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Re: [Discuss-gnuradio] RDS reception

From: mleech
Subject: Re: [Discuss-gnuradio] RDS reception
Date: Fri, 24 Feb 2012 12:02:11 -0500
User-agent: Roundcube Webmail/0.5.1

Actually, I'm using the blks2 WBFM block, which I believe is a PLL demodulator.

Both of my "test" stations are actually roughly 100km away from me, broadcasting from a high point in the Ottawa Valley.

One station  comes in about 10dB stronger than the other, although both have an ERP of about 100kW, and both are located on the same transmit tower.

I'll send you my 'stuff' once I'm more comfortable with it.  The audio quality is actually reasonably good, so I think the FM demodulator portion is just fine.



On Fri, 24 Feb 2012 11:12:28 -0500, Andrew Davis wrote:

I think the problems with SNR is with the FM Demod, my car stereo can
get RDS as the station fades 40 miles away, but with my USRP has
trouble picking up the stereo pilot from a 50kw station one mile away
( -10dBm ). FM demod is hard for software ( I think GNUradio just uses
a zero-crossing detector ).

Do you have the code, I would love to test it.

On Thu, Feb 23, 2012 at 11:33 PM, Marcus D. Leech <address@hidden> wrote:
I managed to get gr-rds built tonight, and was able to test it planted inside my FM receiver. Even for a local station, there are *lots* of errors, and more distant stations get even more.  I'm going to play around a bit with filter bandwidths, etc, to see if I can improve it. Looking at the modulation technique, it's no big surprise--they use a bi-phase baseband coding  (basically like differential Manchester), that drives a DSB-SC modulator with a carrier at  57kHz.  But that signal is generally about 20dB below the other audio sub-channels on the  FM demodulator output. So the SNR is generally *terrible*. I'm rather surprised, after looking at it, that anyone bother deploying RDS. It seems utterly  crappy.  It's generally broadcast with fairly-low SNR, and there's no FEC built in, only CRC. I re-jigged the XML for the demodulator part so you can reset the demodulator state machine  from within a GRC flow-graph (there was a reset() method on the demod, but it wasn't exposed  within the XML framework for GRC). Also, I may re-jig the decoder portion--it currently injects decoded frames into a message queue and  to stdout.   Might be useful to do other things with those messages. Like is there a simple way to  have a message dequeuer within a GRC flow-graph so that one could stuff messages into a variable? -- Principal Investigator Shirleys Bay Radio Astronomy Consortium http://www.sbrac.org _______________________________________________ Discuss-gnuradio mailing list address@hidden https://lists.gnu.org/mailman/listinfo/discuss-gnuradio


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