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.

An input-constrained channel, or simply a constraint, is a set S of words that is generated by a finite labeled directed graph. An encoder for S maps, in a lossless manner, sequences of unconstrained input blocks into sequences of channel blocks, the latter sequences being words of S. In most applications, the encoders are finite-state machines and, thus, presented by state diagrams. In the special case where the state diagram of the encoder is (output) deterministic, only the current encoder state and the current channel block are needed for the decoding of the current input block. In this work, the problem of designing coding schemes that can serve two constraints simultaneously is considered. Specifically, given two constraints S/sub 1/ and S/sub 2/ such that S/sub 1//spl sube/S/sub 2/ and two described rates, conditions are provided for the existence of respective deterministic finite-state encoders /spl epsi//sub 1/ and /spl epsi//sub 2/, at the given rates, such that (the state diagram of) /spl epsi//sub 1/ is a subgraph of /spl epsi//sub 2/ Such encoders are referred to as nested encoders. The provided conditions are also constructive in that they imply an algorithm for finding such encoders when they exist. The nesting structure allows to decode /spl epsi//sub 1/ while using the decoder of /spl epsi//sub 2/. Developments in optical recording suggest a potential application that can take a significant advantage of nested encoders.