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{{short description|Line code used in early magnetic data storage and Ethernet}}
{{Use dmy dates|date=April 2022}}
In [[telecommunication]] and [[computer data storage|data storage]], '''Manchester code''' (also known as '''phase encoding''', or '''PE''') is a [[line code]] in which the encoding of each data [[bit]] is either low then high, or high then low, for equal time. It is a [[self-clocking signal]] with no [[DC component]]. Consequently, electrical connections using a Manchester code are easily [[galvanic isolation|galvanically isolated]].
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Manchester coding is a special case of [[binary phase-shift keying]] (BPSK), where the data controls the [[Phase (waves)|phase]] of a square wave [[Carrier signal|carrier]] whose frequency is the data rate. Manchester code ensures frequent line voltage transitions, directly proportional to the clock rate; this helps [[clock recovery]].
The [[DC component]] of the encoded signal is not dependent on the data and therefore carries no information. Therefore connections may be [[Inductive coupling|inductively]] or [[Capacitive coupling|capacitively]] coupled, allowing the signal to be conveyed conveniently by galvanically isolated media (e.g., Ethernet) using a [[network isolator]]—a simple one-to-one [[pulse transformer]] which cannot convey a DC component.
According to [[Cisco]], "Manchester encoding introduces some difficult frequency-related problems that make it unsuitable for use at higher data rates".<ref>{{citation |url=http://docwiki.cisco.com/wiki/Ethernet_Technologies |title=Ethernet Technologies |publisher=[[Cisco Systems]] |access-date=2017-09-12 |quote=Manchester encoding introduces some difficult frequency-related problems that make it unsuitable for use at higher data rates. |archive-url=https://web.archive.org/web/20181228005303/http://docwiki.cisco.com/wiki/Ethernet_Technologies |archive-date=2018-12-28 |url-status=dead}}</ref><!--Difficulties are in meeting [[Title 47 CFR Part 15]] and other RF emissions requirements.-->
There are more complex codes, such as [[8B/10B encoding]], that use less [[bandwidth (signal processing)|bandwidth]] to achieve the same data rate but may be less tolerant of frequency errors and [[jitter]] in the transmitter and receiver reference clocks.{{
==Encoding and decoding==
[[Image:Manchester encoding both conventions.svg|thumb|650px|An example of Manchester encoding showing both [[Manchester code#Conventions for representation of data|conventions for representation of data]]]]
Manchester code always has a transition at the middle of each bit period and may (depending on the information to be transmitted) have a transition at the start of the period also. The direction of the mid-bit transition indicates the data. Transitions at the period boundaries do not carry information. They exist only to place the signal in the correct state to allow the mid-bit transition.
===Conventions for representation of data===
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