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{{short description|Pattern used within a communications system to represent digital data}}
{{stack|
[[File:NRZcode.png|class=skin-invert-image|thumb|An example of coding a binary signal using rectangular [[pulse
[[File:Ami encoding.svg|class=skin-invert-image|thumb|An example of [[
[[File:Manchester code.svg|class=skin-invert-image|thumb|Encoding of 11011000100 in [[Manchester encoding]] ]]
[[File:Differential_manchester_encoding_Workaround.svg|class=skin-invert-image|thumb|An example of [[
[[File:Biphase Mark Code.svg|class=skin-invert-image|thumb|An example of [[
[[File:MLT3encoding.svg|class=skin-invert-image|thumb|An example of [[MLT-3 encoding]]
}}
{{Modulation techniques}}
In [[
|journal= IEEE Communications Magazine
|date=2022
|title=Innovation in Constrained Codes
|author=K. Schouhamer Immink
|author-link=Kees Schouhamer Immink
|url=https://www.researchgate.net/publication/362866105
|access-date=2022-10-05
}}</ref>
Some signals are more prone to error than others as the physics of the communication channel or storage medium constrains the repertoire of signals that can be used reliably.<ref name="optics">{{Cite journal
|journal= IEEE Journal on Selected Areas in Communications
|volume=19
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|title=A Survey of Codes for Optical Disk Recording
|author=K. Schouhamer Immink
|
|url=https://www.researchgate.net/publication/3234561
|pages=751–764
|
}}</ref>
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After line coding, the signal is put through a physical communication channel, either a [[transmission medium]] or [[data storage medium]].<ref name="paulsen">Karl Paulsen. [http://www.tvtechnology.com/media-servers/0150/coding-for-magnetic-storage-mediums/186738 "Coding for Magnetic Storage Mediums"] {{Webarchive|url=https://web.archive.org/web/20140521215946/http://www.tvtechnology.com/media-servers/0150/coding-for-magnetic-storage-mediums/186738 |date=2014-05-21 }}.2007.</ref><ref>{{citation|author1=Abdullatif Glass |author2=Nidhal Abdulaziz |author3=and Eesa Bastaki |url=http://ro.uow.edu.au/cgi/viewcontent.cgi?article=1285&context=dubaipapers|title=Slope line coding for telecommunication networks|year=2007|page=1537|journal=IEEE International Conference on Signal Processing and Communication|publisher=IEEE|___location=Dubai|quote=Line codes ... facilitates the transmission of data over telecommunication and computer networks and its storage in multimedia systems.}}</ref> The most common physical channels are:
* the line-coded signal can directly be put on a [[transmission line]], in the form of variations of the voltage or current (often using [[differential signaling]]).
* the line-coded signal (the ''[[baseband]] signal'') undergoes further [[pulse shaping]] (to reduce its frequency bandwidth) and then is [[modulation|modulated]] (to shift its frequency) to create an ''[[RF signal]]'' that can be sent through free space.
* the line-coded signal can be used to turn on and off a light source in [[free-space optical communication]], most commonly used in an infrared [[remote control]].
* the line-coded signal can be printed on paper to create a [[bar code]].
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|}
[[File:
Each line code has advantages and disadvantages. Line codes are chosen to meet one or more of the following criteria:
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== 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
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]], [[
* Use a [[scrambler]]. For example, the scrambler specified in {{IETF RFC | 2615}} for [[64b/66b encoding]].
== Polarity ==
Bipolar line codes have two polarities, are generally implemented as RZ, and have a radix of three since there are three distinct output levels (negative, positive and zero). One of the
Unfortunately, several long-distance communication channels have polarity ambiguity. Polarity-insensitive line codes compensate in these channels.<ref>
{{cite patent |
</ref><ref>
{{citation |author=David A. Glanzer |publisher=[[Fieldbus Foundation]] |url=http://www.fieldbus.org/images/stories/enduserresources/technicalreferences/documents/wiringinstallationguide.pdf |archive-url=https://ghostarchive.org/archive/20221009/http://www.fieldbus.org/images/stories/enduserresources/technicalreferences/documents/wiringinstallationguide.pdf |archive-date=2022-10-09 |url-status=live |title=Fieldbus Application Guide ... Wiring and Installation |section=4.7 Polarity |page=10}}
</ref><ref>
{{cite book |author1=George C. Clark Jr. |author2=J. Bibb Cain |url=https://books.google.com/books?id=wgzyBwAAQBAJ |title=Error-Correction Coding for Digital Communications |date=2013 |page=255 |isbn=9781489921741 |publisher=Springer Science & Business Media |quote=When PSK data modulation is used, the potential exists for an ambiguity in the polarity of the received channel symbols. This problem can be solved in one of two ways. First ... a so-called ''transparent'' code. ...}}
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* Pair each code word with the polarity-inverse of that code word. The receiver is designed so that either code word of the pair decodes to the same data bits. Examples include [[alternate mark inversion]], [[Differential Manchester encoding]], [[coded mark inversion]] and [[Miller encoding]].
* [[differential coding]] each symbol relative to the previous symbol. Examples include [[MLT-3 encoding]] and [[NRZI]].
* Invert the whole stream when inverted [[syncword]]s are detected, perhaps using [[ differential signalling#Polarity switching | polarity switching ]]
==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 [[
|journal=Proceedings of the IEEE
|volume=78
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|date=December 1990 |title=Runlength-Limited Sequences
|author=Kees Schouhamer Immink
|
|url=https://www.researchgate.net/publication/2984369
|pages=1745–1759
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|title=EFMPlus: The Coding Format of the MultiMedia Compact Disc
|author=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}}
== Synchronization ==
{{main|Clock recovery}}
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== Other considerations ==
A line code will typically reflect technical requirements of the transmission medium, such as [[optical fiber]] or [[shielded twisted pair]]. These requirements are unique for each medium, because each one has different behavior related to interference, distortion, capacitance and
== Common line codes ==
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* [[Non-return-to-zero]] (NRZ)
* [[Non-return-to-zero, inverted]] (NRZI)
* [[Pulse-position modulation]] (PPM)
* [[Return-to-zero]] (RZ)
* [[TC-PAM]]
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* [https://web.archive.org/web/20130418052107/http://www.electronics.dit.ie/staff/amoloney/lecture-9.pdf Line Coding Lecture No. 9]
* [http://www.fiberoptics4sale.com/wordpress/line-coding-in-digital-communication/ Line Coding in Digital Communication]
* [https://www.ac.uma.es/~guille/codsim2.0/ CodSim 2.0: Open source simulator for Digital Data Communications Model at the University of Malaga written in HTML]
{{Bit-encoding}}
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