Editing Line code (section)

== 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 principle advantages of this type of code is that it can eliminate any DC component. This is important if the signal must pass through a transformer or a long transmission line.

Unfortunately, several long-distance communication channels have polarity ambiguity. Polarity-insensitive line codes compensate in these channels.<ref>
{{cite patent |inventor=Peter E. K. Chow. |url=https://www.google.com.ar/patents/US4387366 |country=US |number=4387366 |title=Code converter for polarity-insensitive transmission systems |pubdate=1983}}
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{{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}}
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{{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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{{cite book |author=Prakash C. Gupta |url=https://books.google.com/books?id=Zr1nAgAAQBAJ |title=Data Communications and Computer Networks |date=2013 |page=13 |isbn=9788120348646 |publisher=PHI Learning Pvt. Ltd. |quote=Another benefit of differential encoding is its insensitivity to polarity of the signal. ... If the leads of a twisted pair are accidentally reversed...}}
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There are three ways of providing unambiguous reception of 0 and 1 bits over such channels:
* 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 ]]