Phase Modulation
Phase modulation (PM) is the process of varying the phase of a carrier signal in proportion to the waveform of the modulating signal. The phase of the carrier is shifted according to the amplitude of the modulating signal, resulting in a waveform that is the sum of the carrier and the modulating signal. PM is used in some digital communication systems, such as spread-spectrum technology.
Quadrature Amplitude Modulation
Quadrature amplitude modulation (QAM) is the process of varying both the amplitude and the phase of a carrier signal in proportion to the waveform of the modulating signal. The amplitude and phase of the carrier are changed according to the amplitude and phase of the modulating signal, resulting in a waveform that is the sum of the carrier and the modulating signal. QAM is used in some digital communication systems, such as digital television and cable modems.