C- QUAM, English Compatible Quadrature Amplitude Modulation, is a common in North America AM stereo method for analog transmission of stereophonic signals over radio stations in the medium-wave band. It was developed in 1979 by Norman Parker and Francis Hilbert at the company Motorola and one in the U.S. and Canada to the AM stereo method most used in the medium-wave band.


Similar to other stereo analog transmission method C- QUAM is compatible with mono radio receivers. Conventional medium wave reception device with amplitude modulation continue to receive a mono signal, for receiving the stereo signal, a C- QUAM -compatible receiver is necessary.

A stereo signal is generally comprised of two separate signals: the left (L) and right (R ) audio channel. In the transmitter, two auxiliary signals at C - QUAM generates by sum and difference:

The first is referred to as a sum signal, it constitutes the mono signal, which ensures the compatibility with mono radio receivers. The second one is referred to as a difference signal and is received by the C- QUAM radios.

These two auxiliary signals using quadrature amplitude modulation (QAM) modulated on the I and to the 90 ° phase-shifted Q- component of the carrier signal, which corresponds to a single-sideband modulation. In this case, the lower side band, the difference signal is transmitted in the upper sideband of the sum signal.

To ensure mono compatibility the sum signal is provided with a constant offset in the sum signal, this is expressed by the summand with 1, which at the same time also represents the highest amplitude occurring. In the adjacent diagram modulation is shifted by the addition of the allowable signal space into a circle with center at 1. For mono receivers is received with the amplitude demodulation primarily the sum signal (L R). This component causes a significant change in the amplitude, in the adjacent diagram, this means that the signal vector shown in red moves vertically. - The difference signal ( L-R) has only a little effect on the amplitude from the diagram, this signal component corresponds to a horizontal course, and results primarily in a change of the phase position φ of the carrier signal. Detection in the receiving device, whether a C- QUAM signal is received, a pilot tone at 20 Hz is added to the difference signal at the transmitter end.

C- QUAM receivers are constructed similarly for single-sideband receiver: The generated in the receiver and synchronized with the received carrier carrier frequency is used for demodulation means for mixing the two auxiliary signals. From the sum and difference signal obtained in the original stereo signal for output can again produce on the receiving device:

Integrated circuits such as the MC13022A are available for the practical construction of C - QUAM receivers.