Home > C-QUAM

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 receiverThe word receiver has a number of different meanings: In communications and information processing, a receiver is the recipient ( observer) of a message ( information), which is sent from a source ( object). Receivers decode messages. A receiver (radio) is to reconstruct the stripped signal and decode it.

As with the subcarrierA subcarrier is separate analog or digital signal carried on a main radio transmission, which carries extra information such as voice or data. More technically, an already- modulated signal, which is then modulated into another signal of higher frequency 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 demodulatedDemodulation is the act of removing the modulation from an analog signal. To demodulate an AM signal, pass it through a diode rectifier. The amplitude variation will integrate into the original modulating signal. There are several ways to demodulate an FM 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 multiplexingIn telecommunications, multiplexing (MUXing) is the combining of two or more information channels onto a common transmission medium using hardware called a multiplexer or (MUX). The reverse of this is known as Inverse multiplexing (see inverse multiplexer 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. 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 being phased out in the United States as it is incompatible with the IBOC digital radio standard. The IBOC system allows transmission in stereo on the AM band, so it is expected that stations using C-QUAM will transition to IBOC as compatible receivers become more commonplace.