Data coding for fast start-up of PRML receivers

In recording systems using partial-response maximum-likelihood detection (PRML) techniques, data sequences are preceded by a preamble consisting of all ones. Coding schemes are disclosed which allow to keep the number of consecutive ones occurring in the coded data sequences at a minimum, while simultaneously restricting the number of consecutive zeros in full and partial data sequences to a low value which is important for improving receiver operation. The disclosed coding schemes and apparatus enable a faster and more reliable discrimination between timing preambles and data sequences, thus allowing to use shorter timing preambles which results in faster receiver start-up and in a reduction of storage overhead for the preambles.

FIELD OF INVENTION 
The present invention is concerned with data coding for systems using 
partial-response signaling with maximum-likelihood sequence detection 
(PRML). Such a signaling and detection scheme is useful e.g. for magnetic 
or optic storage of data, and for data transmission over cables. In 
particular, the invention is concerned with data coding in PRML systems to 
shorten the time needed for recognizing timing preambles that precede the 
data sequences to ensure proper timing and gain acquisition in the 
receiver, and to achieve a more reliable distinction between arbitrary 
data sequences and the timing preamble. 
BACKGROUND 
Rate 8/9 constrained codes for partial-response class-IV signaling with 
maximum-likelihood sequence detection (PRML) have been designed to achieve 
proper operation of gain and timing control loops and reduction of 
path-memory length of Viterbi detectors in PRML receivers. In the presence 
of a precoder between partial-response class-IV (PR-IV) channel and rate 
8/9 encoder, these two objectives are met by restricting the maximum 
run-length of 0's, denoted by G.sub.0, in encoded binary sequences 
{b.sub.n } and limiting the maximum number of consecutive 0's, denoted by 
I.sub.0, in the two subsequences of {b.sub.n } with even or odd time 
indices. An example of a system where G.sub.0 and I.sub.0 are both limited 
by appropriate coding (either to 4 and 4, or to 3 and 6, respectively) is 
disclosed in U.S. Pat. No. 4,707,681 to Eggenberger. 
It would be desirable to have coding schemes which can accomplish a third 
task, i.e. to speed up the start-up procedure of PRML receivers by 
shortening the minimum observation length required to distinguish reliably 
encoded user data from the timing preamble {1,1,1, . . . , 1,1}, which in 
many systems is part of training sequences preceding encoded data 
sequences {b.sub.n }. 
OBJECTS OF THE INVENTION 
It is an object of the invention to devise a coding scheme for PRML systems 
by which the time required for distinguishing timing preambles from coded 
data sequences is reduced. 
It is another object to provide a coding scheme allowing to use shorter 
timing preambles. 
A further object of the invention is a PRML coding scheme allowing the 
selection of code mappings which enable simple implementation of encoders 
and decoders by logic gate arrays. 
Another object of the invention is to provide a coding scheme for systems 
using the PRML technique and using a timing preamble of consecutive ones 
which allows to maintain code constraints that are useful for the proper 
operation of gain and timing control loops and for limiting the path 
memory-length of Viterbi decoders in the receiver. 
SUMMARY OF THE INVENTION 
These objects are achieved by an encoding method and by encoding apparatus 
as defined in claims 1 and 7. Preferred particular embodiments of this 
method and of encoding and decoding apparatus are defined in the dependent 
claims. 
The invention attains the objects by limiting the number G.sub.1 of 
consecutive ones in the encoded data sequence to a relatively low value, 
while maintaining the limitation in the numbers of consecutive zeros in 
the full encoded sequence as well as in the even and odd numbered partial 
sequences. The smaller the maximum run-length of 1's, denoted G.sub.1, in 
encoded sequences {b.sub.n }, the faster data can be discriminated from 
the preamble. Since the timing preamble has to be long enough to be 
reliably detected by PRML receivers during start-up, smaller values of 
G.sub.1 allow shorter training sequences leading to faster synchronization 
of PRML receivers. The presented 8/9 codes, referred to by the 3-tuple 
(G.sub.0, I.sub.0, G.sub.1), permit reliable, fast start-up of PRML 
receivers and the length of the preamble detection window needs typically 
to be less than one half of the window required with earlier codes 
presented in U.S. Pat. No. 4,707,681. 
Several coding schemes are disclosed which allow to select different 
combinations of code constraints so that a combination can be chosen which 
best suits the respective application. These schemes have furthermore the 
property that decoding can be achieved without knowledge of previously 
encoded data, thereby avoiding decoding error propagation in case of 
transmission errors. 
Further disclosed are several code mapping schemes which allow to implement 
the encoders (and decoders) for the presented coding schemes with a small 
number of logic gates. 
These and other advantages will become more apparent from the following 
description of preferred embodiments of the invention with reference to 
the drawings.