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[Discuss-gnuradio] RDS demodulation

From: mleech
Subject: [Discuss-gnuradio] RDS demodulation
Date: Tue, 21 Feb 2012 10:35:32 -0500
User-agent: Roundcube Webmail/0.5.1

I'm having trouble demodulating the RDS signal that comes along with a lot of broadcast FM signals these days.  I know that there's "gr-rds" that has already done this, but I wanted to do my own just for personal education.

I spent a lot of time yesterday playing with it, and I can't get anything but garbage bits out of the DBPSK demodulator in Gnu Radio.  I can't find a freely-available version of the RDS standard, so I've been cribbing from the nearly-impenetrable gr-rds implementation.

What I've done is to carve off a few kHz around 57kHz out of the FM demodulator, downconvert to zero Hz, and then resample to exactly 19.0kHz/8, and feed that into a "Digital" DBPSK block, with symbols-per-sample=4.  I get nothing but rubbish out the back side of this, even for a strong local signal.

Is there something I'm missing in the way the signal gets modulated?



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