资料来源 : Free On-Line Dictionary of Computing

Run Length Limited
     
         (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)