{{Short description|Line code}} frame|CMI line coding
In telecommunication, '''coded mark inversion''' ('''CMI''') is a non-return-to-zero (NRZ) line code. It encodes ''zero'' bits as a half bit time of zero followed by a half bit time of one, and while ''one'' bits are encoded as a full bit time of a constant level. The level used for ''one'' bits alternates each time one is coded.
This is vaguely reminiscent of, but quite different from, Miller encoding, which also uses half-bit and full-bit pulses, but additionally uses the half-one/half-zero combination and arranges them so that the signal always spends at least a full bit time at a particular level before transitioning again.
CMI doubles the bitstream frequency, when compared to its simple NRZ equivalent, but allows easy and reliable clock recovery.
The CMI patent was filed in 1979 with a priority date of July 27, 1978.<ref>{{ cite patent | country = US | number = 4325053 | title = Method and a circuit for decoding a C.M.I. encoded binary signal | gdate = 1982-04-13 | fdate = 1979-07-19 | invent1 = Pierre Le Brozec | invent2 = Francois Ferret | invent3 = Pierre Doussoux }}</ref> It is similar to the Quick CUTS scheme published by Bob Cottis and Mike Blandford in the Amateur Computer Club newsletter in August 1978.<ref name="ACCN">{{cite journal |title=A High Speed CUTS Casette Interface| journal=Amateur Computer Club Newsletter | date=August 1978 | volume=6 | issue=3 | url=http://smrcc.org.uk/members/g4ugm/ACC/Vol6-Issue3.pdf | access-date=September 29, 2022 |archive-url=https://web.archive.org/web/20150721012843/smrcc.org.uk/members/g4ugm/ACC/Vol6-Issue3.pdf |archive-date=2015-07-21}}</ref>
== See also ==
== References == {{Reflist}} * ITU-T G.703 (11/2001), Annex A, A.3. ''Definition of CMI''.
== External links == * {{commonscat-inline}}
{{Bit-encoding}}
Category:Encodings Category:Line codes
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