rtl-sdr over multicast & multiple decoders

Leif Asbrink leif at sm5bsz.com
Mon Dec 31 01:17:55 UTC 2012

Hello James,

You might use Linrad:
Preferrable under Linux where performance is very good.

You can run a master instance of Linrad connected to the dongle
and set it to send fourier transforms (fft1) as UDP to the

You can then runn a large number of linrad slaves that pick 
up the data and produce the sound output of the channels of
your choice. You may use the squelch to make all the channels
quiet when there is no signal. You may route all the channels
to your loudspeaker and you can also save all the channels
as separate .wav files at i.e. 8 kHz sampling rate which means
16 kB/s per channel. In case you want to monitor 10 channels
that is 9.6 megabytes per hour. Normally you would throw those 
files away, but if something happens they might be interesting.
You can also save the raw data, but linrad would save 16 bit 
per sample which means 3.2*2*2=12.8 megabytes per second which
is 46 gigabytes per hour. Still, with a modern disk you can
save 24 hours (1.1 TB) in a single file. In case there was no
interesting event, just delete it. You can use Linrad to save
a small fraction of the big file just as if it were a signal 
from the antenna. 

Linrad is a bit different from other SDR software and many
people find it has a steeper learning curve. Depending on
who you are this may be more or less true. Linrad is quite
general and a bit old-fashioned. You do not have buttons for USB
CW or LSB. The user has to set a suitable bandwidth and place
the BFO at a suitable point outside the filter for those modes.
One can also place the BFO inside the filter or far outside for
other modes just as in radios from 1960 and earlier.


Leif / SM5BSZ

> I've been playing around with rtl-sdr as a way to stream trunking radio 
> and air band traffic.  I've done a bit of research and found people who 
> are using multiple dongles, one to digest the control channel and one or 
> more to tune and receive the voice channels as the talk groups come and 
> go.  They're also using multiple dongles and/or squelch scanning to 
> stream airband signals
> Around here, all of the channels of the trunking system as well as all 
> of the "interesting" fit inside the bandwidth of a dongle running at 
> 3.6Msps.  So, my question is this:
> Would it not be more efficient to have a single tuner dongle outputting 
> all of its samples over multicast (or into a shared memory segment for 
> that matter, if you want to stay on the same host) and then having a 
> bunch of decoder processes listening to the samples and decoding what 
> they need out of them?

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