Broadcast FM subcarrier decoding?

Robert Nickels ranickels at gmail.com
Mon Mar 18 15:31:16 UTC 2013


On 3/17/2013 11:32 AM, Robert Nickels wrote:
> 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 
in addition to the prominent 19 KHz stereo pilot.   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

Another subcarrier is used for RDS (Radio Data System) or RDBS in the US 
digital data channel which can easily seen at 57 KHz on stations 
utilizing this service.  In the US The arithmetic sum of all multiplex 
subcarriers may not exceed 20% modulation and different rules apply than 
for the primary broadcast channel.

73, Bob W9RAN




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