Intelecommunications andelectronics,baud (/bɔːd/; symbol:Bd) is a commonunit of measurement ofsymbol rate, which is one of the components that determine thespeed of communication over adata channel.
It is the unit for symbol rate ormodulation rate insymbols per second orpulses per second. It is the number of distinctsymbol changes (signalling events) made to thetransmission medium per second in a digitally modulated signal or a bd rateline code.
Baud is related togross bit rate, which can be expressed inbits per second (bit/s).[1] If there are precisely two symbols in the system (typically 0 and 1), then baud and bits per second are equivalent.
The baud unit is named afterÉmile Baudot, the inventor of theBaudot code fortelegraphy, and is represented according to the rules forSI units. 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 or is capitalized for another reason, such as in title case.It was defined by the CCITT (now theITU) in November 1926. The earlier standard had been the number of words per minute, which was a less robust measure since word length can vary.[2]
The symbol duration time, also known as theunit interval, can be directly measured as the time between transitions by looking at aneye diagram of the signal on anoscilloscope. The symbol duration timeTs can be calculated as:
wherefs is the symbol rate.There is also a chance of miscommunication which leads to ambiguity.
The baud is scaled using standardmetric prefixes, so that for example
The symbol rate is related togross bit rate expressed in bit/s.The term baud has sometimes incorrectly been used to meanbit rate,[3] since these rates are the same in oldmodems as well as in the simplest digital communication links using only one bit per symbol, such that binary digit "0" is represented by one symbol, and binary digit "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 onebit. A bit (binary digit) always represents one of two states.
IfN bits are conveyed per symbol, and the gross bit rate isR, inclusive of channel coding overhead, the symbol ratefs can be calculated as
By taking information per pulseN in bit/pulse to be the base-2-logarithm of the number of distinct messagesM that could be sent,Hartley[4] constructed a measure of thegross bit rateR as
Here, the denotes the ceiling function of, where is taken to be any real number greater than zero, then the ceiling function rounds up to the nearest natural number (e.g.).
In that case,M = 2N different symbols are used. In a modem, these may be time-limited sinewave tones with unique combinations of amplitude, phase and/or frequency. For example, in a64QAM modem,M = 64, and so the bit rate isN = log2(64) = 6 times the baud rate. In a line code, these may beM different voltage levels.
The ratio is not necessarily an integer; in4B3T coding, the bit rate is4/3 of the baud rate. (A typicalbasic rate interface with a 160 kbit/s raw data rate operates at 120 kBd.)
Codes with many symbols, and thus a bit rate higher than the symbol rate, are most useful on channels such as telephone lines with a limitedbandwidth but a highsignal-to-noise ratio within that bandwidth. In other applications, the bit rate is less than the symbol rate.Eight-to-fourteen modulation as used on audio CDs has bit rate8/17[a] of the baud rate.
