Run Length Limited
Run Length Limited
(storage)IBM invented RLL encoding and used it in mainframe diskdrives. During the late 1980s, PC hard disks began usingRLL. Today, virtually every drive on the market uses someform of RLL.
Groups of bits are mapped to specific patterns of flux. Thedensity of flux transitions is limited by the spatialresolution of the disk and frequency response of the head andelectronics. However, transitions must be close enough toallow reliable clock recovery. RLL implementations varyaccording to the minimum and maximum allowed numbers oftransition cells between transitions. For example, the mostcommon variant today, RLL 1,7, can have a transition in everyother cell and must have at least one transition every sevencells. The exact mapping from bits to transitions isessentially arbitrary.
Other schemes include GCR, FM, Modified Frequency Modulation (MFM). See also: PRML.
http://cma.zdnet.com/book/upgraderepair/ch14/ch14.htm.