Browse Prior Art Database

(1,6) Rll Code with a Lookahead of 8

IP.com Disclosure Number: IPCOM000119666D
Original Publication Date: 1991-Feb-01
Included in the Prior Art Database: 2005-Apr-02
Document File: 4 page(s) / 153K

Publishing Venue

IBM

Related People

Franaszek, PA: AUTHOR [+2]

Abstract

The tables that follow this description are the encoding and decoding tables for a (1,6) fixed-length run-length limited code with a rate 2/3 that requires a lookahead of 8 bits to encode. General methods for constructing codes for run-length limited channels are described in *. Such channels typically constrain codewords so that runs of consecutive 0s are constrained in length. A (1,6) code, for example, is a code for which runs of consecutive 0s have a length at least 1 and do not exceed 6.

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(1,6) Rll Code with a Lookahead of 8

      The tables that follow this description are the encoding
and decoding tables for a (1,6) fixed-length run-length limited code
with a rate 2/3 that requires a lookahead of 8 bits to encode.
General methods for constructing codes for run-length limited
channels are described in *.  Such channels typically constrain
codewords so that runs of consecutive 0s are constrained in length.
A (1,6) code, for example, is a code for which runs of consecutive 0s
have a length at least 1 and do not exceed 6.

      A class of codes for this channel for which encoding depends on
a finite past history of states visited by the encoder and on the
next few input symbols is described in *. One code in this class, the
stationary code, has the property that the coding rules are the same
for each input symbol.  It is not difficult to show that a
fixed-length stationary (1,6) code with a rate of 2/3 (two
information bits per three bits of encoder output) must depend on the
present state and at least the next eight bits of input. The tables
below describe a (1,6) code with a rate of 2/3 and an encoding
lookahead of 8 bits.  No code with this property has been discovered
prior to this disclosure, and this code achieves the theoretical
minimum encoding lookahead.

      To encode sequences using the tables at the end of this
article, given the present state of the encoder, observe the next 8
input bits.  In almost all cases, the next 6 bits determine the next
state of the encoder.  The next state of the encoder determines what
three bits are output from the encoder.  The few cases that depend on
8 bits are indicated by an asterisk in the tables.  The state diagram
of the encoder that corresponds to encoder states is shown in the
figure above.  The figure lists channel states as states numbered
from 1 through 7, and for each channel state it gives one or more
encoder states that represent this channel state.

      Because state 1 in the figure can be reached by two different
paths of length 3 from each of states 2, 3, 4, and 5, each of
the encoder states for states 2, 3, 4, and 5 have two successor
states that correspond to state 1.  The encoder associates each of
the two paths that end on state 1 with a different state 1,...