RLL

   (Run-Length Limited) A method of encoding data as a series of
magnetic flux reversals on disk or tape.  RLL can achieve higher data
densities than MFM recording because it encodes the data such that
(on average), fewer than one flux reversal per data bit is required
(however, timing becomes more critical).  RLL is actually an entire
family of encoding methods, specified with two numbers indicating the
minimum and maximum distances between one bits (flux transitions).  The
variant normally called RLL is RLL-2,7; RLL-1,7 and RLL-3,9 are also in
use.  MFM is in effect RLL-1,3.  See also FM, MFM.