Variable-length constrained-sequence codes
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Abstract
The use of variable-length codes to construct capacity approaching constrained sequence codes is examined. Constrained sequence codes are commonly used for digital storage and transmission, but have historically been implemented using block codes. A new technique is developed for constructing variable-length constrained sequence codes. These codes are based on Huffman encoding various partial extensions, a technique that is shown to be optimal in the sense that no other technique can result in a higher code rate given the same partial extension. Partial extensions are exhaustively searched in order to identify the highest rate code within a particular bound. Examples of the technique are provided for various constraints, including (d,k) and DC-free constraints. These examples are shown to have average rates within 1% of capacity for their respective constraints. Tables are also provided listing the optimal sourceword to codeword length mappings for various other constraints.