Run Length Limited

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 FrequencyModulation (MFM). See also: PRML.

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(2003-08-12)