Quadrature amplitude modulation (QAM) is a technique used to transmit two digital bit streams or two analog signals by modulating or changing the amplitudes of two carrier waves so that they differ in phase by 90 degrees, a quarter of a cycle, hence the name quadrature. One signal is called the "I" signal and the other is the "Q" signal, which can be mathematically represented by a cosine and a sine wave, respectively.

QAM combines the two carriers and sends the combined signals in a single transmission to be separated and extracted at the destination. The signals are demodulated, and the data are then extracted from each and recombined to form the original modulating information. Examples of technologies using QAM are the PAL and NTSC television systems, where the different channels, which are provided by QAM, enable the transmission of the components of chroma or color information to TV sets.