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 Frequency Modulation (MFM). See also:
PRML.
http://cma.zdnet.com/book/upgraderepair/ch14/ch14.htm.
(2003-08-12)
Nearby terms:
runes « runic « run-length encoding « Run Length
Limited
» run time » run-time environment » run-time error
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