CONSTRUCTIONS OF ALMOST BLOCK-DECODABLE RUNLENGTH-LIMITED CODES

The correspondence describes a new technique for constructing fixed-length (d, k) runlength-limited block codes. The new codes are very close to block-decodable codes, as decoding of the retrieved sequence can be accomplished by observing (part of) the received codeword plus a very small part (usually only a single bit) of the previous codeword. The basic idea of the new construction is to uniquely represent each source 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 or equal to d, merging bits between every pair of adjacent words. An essential element of the new coding principle is look ahead. The merging bits are governed by the state of the encoder (the history), the present source word to be translated, and by the upcoming source word. The new constructions have the virtue that only one look-up table is required for encoding and decoding.