Hello,
Here is a bit of code that will receive SCA analog subcarriers of FM
Broadcast Stations using the USRP. It is a modified version of
usrp_wfm_rcv.py.
Common SCA frequencies are 67 kHz and 92 kHz. SCA is used for Reading
Services for the Blind, Background Music, Foreign Language Services, and
other services. Remember you may hear static when tuned to a FM station
because this code only outputs SCA audio.
The USRP gain is critical for good decoding. Adjust for minimum noise.
I use the Post FM Demod FFT to check for SCA subcarriers and to adjust
the USRP gain for the lowest noise floor. The stereo pilot at 19 KHz,
the stereo difference signal around 38 KHz, and RDS at 57 KHz are also
displayed on the Post FM Demod FFT if present.
The range below 67 kHz is used for SCA only when Stereo is not used.
The SCA recieve range is not as far as the main FM carrier receive range
so tune in strong local stations first.
I tried to comment the code with the various parameters. There seems to
be several choices for a couple of them. I coded the common ones I see
here.
In the local area there are a couple of stations using digital SCA.
These look similar to narrow DRM signals and I wonder if they are using
OFDM.
Have fun, I am!
73 Eric