Line code: Difference between revisions

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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-amplitude modulation]] with polar [[non-return-to-zero]] code]]

[[File:Ami encoding.svg|class=skin-invert-image|thumb|An example of [[bipolar encoding]], or AMI.]]

[[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 [[differential Manchester encoding]]]]

[[File:Biphase Mark Code.svg|class=skin-invert-image|thumb|An example of [[biphase mark code]] ]]

[[File:MLT3encoding.svg|class=skin-invert-image|thumb|An example of [[MLT-3 encoding]]]]

}}

{{Modulation techniques}}

In [[telecommunicationtelecommunications]], a '''line code''' is a pattern of voltage, current, or photons used to represent digital data [[transmission (telecommunications)|transmitted]] down a [[communication channel]] or written to a [[storage medium]]. This repertoire of signals is usually called a '''constrained code''' in data storage systems.<ref>{{Cite journal

|journal= IEEE Communications Magazine

|date=2022

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[[File:Digital signal encoding formats-en.svg|class=skin-invert-image|framed|center|An arbitrary bit pattern in various binary line code formats]]

Each line code has advantages and disadvantages. Line codes are chosen to meet one or more of the following criteria:

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== 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 principleprincipal 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>

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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 [[telecommunicationtelecommunications]] and storage systems that move a medium past a fixed [[recording head]].<ref>{{Cite journal

|journal=Proceedings of the IEEE

|volume=78

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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]]