On 3/17/2013 9:48 AM, Alan Corey wrote:
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
 
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Radio Astronomy - the ultimate DX

Many, if not most, FM stations US have subcarriers, if only to carry the stereo (L-R) signal and the bit that turns the "stereo" indication on.  On the two SCA channels for alternate programming, you will find "elevator music," news broadcasts, audio books for the blind, and alternate language programming on them.  Since they're usually done up as a subscription service, they don't have commercials.  I've never decoded them, but I used to work at the transmitter site for an FM radio station.  Our SCA's carried "ambient music" (for which I had to change the tape if one ran out) and some sort of agricultural news broadcast.