Run Length Limited

Run Length Limited codes, or RLL codes are widely used in hard disk drives and notably digital optical discs, such as CD, DVD and BluRay disc.

=History=

RLL encoding was invented by IBM engineers, who used it in mainframe disk drives. During the late 1980s, IBM PC compatible hard disks began using RLL. RLL codes have found almost universal application in optical disc recording practice since 1980. In consumer electronics, there is the Eight-to-Fourteen Modulation or EFM code (rate = 8/17, d=2, k=10), which is employed in the Compact Disc (CD), and the EFMPlus code (rate = 8/16, d=2, k=10) used in the DVD. See below.

=Technical overview=

The length of time usually expressed in channel bits between consecutive transitions 0 to 1 or 1 to 0 is known as the runlength. For instance, the runlengths in the word 0111100111000000 are of length 1, 4, 2, 3, and 6. Run length limited sequences are characterized by two parameters, (d+1) and (k+1), which stipulate the minimum (with the exception of the very first and last runlength) and maximum runlength, respectively, that may occur in the sequence. The parameter d controls the smallest pit/land recorded on an optical disc. The maximum runlength parameter k ensures adequate frequency of clocking information for synchronization of the read clock. The grounds on which d and k values are selected, in turn, depend on various factors such as the channel response, the desired data rate (or information density), and the jitter and noise characteristics.

=Other encodings=

Other schemes include Group Code Recording, FM and MFM (Modified Frequency Modulation).

=See also=

*PRML

=External links=

[http://cma.zdnet.com/book/upgraderepair/ch14/ch14.htm#Heading10 Chapter 14: Hard Disk Drives] - QUE Upgrading & Repairing PCs, Eighth Edition