Broadcast FM subcarrier decoding?

Robert Nickels ranickel at comcast.net
Sun Mar 17 16:32:20 UTC 2013


On 3/17/2013 9:48 AM, Alan Corey wrote:
> I can see what look like they might be subcarriers on either side of 
> the main signal.  Anyone decoded those?

Hi Alan,

Depending on where you live, there could be several subcarriers 
present.   From your description I'm pretty sure you're referring to Sub 
Carrier Authorization (SCA) which has been mainly used for background 
music and an audio book reading service for the blind as well as other 
voice and  data services.  SCA uses 67 or 92 KHz subcarriers which can 
be seen on the composite FM signal.  If you have a soundcard with 
sufficient bandwidth  you can send the output of SDR# (in NBFM, 150 KHz 
mode) to another instance of SDR# *(sound card, DSB mode)  using Virtual 
Audio Cable.     My soundcard won't go high enough but here's a set of 
screen images from a system that can, where the various elements on the 
upper side of the FM channel center are annotated: 
http://img208.imageshack.us/img208/5922/fmspectrum.png

The RDS (Radio Data System) or RDBS in the US digital data channel is 
easily seen at 57 KHz,   The level of all these subcarriers is much 
lower than the stereo pilot so you'll need a strong signal for decoding.

73, Bob W9RAN




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