Redigerer Baud

'''Baud''' (Bd) angir et antall informasjonsbærende, adskilte informasjonsenheter (symboler) i en informasjonsstrøm. Baud brukes for å angi informasjonshastighet (symbolhastighet) ved signaloverføring. 1 baud betyr 1 symbol per sekund. Ett slikt symbol kan representere flere bit. Baud er oppkalt etter [[Émile Baudot]] (1845–1903) som oppfant [[baudotkode]]n.<ref name="linfo"/>

I eldre typer [[modem]]er overførte en ett [[bit]] per symbol, og baudraten tilsvarte bitraten. Da bithastigheten skulle økes over 2400b/s var en begrenset av det analoge telefonnettets frekvensbånd (300Hz-3300Hz) og kunne ikke fortsette å øke baudhastigheten tilsvarende. Løsningen ble da å overføre flere bit per symbol og beholde symbolhastigheten på 2400Bd. I f.eks. de populære 28800b/s modemene var baudraten 2400Bd og man overførte dermed 12bit per symbol. For å overføre 12bit per symbol måtte en ha 2^12=4096 forskjellige symboler å velge mellom hver gang et nytt symbol skulle overføres.

Det er også mulig å ha færre enn ett bit per symbol. Dette kan være aktuelt i lavhastighets krypterte og redundante kodesystemer. 

I dagligtale har baud feilaktig blitt brukt synonymt med bitrate og også i noen tilfeller synonymt med byterate. Baud og baudrate er fortsatt nyttige begreper som brukes av fagfolk innen kommunikasjonsteknolgi, men bør unngås brukt av folk flest. Årsaken til det siste er både menigmanns feilaktige bruk og forståelse av begrepet og at begrepene bitrate og byterate inneholder den nyttige informasjonen for den jevne bruker.

Baud skrives som [[SI-systemet|SI-enheter]], forkortet '''Bd''' (med stor B), og ellers '''baud''' med små bokstaver. 

<!-- Engelsk kilde, skal oversettes

In [[telecommunications]] and [[electronics]], '''baud''' ({{pronEng|ˈbɔːd}}, unit symbol "Bd") is synonymous to ''symbols/s'' or ''pulses/s''. It is the unit of [[symbol rate]], also known as ''baud rate'' or ''modulation rate''; the number of distinct [[symbol (data)|symbol]] changes (signalling events) made to the transmission medium per [[second]] in a digitally [[modulation|modulated]] signal or a [[line code]]. The baud rate is related to but should not be confused with [[gross bit rate]] expressed in bit/s. 

The '''symbol duration time''', also known as [[Unit interval (data transmission)|unit interval]],  can be directly measured as the time between transitions by looking into an [[eye diagram]] of an [[oscilloscope]]. The symbol duration time ''T''<sub>s</sub> can be calculated as: 

:<math>  T_s  =   {1 \over f_s},  </math>

where ''f''<sub>s</sub> is the symbol rate.

:'''A simple example''': A baud rate of 1 kBd = 1,000 Bd is synonymous to a symbol rate of 1,000 symbols per second. In case of a modem, this corresponds to 1,000 tones per second, and in case of a line code, this corresponds to 1,000 pulses per second. The symbol duration time is 1/1,000 second = 1 millisecond.
 
The baud unit is named after [[Emile Baudot]], the inventor of the [[Baudot code]] for [[telegraphy]], and is represented as [[International System of Units|SI units]] are.  That is, the first letter of its symbol is uppercase (Bd), but when the unit is spelled out, it should be written in lowercase (baud) except when it begins a sentence.

==Relationship to gross bit rate==
The symbol rate is related to but should not be confused with gross bit rate expressed in bit/s. 

The term baud rate has sometimes incorrectly been used to mean bit rate, since these rates are the same in old [[modem]]s as well as in the simplest digital communication links using only one bit per symbol, such that binary "0" is represented by one symbol, and binary "1" by another symbol. In more advanced modems and data transmission techniques, a symbol may have more than two states, so it may represent more than one binary bit (a binary bit always represents exactly two states). 

If ''N'' bits are conveyed per symbol, and the gross bit rate is ''R'', inclusive of channel coding overhead, the symbol rate can be calculated as: 

:<math>  f_s  =   {R \over N}. </math>

In that case ''M''=2<sup>''N''</sup> different symbols are used. In a modem, these may be sinewave tones with unique combinations of amplitude, phase and/or frequency. For example, in a [[64QAM]] modem, ''M''=64. In a line code, these may be ''M'' different voltage levels. 

By taking information per pulse ''N'' in bit/pulse to be the base-2-[[logarithm]] of the number of distinct messages ''M'' that could be sent, [[Ralph Hartley|Hartley]]<ref>{{cite book | title = Information Theory; and its Engineering Applications | author = D. A. Bell | edition = 3rd ed. | year = 1962 | publisher = Pitman | location = New York}}</ref> constructed a measure of the [[gross bitrate]] ''R'' as:  

:<math>  R  =   f_s \log_2(M), \,  </math>

where ''f''<sub>''s''</sub> is the baud rate in symbols/second or pulses/second. (See [[Hartley's law]]).
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==Se også==
* [[Båndbredde]]
* [[Bitrate]]
* [[Modulasjon]]
* [[Modem]]
* [[Nyquist-rate]]
* [[PCM]]

== Referanser ==
<references>
<ref name="linfo">{{Kilde www 
 |url = http://www.linfo.org/baud.html
 |tittel = Baud Definition
 |besøksdato = 2018-04-15
 |utgiver = The Linux Information Project
 |dato = 2005-11-08
}}

</ref>
</references>

== Kilder ==
* [https://web.archive.org/web/20070720131009/http://www.kunnskapsnett.no/innhold/baudbps.htm Johan Futsæther:''Forskjellen på BAUD og bps'', 1992]

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* {{cite web 
| title=On the origins of serial communications and data encoding
| url=http://www.compkarori.com/dbase/bu07sh.htm
| accessmonthday=January 4 
| accessyear=2007
}}

[[Kategori:Data transmission]]
[[Kategori:Units of measure]]
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[[Kategori:Datamåleenheter]]