Pulse or digital communications – Receivers – Particular pulse demodulator or detector
Reexamination Certificate
1998-11-03
2001-11-20
Chin, Stephen (Department: 2734)
Pulse or digital communications
Receivers
Particular pulse demodulator or detector
C375S262000, C714S795000
Reexamination Certificate
active
06320916
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to digital magnetic recording/reproducing apparatuses such as magnetic disk recorders, and particularly to a signal processing system and apparatus for recording digital data at high density.
The demand for high-density recording and fast processing in magnetic disk recorders has more and more escalated. The signal processing techniques in the signal recording and reproducing system to support these requirements have also been developed towards the high-density and fast recording. As to the recording code, the coding rate R is increased, and now the main current is R={fraction (8/9)}. In addition, recently a higher rate recording code of R={fraction (16/17)} has started to be practically used. Moreover, the partial response equalization system is employed in order to prevent the signal-to-noise ratio from being reduced by the increase of the intercode interference involved with the high-density recording. The PR4ML (Partial Response Class 4 with Maximum Likelihood Detection) is utilized to detect a signal sequence nearest to the reproduced signal by the Viterbi algorithm (best likelihood sequence estimation) using a known interference formed on the reproducing channel. A device for this purpose is already incorporated as an LSI (Large Scale Integrated Circuit) a magnetic disk product. It is known that if the input signal sequence to the PR4ML processor is binary data of 0, 1, the minimum squared Euclidean distance (MSED) between signal sequences produced by the PR4ML is 2. Therefore, the PR4ML is improved 3 dB in its tolerance to noise than the peak detection system (MSED=1) for deciding magnetic information by only 0, 1 without the best likelihood sequence estimation.
In order to achieve higher-density recording than the PR4ML, it is necessary to use such a signal processing technique as to increase the MSED. For this purpose, there are EPR4ML (Extended PR4ML), and EEPR4ML (Extended EPR4ML). These are the extensions of the PR4ML idea. The values of MSED for those extensions are known to be 4, 6 (binary conversion), respectively. In addition, the channel state number is 8 for EPR4ML, and 16 for EEPR4ML.
FIG. 2
is a block diagram of a conventional digital magnetic recording/reproducing apparatus. As illustrated, on the recording side, an information sequence of “0”s and “1”s as digital data is converted into a high rate code such as R={fraction (8/9)} or {fraction (16/17)} by a recording coder
201
. The recording code, as well known, has a limited number of successive “0”s provided to prevent the timing extraction and gain control (not shown in
FIG. 2
) in the reproducing section from being reduced in their performances. The recording coded sequence is further supplied to a precoder
202
, thereby being converted into a code in which 1/(1+D) is treated as a transfer function. Only when data of “1” is supplied to the precoder
202
, the output is changed from just the preceding value. Here, D is the delay operator, and the delay time is equal to the bit distance. The precoder
202
is able to suppress the decoded error propagation length after the Viterbi detection on the reproducing section to a limited value. The precoded sequence is supplied through an amplifier
203
to a record head
204
, by which it is recorded on a magnetic recording medium
205
as magnetic information.
On the reproducing side, the magnetic information recorded on the magnetic recording medium
205
is reproduced by a reproduce head
206
, and supplied to an amplifier
207
, thereby being converted into an analog electric signal. This signal is supplied to an A/D converter
208
, which samples it at each bit interval, thereby converting it into a digital signal. The digital signal is fed to a PR equalization circuit
209
, which then equalizes it into partial response channels such as PR4, EPR4 and EEPR4. The PR equalization can be easily realized by a well known transversal filter. The output from the PR equalization circuit
209
contains noise added up to a signal level determined by the PR channel characteristic. Here, the noise contains the medium noise, the noise mixed from the reproduce-side head, and the noise caused by A/D quantization. These noises, when passed through the PR equalization circuit
209
, become colored noises with a correlation. The equalized signal with the noise added is supplied to a Viterbi detector
210
that makes MLSE (Maximum Likelihood Sequence Estimation). Thus, it produces the most probable data sequence, or a data sequence most likely to resemble the input signal sequence. Since the reverse characteristic (1+D) to the precoder
202
can be produced as NRZI (Non Return to Zero Inverted) within the Viterbi detector as well known, a postcoder can be removed. The data sequence from the Viterbi detector
210
is decoded into the information sequence by a recording decoder
211
.
Thus, this conventional digital magnetic recording/reproducing apparatus employs a high-rate recording code, and combines the partial response and Viterbi detection, thereby increasing the signal-to-noise ratio of the reproduced signal for high-density recording.
In recent years, other various signal processing systems for higher-density recording have been discussed in addition to the above-mentioned conventional apparatus. As a powerful one of those systems, there is a list Viterbi algorithm (hereinafter, referred to as LVA). In this system, the Viterbi detector
210
detects the most reliable data sequence (best sequence), secondly reliable data sequence (2nd best sequence), thirdly reliable data sequence (3rd best sequence), . . . , and nth reliable data sequence (nth best sequence), produces these as proposed sequences, or candidates, detects decoding error of each candidate by use of CRC (Cyclic Redundancy Check) or the like, and generates a decoded output of candidates with no decoding error. The LVA is able to greatly improve the decoding error characteristic of the Viterbi detector. The details of the LVA are described in N. Seshadri et al., “List Viterbi Decoding Algorithms with Applications”,
IEEE Transactions on Communications
, Vol. 42, No. 2/3/4, Feb./Mar./Apr. 1994, pp. 313-323. The LVA was contrived for the purpose of originally applying to the communications field such as mobile radio communication, but it has not yet been applied to a magnetic recording/reproducing apparatus. The present invention fundamentally applies the LVA to a magnetic recording/reproducing apparatus as will be described in the later sections of embodiments. However, a simple application of the conventional idea directly to the apparatus cannot achieve high-density recording. This will be described in detail later.
The basic idea of the LVA will be described as an example of EPR
4
channel with reference to the trellis diagram of FIG.
3
.
FIG. 3
shows the structure of EPR
4
channel with respect to time (here, time 0 to 8). The number of channel states is 8. The states 000, . . . , 111 are expressed as 0, . . . , 7, respectively. When −1 and +1 (which correspond to binary data 0, 1, respectively) are applied to the respective states, the associated PR equalization signals (expressed in binary conversion values as illustrated) are produced, making the channel states transitional.
It is now assumed that the state at time 0 is 1. When data
1
is applied at time 1, the PR equalization output of 2 is produced, and thus the channel state is shifted from 1 to 3.
The equalization signal of EPR4 channel takes five values of 2, 1, 0, −1, and −2. In
FIG. 3
, it is assumed that the line A indicates the state transition sequence (path) corresponding to the correct data sequence. However, as a result of the Viterbi detection, the decoded signal may sometimes be in error as indicated by line B. As will be seen from the figure, the squared Euclidean distance is 4 between the correct decoded path (the equalization signal value sequence is 0, 0, 0, 1, 1, −1, 0, 1) and the erroneous decoded pa
Kobayashi Naoya
Kondo Masaharu
Mita Seiichi
Moriyasu Takashi
Sawaguchi Hideki
Antonelli Terry Stout & Kraus LLP
Chin Stephen
Fan Chieh M.
Hitachi , Ltd.
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