CONSTRUCTIONS OF ALMOST BLOCK-DECODABLE RUNLENGTH-LIMITED CODES

The correspondence describes a new technique for constructing fixed-le ngth (d, k) runlength-limited block codes. The new codes are very clos e to block-decodable codes, as decoding of the retrieved sequence can be accomplished by observing (part of) the received codeword plus a ve ry small part (usually only a single bit) of the previous codeword. Th e basic idea of the new construction is to uniquely represent each sou rce word by a (d, k) sequence with specific predefined properties, and to construct a bridge of beta, less than or equal to beta less than o r equal to d, merging bits between every pair of adjacent words. An es sential element of the new coding principle is look ahead. The merging bits are governed by the state of the encoder (the history), the pres ent source word to be translated, and by the upcoming source word. The new constructions have the virtue that only one look-up table is requ ired for encoding and decoding.