Run-length limited coding

Run-length limited coding (often abbreviated to RLL) is the name for a group of coding system in which a group of N data bits is stored as N+M bits, with the encodings from the first to the second chosen to limit the maximum number of sequential bits of one value of another.

RLL is used with encoding systems such as NRZI, in which long runs of a particular value would otherwise not exhibit level transitions, making it hard to keep the clock synchronized.