RDS demodulation

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?

-Marcus

exactly 19.0kHz/8?
Are you feeding it the stereo pilot or generating your own?

Have you tried looking at the signal with an FFT scope to see if there
is anything there or if it looks distorted or something?

I’m not feeding it the stereo pilot. I just use a fractional
resampler to resample the few kHz at 19.0kHz/8 --this gives 4
samples/symbol into the DBPSK block.

The spectrum looks roughly
correct–a double-humped structure either side of zero.

On Tue, 21 Feb
2012 12:27:54 -0500, Andrew D. wrote:

exactly 19.0kHz/8?
Are
you feeding it the stereo pilot or generating your own?

Have you
tried looking at the signal with an FFT scope to see if there
is
anything there or if it looks distorted or something?

On Tue, Feb
21, 2012 at 10:35 AM, wrote:

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? -Marcus
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If I remember correctly, the original RDS feed the stereo pilot to the
DBPSK block as it had the correct symbol phase. From this
https://www.cgran.org/raw-attachment/wiki/RDS/rds.png it looks just
like band passing the pilot and dividing to the correct rate.

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