Feature request : SCA demod. Digital secondary audio transmitted.

[chris451gh]

ON US and Canadian Analog FM broadcast (88.1-107.9 steo 200khz) a secondary carrier authorization analog nbfm signal is transmitted.
Similar to the way RDS is sent, a narrowband fm subcarrier is transmitted on top of the main wide fm carrier.
A narrowband demod is tuned to 67khz to receive this second carrier. A bandwidth of 15khz is adequate to receive this,
usually voice or low quality music.
The carrier level is the same as the 19khz pilot. This subcarrier and ocasionally another one at another subcarrier frequency is transmitted. I've seen 67khz and 87Khz. The total bandwidth is less than 200khz.
Additionally, HD Radio may be transmitted, for a total bandwidth of 400khz. Yes this does eliminate reception of an adjacent channel.
To get the SCA a wideband FM demod is applied to the main carrier, with a 90khz low pass filter.
Then a secondary NBFM demod is attached to the WFM demod and tuned to 67khz10% and or 92khz10%, with the RDS at 57khz 5%
This audio is then routed similar to the main audio supplying the Sca content.

https://www.sigidwiki.com/wiki/HD_Radio_(FM)
https://www.sigidwiki.com/wiki/FM_Broadcast_Radio