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[quote:87a40eba8a=”PhilB”]Part 15 FM is terminally crippled by the FCC power limits. It is unlikely that anyone dabbling in this would also know of a way to inject SCA into their signal. Just my oppinion.[/quote:87a40eba8a]
A simple understanding how FM stereo is made should give a clue, I would think- Namely, that the pilot and stereo information are subcarriers which ride in audio frequencies above the mono baseband audio.
Those who know how to add RDS (which seems to be quite a few) would know how to add SCA, similarly. All SCA subcarriers are are narrowband FM carriers centered at +67kHz and +92kHz, with deviation of about +/-8kHz.
Some FM designs do allow access to the final FM modulator, after the pilot and stereo portions are matrixed, modulated and inserted.
Here’s what the baseband frequency spectrum looks like, courtesy of one such SCA radio manufacturer: