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The Motorola CQUAM (Compatible Quadrature AM) system is used for AM stereo broadcasts today, although other systems have been developed in the past. AM stereo first appeared in the 1920s, but never enjoyed widespread use. Neither does CQUAM, though. Two signals are broadcast, one containing the sum of the two channels (L+R) and the other containing the difference (L-R). The two stereo channels (L and R) are reconstructed from the broadcast signals using: (L+R) + (L-R) = 2L (L+R) - (L-R) = 2R CQUAM uses two phases of the carrier frequency, 90 degrees out of phase, to carry the two signals. The in-phase carrier transmits the (L+R) signal. This can be decoded by existing mono AM radios. The (L-R) signal is broadcast on the out-of-phase carrier. A 25 Hz tone is transmitted along with the (L-R) signal. This pilot tone tells the AM receiver that a stereo signal is present for decoding.
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