Run Length Limited
Run Length Limited, commonly RLL, is the most popular scheme for encoding data on hard disks. RLL packs up to 50% more data on a disk than its predecessor in the PC market, MFM.
History
RLL encoding was invented by IBM, who used it in mainframe disk drives. During the late 1980s, PC hard disks began using RLL. Today, some form of RLL is almost universal.
Technical overview
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 encodings
Other schemes include Group Code Recording, FM and MFM (Modified Frequency Modulation).
See also
