A modulator-demodulator (modem) is an electronic device that modulates transmitted signals and demodulates received signals. Modems are communications devices which employ digital modulation techniques to transmit binary data over analog communications channels. The modem generally provides an interface between digital devices and an analog communications system to thus make possible analog transmission of digital information between two terminals or stations. Such transmissions may be over a transmission link such as a telephone line, cellular communication link, satellite link, and cable TV, each of which being generally band-limited. Data modems are commonly used to send and receive data via telephone lines or other communications facilities. In a typical modem connection, clock synchronization time is insignificant when compared to the time it takes for the two modems to transfer data. Modulator/demodulators (modems) are essential components in most communication systems, including digital radios. A modem is used to provide wireless transmission between transmitting and receiving stations. Typically, two modems communicate over a single channel, with one modem at each end of the channel. Signal processing structures implemented at each modem provide encoding, modulation, filtering, interpolation, echo cancellation, signal detection, equalization, demodulation, and decoding functions. The advent of digital transmission systems intended primarily for broadcasting television signals, in particular but not exclusively satellite television systems, has opened up the possibility of using such systems for other purposes. One of these is to provide interactivity with the end user. In transmitting digital radio signals, most satellite communication systems utilize constant envelope modulating techniques. Constant envelope techniques are common because saturating high power amplifiers are typically used in these applications and it is desirable to minimize signal distortion.
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