operation of the read head
8
and an equalizer
9
has the differential characteristic of (1+D). Therefore, the recording/reproducing system
5
and the equalizer
9
carry out a process of {(1−D)(1+D)}, thereby accomplishing PR class IV impulse response.
The equalizer
9
performs PR equalization on an analog read signal generated by the read head
8
, thereby producing an equalization signal S
3
. The equalization signal S
3
is a multi-value signal as shown in FIG.
2
.
A comparator
10
compares the equalization signal S
3
from the equalizer
9
with threshold values A and B (shown in
FIG. 2
) and produces a decoded signal Dout of “0” or “1” which coincides with the input signal Din. When the level of the equalization signal S
3
is higher than the threshold value A or is lower than the threshold value B, “1” is output otherwise “0” is output.
For digital signal recording/reproducing apparatuses which use recording media, such as a floppy disk, that needs swapping, however, the format of signals to be recorded on recording media is specified. Since the specifications states that the input signal Din is recorded on the recording medium using only a write buffer, the digital signal recording/reproducing apparatuses cannot use a precoder.
A digital signal recording/reproducing apparatus of a peak detection type which does not use a precoder cannot implement PR-system based recording and reproduction operations using a precoder. As a solution to this shortcoming, Japanese Unexamined Patent Publication Nos. 5-325425, 5-307837, 8-147893 and 8-77712 disclose apparatuses capable of adapting the PR system without having a precoder at the preceding stage of the recording/reproducing system. However, decoding circuits in the apparatuses described in these publications are complicated.
By contrast, the recording/reproducing apparatus
100
in
FIG. 1
is of a linear type, where the characteristics of the system do not change even if the order of the individual blocks is changed.
One possible modification is a recording/reproducing apparatus
110
shown in
FIG. 3
which has a precoder equivalent circuit
11
located after the comparator
10
. The write buffer
4
, recording/reproducing system
5
and comparator
10
in
FIG. 3
are identical to those in FIG.
1
.
The operation of the recording/reproducing apparatus
110
in
FIG. 3
will now be discussed referring to the waveform diagram of FIG.
4
. The write buffer
4
receives the input signal Din and provides an output signal S
4
according to the characteristic of (1−D) to the recording/reproducing system
5
. The recording/reproducing system
5
provides its output signal having a characteristic of (1−D)(1+D) to the equalizer
9
, which in turn provides an equalization signal S
5
to the comparator
10
. The comparator
10
performs a comparison operation and provides an output signal S
6
to the precoder equivalent circuit
11
. The precoder equivalent circuit
11
performs an operation of 1/(1+D) on the output signal S
6
, thus generating a decoded signal Dout which is substantially the same as the input signal Din.
Even if the input signal Din has been recorded on the recording medium
7
without using a precoder, the signal that is read by the recording/reproducing system
5
can be decoded by the PR system using the PR equalizer
9
and the precoder equivalent circuit
11
.
If the operation of the recording/reproducing system
5
or the equalizer
9
in the digital signal recording/reproducing apparatus
110
causes an arbitrary bit b
1
in the equalization signal S
5
to contain noise n
1
which is greater than the threshold value as shown in
FIG. 5
, however, the comparator
10
performs an operation different from the one illustrated in
FIG. 4
so that the bit b
1
of the output signal S
6
is inverted. Then, the feedback loop of the precoder equivalent circuit
11
inverts all the bits in the decoded signal Dout after the bit b
1
, causing an error to propagate endlessly. This type of digital signal recording/reproducing apparatus is therefore not practical.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the present invention to provide a digital signal decoding apparatus which performs a decoding operation using a PR system while suppressing propagation of an error without a precoder at the preceding stage of a recording/reproducing system or a transmission system.
In a first aspect of the present invention, a method of decoding a digital signal is provided. First, the digital signal is processed using a delay operation element in accordance with a frequency characteristic of 1+D, where D is an output signal of the delay operation element. The processed digital signal is converted to a conversion signal of at least three values having polarities of 0, positive and negative. A decoded signal is generated by processing the conversion signal in accordance with a frequency characteristic of 1/(1+D). Then, it is detected if there is a conversion error in the conversion signal and a correct signal is generated by restricting propagation of the conversion error to the decoded signal when the conversion error is detected.
In a second aspect of the present invention, a method of decoding a digital signal is provided. First, a 1/(1−D) process is performed on the digital signal using a delay operation element, where D is an output signal of the delay operation element to generate a write signal. The write signal is written on a recording medium in accordance with a (1−D) characteristic. The write signal written on the recording medium is read in ac
Iijima Masao
Okada Koji
Armstrong Kratz Quintos Hanson & Brooks, LLP
Fujitsu Limited
Tran Khai
Tran Khanh Cong
LandOfFree
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