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modem [modulator/demodulator], a device used to transmit and receive digital data over a communications line normally used for analog signals. A modem attached to a computer converts digital data to an analog signal that it uses to modulate a carrier frequency. This frequency is transmitted over a line, frequently as an audio signal over a telephone line, to another modem that converts it back into a copy of the original data. Synchronous data transmission uses timing signals in the data stream along with transmitted bits of uniform duration and interval. This permits the receiving modem to ignore spurious signals that do not conform to the anticipated signal. Asynchronous data transmission relies instead on various error-correcting protocols. The 8-bit parity protocol, for example, transmits a control bit after every 8 data bits that indicates whether the sum of the data bit values was odd or even. Although most modems are either of the synchronous or asynchronous variety, some employ both methods of communication. See also baud; code; modulation.
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