Feature Request: SCA subcarrier detection/demodulation capability

[markjfine]

Addition to the Radio module to detect/demodulate SCA programming if present on 67 kHz or 92 kHz subcarriers. I assume detection could be similar to the existing RDS capability on 57kHz, but with audio demodulation instead of data handling.

[AlexandreRouma]

I was thinking of potentially adding a way to decode these, but the issue is there is essentially no standard on how this is included in the MPX. The modulation, bandwidth and subcarrier frequency are all left to the discretion of the service. I'll think about it more. With 2.0 there may be a way to just point another radio module at the MPX output of the first, which should basically allow to do this.

[markjfine]

Hmm. The data stations might be a problem, but I doubt anyone would mess with the internals of those. The 57kHz channel is likely all RDS now so I wouldn't worry about that channel. What triggered me to think about this is seeing some of the RDS waveforms showing the SCA riding on it.

Back in the day, I used to use a simple LM565 PLL for the 67 and 92kHz ones, fed by a simple Chebyshev filter, so a simple, fixed NFM demodulator at either 20-25kHz wide for either channel should be fine.

As far as channel selection, you could feasibly add an SCA switch, then have the user select either 67 (low) or 92 (high). You could get fancy and try to detect them, but that might be a level of complexity that's too far for now.