C-QUAM

C-QUAM is the method of AM stereo broadcasting used in Canada, the United States and most other countries. It was invented in 1977 by Norman Parker, Francis Hilbert, and Yoshio Sakaie, and published in an IEEE journal.

Using circuitry developed by Motorola, C-QUAM uses quadrature amplitude modulation (QAM) to encode the stereo separation signal. This extra signal is then stripped down in such a way that it is compatible with the envelope detector of older receivers (hence the name C-QUAM, i.e. Compatible QUadrature Amplitude Modulation). A 25 Hz pilot tone is added to trigger receivers; it is not necessary for the reconstruction of the original audio sources.

How it works

The C-QUAM signal is composed of two distinct modulation stages: a conventional AM version and a compatible quadrature PM version.

Stage 1 provides the transmitter with a summed L+R mono audio input. This input is precisely the same as conventional AM-Mono transmission methods and ensures 100% compatibility with conventional 'envelope detector' receivers.