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In the stereo system the DSBSC demodulation problem is dealt
with by including a 19 kHz pilot tone in the broadcast. This tone is
generated by a divide-by-two frequency converter circuit, which takes
the 38 kHz carrier and produces the 19 kHz pilot tone. The 19 kHz pilot
tone falls midway in the spectral region between the mono sum signal
(up to 15 kHz) and below the DSBSC difference signal information.
The DSBSC signal extends from 23 kHz to 53 kHz, since the input
modulating signals are band limited to 15 kHz. The DSBSC output is
added to the baseband (L and R sum) signal and the 19 kHz pilot tone
before being sent to the FM modulator.
A mono FM receiver ignores the stereo information by using a
filter after its FM demodulator to block everything ...