This might be a better question for the Gnuradio list, but for years I've known that some broadcast FM stations actually transmit a second audio program which is somehow modulating a subcarrier in the main transmission. It probably carries mostly elevator music type junk but is rumored to be commercial-free (a rare thing in the US!). I remember seeing dedicated receivers advertised for this, but never actually bought one because I wasn't sure it was active in my area. These date back to Lafayette Electronics days I think and ever since so 40 years or more.

I've noticed (in SdrSharp) that on some FM stations I can see what look like they might be subcarriers on either side of the main signal. Anyone decoded those?

Alan

-----
Radio Astronomy - the ultimate DX