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Back awhile I was involved sending programming to a monophonic FM public station, and one night we mixed a 19kHz tone into the broadcast at 10% modulation.
The stereo light on our FM receiver lit up. Is that exciting, or did we just think it was.
The stereo light on that FM receiver might have lit, but that 19 kHz signal should not have been transmitted by that monophonic FM station.
The FCC does not permit a monophonic-only, analog FM broadcast station to transmit a baseband signal at/near 19 kHz.
The reason for this is to prevent an analog FM stereo receiver from enabling its L-R subchannel demodulator under those conditions — which would degrade the signal-to-noise ratio at the output of the receiver.