Coding for magnetic and optical recording

A widely-used model for describing the read-write requirements of magnetic and optical storage devices is the so-called constrained system, which is defined as the set of sequences obtained by reading the labels of paths in a labeled finite directed graph. In order to transform an arbitrary stream of data into a sequence which satisfies the constrained-system specifications, the raw data has to undergo an encoding, or modulation stage. The study of encoders for constrained systems has evolved quite significantly in recent years. This paper summarizes some of the encoding methods for constrained systems. We present some of the classical schemes, and we mention a recent encoding scheme that has been suggested by Ashley, Marcus, and Roth (1993).