C-QUAM is the method of AM stereo broadcasting used in Canada and most other countries, and most of the United States. 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 older receivers (hence the name C-QUAM). A 25 Hz pilot tone is added to trigger receivers to reconstruct the stripped signal and decode it.
As with the subcarrier used for FM stereo, the audio in the C-QUAM signal is the stereo difference -- the left channel "minus" the right channel (L - R). (This "subtraction" is accomplished by simply reversing the polarity of the right channel before mixing it with the left.) The main audio is the stereo sum, or left channel plus right channel (L + R). Once fully demodulated at the receiver, adding the two together yields the left channel again (L+R + L-R = 2L), and subtracting the difference then gives the right (L+R - L-R = 2R). This method of multiplexing audio is common to all analogue stereo systems.
C-QUAM is not perfect, however, in large part because it exhibits platform motion, with the audio "center" rocking back and forth as if changing the balance knob. This effect is potentially bothersome, especially in a moving vehicle where the received signal changes rapidly, and occupants (particularly the driver) would be more prone to its effects. Also, since some stereo information is contained in the sidebands, adjacent channel interference can cause problems. Some other FCC-approved systems are still in use in the U.S., mainly the independent sideband (ISB) standard invented by Leonard Kahn.
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