FM Stereo Modulation
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.