C-QUAM (Compatible QUadrature Amplitude Modulation) is the method of AM stereo broadcasting used in Canada, the United States and most other countries.
Using circuitry developed by Motorola, C-QUAM uses quadrature amplitude modulation (QAM) to encode the stereo separation signal.
A 25 Hz pilot tone is added to trigger receivers; unlike its counterpart in FM radio, this carrier is not necessary for the reconstruction of the original audio sources.
[1] 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.
Among those stations are WXYG/540: Sauk Rapids, MN; CFCB/570: Corner Brook, NL; CFCO/630: Chatham, Ontario (covering SW Ontario, Eastern Michigan and Northern Ohio); WNMB/900: North Myrtle Beach, South Carolina; WBLQ/1230: Westerly, Rhode Island; WIRY/1340: Plattsburgh, New York; WAXB/850: Ridgefield, Connecticut and WYLD-AM/940: New Orleans, Louisiana.
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[according to whom?].
There has been a move to bring back C-QUAM in the last few years, due to the poor sound quality of digital audio encoding at low bit rates.
Providing for a full, rich stereo sound is simply not possible with digital audio encoding[dubious – discuss].