A method for representing digital data with analogue signals by changing the phase of the analogue 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 analogue 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.

Phase shift keying. 2-phase modulation

See also digital data transmission.