Error detection/correction and fault detection/recovery – Pulse or data error handling – Error detection for synchronization control
Reexamination Certificate
1999-10-21
2003-03-11
Decady, Albert (Department: 2133)
Error detection/correction and fault detection/recovery
Pulse or data error handling
Error detection for synchronization control
C714S795000
Reexamination Certificate
active
06532567
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a method of Viterbi detecting and an apparatus for Viterbi detecting. More particularly, the invention relates to a method and apparatus which are suitable for Viterbi detecting by the use of a detection trellis that has a time-variant structure.
To transmit (or record) data, a block sync signal is usually added to the head of each data block (hereinafter referred to as lock). The block sync signal is detected when the data is received (or reproduced), thereby to detect the head of the block. The term lock, as used here, means a data unit that consists of a plurality of data words or code words. A block recorded on DAT (Digital Audio Tape), for example, is composed of a 1-symbol ID (Identity) data item, a 1-symbol ID parity and a 32-symbol data word. That is, this data block consists of 36 symbols. Each symbol consists of 8 bits before it is encoded to an 8/10 recording code, and by 10 bits after it is encoded to an 8/10 recording code. To record the block, the 8-bit symbols forming the block are encoded to 8/10 codes. Thereafter, a sync signal (sync word), which is a 1-symbol (10 bits) and which does not exist in the sequence of 8/10 codes, is added to the head of the block. To reproduce the block, the sync word is detected, thus finding the head of the block. Then, the ID data, parity and 32-symbol data word are decoded, symbol by symbol. The 8/10 codes have been generated to accomplish NRZI (nonreturn-to-zero interval) recording. The maximum length Tmax of a train of identical bits, existing in the NRZI-converted sequence of 8/10 codes, is 4. Two or more identical bit trains having Tmax would not follow one after another. The sync word contains a pattern in which Tmax continuously appears at least twice, though Tmax does not appear in the sequence of codes. The Tmax distinguishes the sync word from the sequence of codes.
In recent years, the TCPR (Trellis-Coded Partial Response) system has been studied with enthusiasm in the field of magnetic recording. In this system, the partial response characteristic and the code characteristic are integrated, thereby to increase the Euclidean distance between the output sequences on a transmission path (recording/reproducing path), i.e., free square Euclidean distance d
2
free
. An increase of distance d
2
free
is equivalent to a rise of signal level. Hence, the TCRP system enhances SNR (Signal-to-Noise Ratio) at the time of detecting data. Codes used in the TCRP system are generally called Trellis signals. The assignee of the present application has proposed 16/20 trellis codes in Japanese Patent Application No. 10-207372.
FIG. 5
shows an ADS trellis illustrating the transition of an ADS (Alternating Digital Sum) that a 20-bit code word assumes. The ADS can range from 0 to 10 for the code word. However, the ADS at the start point and the ADS at the end point can take only two values, i.e., 3 and 7. Further, the ADSs at time
7
are limited to the values shown in
FIG. 5
, in accordance with whether the value of the ADSs at the start points is 3 or 7. An ADS is the sum of symbols acquired to the present time, from a time in the infinite past or from the start of encoding the symbols, each symbol being −1 or +1 allocated to one code bit, and every other symbol having been multiplied by 1. If the ADs for a sequence of codes is set within a specific range, the Nyquist frequency component of the code power density, i.e., the frequency component which is half (½) the code bit rate, can be reduced to null.
It is known that the distance d
2
free
increases when the null point of the code power density is made to coincide with the null point of the transfer function of the transmission path. The power density of a 16/20 code has the null point at the Nyquist frequency. This code is therefore a trellis signal to a partial response whose transfer function has a null point at the Nyquist frequency.
FIG. 6
depicts a detector trellis that is used to Viterbi detect in the TCPR system, by the use of a class-1 partial response. The detector trellis has a structure, which is a combination of the characteristic of the code and the characteristic of the class-1 partial response. (The characteristic of the code is the ADS trellis of
FIG. 5
, hereinafter referred to as code trellis. The transfer function of the class-1 partial response has a null point at the Nyquist frequency.) The structure shown in
FIG. 6
is time-variant in units of code words. This is why a sequence can be detected in the TCPR system only when the boundary between the adjacent code words is determined.
FIG. 7
shows a reproduction apparatus of a TCPR system, in which a time-variant trellis is utilized. The signal reproduced from a medium
31
and amplified by a regenerative amplifier
32
is equalized to a prescribed partial-response characteristic by means of an equalizer
33
. Then, a PLL
34
extracts a clock signal from the output signal of the equalizer
33
. The PRML(Partial Response Maximum Likelihood) Viterbi detector
36
, sync word detector
37
, TCPR Viterbi detector
38
and decoder
39
, all incorporated in the reproduction apparatus, operate in accordance with the clock signal. The signal equalized to the prescribed partial-response characteristic and output from the equalizer
33
is sampled by a sampling circuit
35
and then input to the TCPR Viterbi detector
38
via a delay element
310
. The PRML Viterbi detector
36
detects, using a detector trellis that has no characteristics of a time-variant trellis. Therefore, the PRML Viterbi detector
36
need not determine the boundary between adjacent code words and can detect data asynchronously. The bit train the PRML Viterbi detector
36
has detected is input to the sync word detector
37
. The sync word detector
37
detects a sync word from the bit train and supplies the sync word to the TCPR Viterbi detector
38
. The TCPR Viterbi detector
38
detects, using a detector trellis that has the characteristics of a time-variant trellis (i.e., the trellis shown in FIG.
6
). Hence, the TCPR Viterbi detector
38
cannot correctly operate unless the boundary between adjacent code words is determined. The TCPR Viterbi detector
38
is thereby synchronized by the input sync word and starts detecting one block of data. The data thus detected by the TCPR Viterbi detector
38
is input to the decoder
39
, which decodes code words. The delay element
310
is provided to delay the output of the equalizer
33
by the same time the sync word detected has delayed due to the internal delays of the PRML Viterbi detector
36
and sync word detector
37
.
The accuracy of detecting the block sync word greatly influences the reception of data or the quality of the data reproduced. For example, one block of data will be lost in its entirety if the sync signal is not detected due to disturbance. If the sync signal is detected incorrectly, data will be lost for the period between the time when the sync signal is incorrectly detected and the time when the sync signal is correctly detected. In the case of DAT (Dynamic Address Translation), the ID data will be erroneously detected, too, if the sync word is detected incorrectly. The ID data contains address data. If the incorrectly detected address data is used, thereby writing data into a memory, the data of any other block may be destroyed.
The characteristics of a trellis code are not imparted to the PRML Viterbi detector
36
shown in FIG.
7
. Therefore, the reproduction apparatus of a TCPR system cannot obtain a gain. It follows that the output from the PRML Viterbi detector
36
is inevitably inferior to the output of the TCPR Viterbi detector
38
in terms of quality.
No TCPR system is employed in DAT. Only one data detector is used, and the accuracy of detecting a sync word is the same as that of detecting data. In the reproduction apparatus of
FIG. 7
, however, the accuracy of detecting the sync word is much lower than that of detecting data. This will impose a very adverse influence on the receptio
De'cady Albert
Frommer William S.
Frommer & Lawrence & Haug LLP
Simon Darren M.
Sony Corporation
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