Modem

Modem is a contraction of Modulator-Demodulator. In its most common usage, it is a device that takes digital signals to be transmitted, converts them to an analog waveform appropriate for the transmission media, and sends that waveform into the transmission media. In the opposite direction, it converts received analog data to digital data. The digital data transmitter and receiver is usually a computer, but could be a specialized device such as a digital telephone.

Most terrestrial transmission systems are now digital, so modems are far less important than they once were. There are still low-speed applications, such as credit card authorization, where a modem might be used on a dial-up telephone line. Even ostensibly analog telephone lines, however, may be converted to digital format once they leave the customer premises, and the digital format used may be quite good for voice, but incompatible with modem formats.

A wide range of modulation techniques have been used on modems. Modulation schemes for putting relatively high-speed signals (i.e., 9.6 to 53 Kbps) onto telephone lines can be quite complex.

Specialized modems are still often used over military tactical radio systems, especially at high frequency (HF) or very high frequency (VHF).

The term "digital modem" is occasionally encountered, but that is usually some type of reformatter of digital signals, rather than a true modulator of a carrier wave.