The iPod's dirty little secret

Muzak was carried on another subcarrier frequency of the FM station. The main frequency carries the L+R signal. At 38 KHz above center, the L-R signal is transmitted. Using sum and difference (L+R) + (L-R) = 2L and (L+R) - (L-R) = 2R. That's how they extract the stereo signal. A silent 19 KHz pilot operates the "stereo" lamp on the receiver and provides an accurate frequency reference for the 38 KHz needed to demodulate the L-R. Fancy RDS radios send the station name and title of a song on the 57 KHz subcarrier at a data rate of 1200 bps. Muzak runs on 67 KHz on selected FM stations, but has been mostly moved to satellite services. Another SCA carrier at 92 KHz is available for music or data as well.

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