Distance preserving run-length limited codes

A subset of the RLL (run-length limited) codes called distance preserving RLL codes is introduced. In addition to satisfying a run-length constraint, these codes have the property that the Hamming distance between any two encoder output sequences is at least as large as the Hamming distance between the corresponding encoder input sequences. Thus, when used in combination with a binary ECC code, a distance preserving RL code does not reduce the overall Hamming distance of the ECC/RLL combination to something below the Hamming distance of the ECC code alone. It is shown how distance preserving RLL codes can be used with binary convolutional codes to create combined ECC/RLL codes with the distance properties of the original convolutional code