Editing Line code (section)

== Disparity ==
Most long-distance communication channels cannot reliably transport a [[DC component]]. The DC component is also called the ''disparity'', the ''bias'', or the [[DC coefficient]]. The disparity of a bit pattern is the difference in the number of one bits vs the number of zero bits. The ''running disparity'' is the [[running total]] of the disparity of all previously transmitted bits.<ref>{{cite web |author=Jens Kröger |url=https://www.psi.ch/mu3e/ThesesEN/BachelorKroeger.pdf |archive-url=https://ghostarchive.org/archive/20221009/https://www.psi.ch/mu3e/ThesesEN/BachelorKroeger.pdf |archive-date=2022-10-09 |url-status=live |title=Data Transmission at High Rates via Kapton Flexprints for the Mu3e Experiment |date=2014 |page=16}}</ref> The simplest possible line code, [[Unipolar encoding|unipolar]], gives too many errors on such systems, because it has an unbounded DC component.

Most line codes eliminate the DC component{{snd}} such codes are called [[DC-balanced]], zero-DC, or DC-free. There are three ways of eliminating the DC component:

* Use a [[constant-weight code]]. Each transmitted [[Code word (communication)|code word]] in a constant-weight code is designed such that every code word that contains some positive or negative levels also contains enough of the opposite levels, such that the average level over each code word is zero. Examples of constant-weight codes include [[Manchester code]] and [[Interleaved 2 of 5]].
* Use a [[paired disparity code]]. Each code word in a paired disparity code that averages to a negative level is paired with another code word that averages to a positive level. The transmitter keeps track of the running DC buildup, and picks the code word that pushes the DC level back towards zero. The receiver is designed so that either code word of the pair decodes to the same data bits. Examples of paired disparity codes include [[alternate mark inversion]], [[8b/10b]] and [[4B3T]].
* Use a [[scrambler]]. For example, the scrambler specified in {{IETF RFC | 2615}} for [[64b/66b encoding]].