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## phase shift keying

phase shift keying (PSK) A method for representing digital data with analog signals by changing the phase of the analog carrier to represent the digital information (see diagram). It is a type of modulation.

There are two ways of detecting the phase information in a signal. Fixed-reference PSK assigns a meaning to each phase position. The demodulator uses a signal source of the same frequency to compare with the incoming signal and detect its phase. Differential PSK assigns meaning to phase changes, e.g. a phase change of 180° could be taken to mean a 1, while no phase change means a 0. No comparison with another wave is needed in the demodulator.

The amount of information associated with a phase or phase change depends on the number of discrete phases that the carrier may assume. If the carrier may assume two phases then each phase or phase change represents a single bit. If four phases are used then each phase or phase change represents a different combination of two bits. The greater the number of discrete phases, the more difficult it is to generate, transmit, and detect the analog signal, thus the cost is higher; for this reason, modems that require eight or more discrete signals usually combine the phase changes with changes in amplitude in order to make the signals more distinct.

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## phase modulation

phase modulation: see modulation.

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## phase modulation

phase modulation (PM) See modulation.