An Endec for high rate quadbit modulo 2 codes

Quadbit Module 2 (QM2) constraint codes are similar to Tribit Module 2 codes, and have a Shannon capacity of 0.9613. QM2 codes disallow all pentabits and longer in NRZI space, and allow quadbits to occur only on even time indices. This constraint removes a class of error events, and improves performance. In order to achieve rates close to capacity, larger block sizes must be used. This paper describes a method for encoding high rate QM2 codes. The method uses sets of subsequences to form the large block size codewords, and is memory efficient, requiring less than 256 bytes for a rate 15/16 code.