The baud unit is named after Émile Baudot, the inventor of the Baudot code for telegraphy, and is represented in accordance with the rules for SI units. That is, the first letter of its symbol is uppercase, but when the unit is spelled out, it should be written in lowercase except when it begins a sentence. It was defined by the CCITT in November 1926. The earlier standard had been the number of words per minute. One baud was equal to one pulse per second, a more robust measure as word length can vary.
Definitions
The symbol duration time, also known as unit interval, can be directly measured as the time between transitions by looking at an eye diagram of the signal on an oscilloscope. The symbol duration time Ts can be calculated as: where fs is the symbol rate. There is also a chance of miscommunication which leads to ambiguity. In digital systems with binary code, 1 Bd = 1 bit/s. By contrast, non-digital systems use a continuous range of values to represent information and in these systems the exact informational size of 1 Bd varies. The baud is scaled using standard metric prefixes, so that for example
The symbol rate is related to gross bit rate expressed in bit/s. The term baud has sometimes incorrectly been used to mean bit rate, since these rates are the same in old modems 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 one bit. A bit always represents one of two states. If N bits are conveyed per symbol, and the gross bit rate is R, inclusive of channel coding overhead, the symbol rate fs can be calculated as 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, Hartley constructed a measure of the gross bitrateR as In that case, 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 a 64QAM modem,, and so the bit rate is times the baud rate. In a line code, these may be M different voltage levels. The ratio is not necessarily even an integer; in 4B3T coding, the bit rate is of the baud rate. 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 limited bandwidth but a high signal-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 rate of the baud rate.
