Phase modulation (PM) is the encoding of information into a carrier wave by variation of its phase in accordance with an input signal.
Note that phase modulation can be regarded as a special case of frequency modulation where the carrier frequency modulation is the time derivative of the PM modulating signal.
Phase modulation can also be regarded as a special case of quadrature amplitude modulation.
Phase-shift keying refers to the simple case of phase modulation by a simple signal with a discrete number of states, such as in morse code or radioteletype applications. For example, with only two states, the technique is Binary Phase Shift Keying (BPSK). With four states, it's known as Quadrature Phase Shift Keying (QPSK), with eight states, it's known as 8-PSK, 16 states is 16-PSK, and so on.
See also
- modulation for a list of other modulation techniques