Nested block decodable runlength-limited codes

Consider a (d/sub 1/, k/sub 1/)-runlength-limited (RLL) constraint that is contained in a (d/sub 2/, k/sub 2/)-RLL constraint, where k/sub 1//spl ges/2d/sub 1/ and d/sub 2/>0, and fix a codeword length q>k/sub 2/. It is shown that whenever there exist block-decodable encoders with codeword length q for those two constraints, there exist such encoders where one is a subgraph of the other: furthermore, both encoders can be decoded by essentially the same decoder. Specifically, a (d/sub 1/, k/sub 1/)-RLL constrained word is decoded by first using a block decoder of the (d/sub 2/, k/sub 2/)-RLL encoder, and then applying a certain function to the output of that decoder.