Scanning with rtl_fm

Alan Corey alancorey at
Sun Apr 28 15:43:44 UTC 2013

I'm using rtl_fm in a scanning (nbfm) situation, and even with the antenna input disconnected and terminated, I get a burst of noise that sounds like a squelch tail maybe once each scan cycle.  They're a little over 1 second apart.  I've tried setting the squelch up to 10000 where I don't get signals anymore, but I still get the noise.

I discovered that if you scan and have the output be a raw audio file, nothing gets recorded when nothing breaks squelch.  This makes time-lapse recordings where all the pauses are taken out.  Very nice, except for the noise bursts.

I'm using:
rtl_fm -N -S -l 132 -p 102 -f 854040000 -f 854240000 -f 854315000 -f 854415000 -f \
854490000 -f 854540000 -f 855165000 -f 855240000 -f 858790000 -f 867350000 \
-f 867715000 /tmp/`date +"%Y-%m-%d_%H%M.raw"`

and recorded overnight, but I mostly got noise bursts.


Radio Astronomy - the ultimate DX
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