FM Stereo Modulation

Cuthbert Nyack
FM modulation used in the 88 to 108MHz band takes a signal with frequency range ~50Hz to ~15kHz and modulates a carrier in the FM band with a modulation index of ~5. This gives a bandwidth of 2 x 75kHz. Each station is allowed a bandwidth of 200kHz allowing 100 stations in the FM band. When FM stereo was introduced it had to fit in the existing band and the system used is illustrated in the diagram below.
L corresponds to left signal and R to the right. A mono receiver sees only the L + R signal. The information which can be used to produce the stereo signal is contained in the L + R and the L - R signals. The L - R signal is added to the baseband signal as a DSB signal with suppressed carrier. A pilot tone at 19kHz is used by the receiver to regenerate the carrier for demodulating the DSB L - R signal. The baseband signal shown above is used to frequency modulate a carrier.
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COPYRIGHT 1996 Cuthbert Nyack.