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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 
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.
Known problems 
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.
User base 
As of September 2012[update], there are still several AM radio stations in North America broadcasting in C-QUAM stereo. Among those stations are CFCO/630: Chatham, Ontario (covering SW Ontario, Eastern Michigan and Northern Ohio); WLS/890 (now during both day and night hours): Chicago, Illinois; WNMB/900: North Myrtle Beach, South Carolina; WBLQ/1230: Westerly, Rhode Island; WIRY/1340: Plattsburgh, New York; WAXB/850: Ridgefield, Connecticut; and WLAD/800: Danbury, Connecticut. In addition to FCC-Licensed C-QUAM AM broadcast stations, low-powered (<100 mW) Part 15 C-QUAM stereo transmitters are available for sale for use in the United States. In Rome, Italy, there is Broadcastitalia on 1485 kHz.
Competition from IBOC Hybrid Digital Systems 
While C-QUAM is an accepted international standard for AM Radio broadcasting, it is incompatible with the IBOC (In-band on-channel) "HD radio" system, so a broadcaster must choose what system they will use. The IBOC system allows transmission of an audio frequency range extending to approximately 15 kHz, 2-ch Stereo on the AM band, but with significant digital artifact and aliasing due to substantial codec inadequacy.
In addition, C-QUAM patents have expired. iBiquity still controls IBOC intellectual property through patents, through licensing fees for both the use of the technology, and any modifications to be made, even if the broadcaster in question has purchased the equipment outright and made costly modifications to their transmitter plant in order to implement it.
A very few AM radio stations that broadcast with IBOC HD Radio during the day switch to C-QUAM AM Stereo during nighttime operation to reduce sideband digital (hash) interference and to provide long-range stereo reception. Many HD radio tuners have a limited ability to decode C-Quam stereo transmissions, (typically with lower bandwidth), and as a result, reduced audio quality than what could be expected from a specifically designed AMAX/C-QUAM only tuner. C-QUAM AM Stereo transmissions have the same range as AM Monural transmission, a key benefit.