A unit of signaling speed, used to describe how quickly information can be transmitted over a line, originally a telegraph line. A rate in baud is the maximum number of times the system can change the state of the line in one second. It is thus the reciprocal of the length in seconds of the shortest element in the signaling code.

The baud is named after J. M. E. Baudot (1845 – 1903), who invented the International Telegraph Code No. 1 (also known as the Baudot code) sometime around 1880.

The baud is often used erroneously in describing the transmission speed of computer modems. Baud rates are not the same as bits per second rates. In the early 1900s many telegraph lines had more than two possible states. For example, some telegraph systems could send and detect four different voltage levels, so that in the duration of the shortest signaling element two bits of information could be represented (00, 01, 10, or 11). If a line has only two states, each signal element represents a single bit, and the rate in baud will be equal to the rate in bits per second. This was true of the earliest, 300-baud, personal computer modems. But it was not the case with “1200-baud” modems, most of which sent at 300 baud with a data transmission scheme that transmitted 4 bits per baud, nor with “2400-baud modems,” which run at 600 baud with 4 bits per baud. Modems running under the V.32bis protocol send 6 bits per baud at 2400 baud, for a transmission speed of 14,400 bps, but they are not “14.4K-baud modems.” To avoid confusion, when signal elements represent only 0's and 1's, the transmission speed should be described in bits per second.

ANSI Standard X3.12-1970.

ANSI-IEEE Standard C37.1-1979. Standard Definition, Specification and Analysis of Manual, Automatic, and Supervisory Station Control and Data Acquisition.

IEEE Standard 599-1983. Standard Glossary of Power Systems Data Transmission and Related Channel Terminology.

IEEE Standard 145-1983. Definitions of Terms for Antennae.

ISO Standard 2382/V, VI. Vocabulary for Information Processing.

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