Editing Line code (section)

==Run-length limited codes==
For reliable [[clock recovery]] at the receiver, a [[Run-length limited|run-length limitation]] may be imposed on the generated channel sequence, i.e., the maximum number of consecutive ones or zeros is bounded to a reasonable number. A clock period is recovered by observing transitions in the received sequence, so that a maximum run length guarantees sufficient transitions to assure clock recovery quality.

RLL codes are defined by four main parameters: ''m'', ''n'', ''d'', ''k''. The first two, ''m''/''n'', refer to the rate of the code, while the remaining two specify the minimal ''d'' and maximal ''k'' number of zeroes between consecutive ones. This is used in both [[telecommunications]] and storage systems that move a medium past a fixed [[recording head]].<ref>{{Cite journal 
|journal=Proceedings of the IEEE
|volume=78 
|issue=11 
|date=December 1990 |title=Runlength-Limited Sequences
|author=Kees Schouhamer Immink
|author-link=Kees Schouhamer Immink 
|url=https://www.researchgate.net/publication/2984369
|pages=1745–1759
|quote=A detailed description is furnished of the limiting properties of runlength limited sequences.
 |doi=10.1109/5.63306}}</ref>

Specifically, RLL bounds the length of stretches (runs) of repeated bits during which the signal does not change.  If the runs are too long, clock recovery is difficult; if they are too short, the high frequencies might be attenuated by the communications channel.  By [[Modulation|modulating]] the [[data]], RLL reduces the timing uncertainty in decoding the stored data, which would lead to the possible erroneous insertion or removal of bits when reading the data back. This mechanism ensures that the boundaries between bits can always be accurately found (preventing [[bit slip]]), while efficiently using the media to reliably store the maximal amount of data in a given space.

Early disk drives used very simple encoding schemes, such as RLL (0,1) FM code, followed by RLL (1,3) MFM code which were widely used in [[hard disk drive]]s until the mid-1980s and are still used in digital optical discs such as [[CD]], [[DVD]], [[Minidisc|MD]], [[Hi-MD]] and [[Blu-ray]] using [[Eight-to-Fourteen Modulation|EFM]] and [[EFMPLus]] codes.<ref>{{Cite journal 
|journal=IEEE Transactions on Consumer Electronics
|volume=CE-41 
|date=1995
|title=EFMPlus: The Coding Format of the MultiMedia Compact Disc
|author=Kees Schouhamer Immink
|author-link=Kees Schouhamer Immink 
|url=https://www.researchgate.net/publication/3179483
|pages=491–497
|quote=A high-density alternative to EFM is described.}}</ref> Higher density RLL (2,7) and RLL (1,7) codes became the [[de facto standard]]s for hard disks by the early 1990s.{{citation needed|date=August 2019}}