Phase modulation can also be regarded as a special case of quadrature amplitude modulation, where the amplitude of the modulating signal is constant, with only the phase varying. This can also be extended to frequency modulation, as this can be regarded as a special case of phase modulation.
QAM is used in NTSC and PAL television systems, where the in-phase and 90° components carry the components of chroma (colour) information. "Compatible QAM" or C-QUAM is used in AM stereo radio to carry the stereo difference information.
It is also used extensively in modems, and other forms of digital communication over analogue channels. In digital applications, the modulating signal is generally quantised in both its in-phase and 90° components. The set of possible combinations of amplitudes, as shown on an x-y plot, is a pattern of dots known as a QAM constellation.
This constellation, and therefore the number of bits which can be transmitted at once, can be increased for higher bitrates and faster throughput, or decreased for more reliable transmission with fewer bit errors. The number of "dots" in the constellation is given as a number before the QAM, and is always an integer power of two -- from 21 (2QAM) to 212 (4096QAM). 64QAM is often used in digital cable television and cable modem applications.