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C-QUAM
From Wikipedia, the free encyclopedia
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). A 25 Hz pilot tone is added to trigger receivers. It is NOT necessary for the reconstruction of the original audio sources.
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 pre-AMAX it exhibited 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. This has been alleviated in subsequent revisions. Also, since some stereo information is contained in the sidebands, adjacent channel interference can cause problems. Finally, when only part of a sideband is attenuated (as often happens to skywave signals reflecting off the ionosphere), an effect known as selective fading, very unpleasant effects result; hence, the C-QUAM system is not often if ever used for shortwave broadcasting, nor by stations which receive a great deal of skywave interference.
C-QUAM is incompatible with IBOC digital radio. The IBOC system allows transmission in about 15kHz stereo on the AM band with significant digital artifacting due to the 36kb bit rate.