<storage> (RLL) The most popular scheme for encoding data on
magnetic disks. RLL packs up to 50% more data on a disk
than MFM.
IBM invented RLL encoding and used it in mainframe disk
drives. During the late 1980s, PC hard disks began using
RLL. Today, virtually every drive on the market uses some
form of RLL.
Groups of bits are mapped to specific patterns of flux. The
density of flux transitions is limited by the spatial
resolution of the disk and frequency response of the head and
electronics. However, transitions must be close enough to
allow reliable clock recovery. RLL implementations vary
according to the minimum and maximum allowed numbers of
transition cells between transitions. For example, the most
common variant today, RLL 1,7, can have a transition in every
other cell and must have at least one transition every seven
cells. The exact mapping from bits to transitions is
essentially arbitrary.
Other schemes include GCR, FM, Modified Frequency
Modulation (MFM). See also: PRML.
