[aprssig] Help needed: APRS receive with RTL_SDR and Direwolf

Greg D ko6th.greg at gmail.com
Sat Aug 17 22:50:25 EDT 2019

Thanks, Andrew.  Any clues would be helpful.

The link times out, however.  No response.  Did he/she link to another
document somewhere with the info?

I've been poking at it some more, and part of the problem appears that
the option '-E offset 0' is causing the audio output stream to be muted
somehow on the RPi.  But not forcing the offset means that I'm not on
the right frequency, as it appears to be picking up a nonsense offset
from somewhere / sometime else.  I can fudge that with some reverse
engineering, but direwolf still doesn't decode anything.

Capturing the output from the frequency-fudged rtl_fm into a file and
playing it on my PC does yield a packet signal, but it's badly
distorted.  Need to find some additional controls.  So, making some slow

Greg  KO6TH

spam8mybrain wrote:
> Hi, Greg.
> One of the users of my software pulled this stunt to use an RTL_SDR
> dongle. See if this (with appropriate adjustments) works for you.
> https://blog.pwarren.id.au/tech/2014/02/11/APRS-to-YAAC.html
> I make no guarantees; I haven't tried it myself yet.
> Andrew, KA2DDO
> author of YAAC
> -------- Original message --------
> From: Greg D <ko6th.greg at gmail.com>
> Date: 8/12/19 00:42 (GMT-05:00)
> To: TAPR APRS Mailing List <aprssig at lists.tapr.org>
> Subject: [aprssig] Help needed: APRS receive with RTL_SDR and Direwolf
> Hi folks,
> I'm trying to set up a "quick" receive iGate with a Raspberry Pi, an
> RTL_SDR dongle, and Direwolf.  No joy as yet.  However, when I run the
> SDR dongle as a server with rtl_tcp and aim GQRX at it with its internal
> packet decoder tool, that decodes just fine.  Direwolf also works fine
> with a USB sound card and a cell phone mike near my HT, so I'm guessing
> the issue is with rtl_fm.  What setting have I bungled or missed in
> doing this?
> The run string I'm using is this, or variations on the theme (I've tried
> a bunch of different stuff, without luck).
> $ rtl_fm -f 144.39M -M nbfm -E offset 0 -g 11 -p 60 -r 24000 - |
> direwolf -c direwolf.sdr.conf -r 24000 -p -D 1 -
> Here's what it displays.  Note the error from line 155 also is seen when
> running Direwolf from a sound card, and that decodes just fine, so I
> don't think it's a problem with the Direwolf config.  And, yes, it is an
> old version of Direwolf, but for reasons too convoluted to go into here,
> that's what I've got.
> Dire Wolf version 1.0a
> Line 155: Missing tone frequency for space.
> Audio input device for receive: stdin
> Audio out device for transmit: null
> Found 1 device(s):
>   0:  Generic, RTL2832U, SN: 77771111153705700
> Using device 0: Generic RTL2832U
> Channel 0: 1200 baud, AFSK 1200 & 2200 Hz, A, 24000 sample rate.
> Note: PTT not configured for channel 0.
> Virtual KISS TNC is available on /dev/pts/0
> WARNING - Dire Wolf will hang eventually if nothing is reading from it.
> Created symlink /tmp/kisstnc -> /dev/pts/0
> Ready to accept KISS client application on port 8001 ...
> Ready to accept AGW client application on port 8000 ...
> Found Rafael Micro R820T tuner
> Tuner gain set to 12.50 dB.
> Tuner error set to 60 ppm.
> WARNING: Failed to set offset tuning.
> Tuned to 144390000 Hz.
> Oversampling input by: 42x.
> Oversampling output by: 1x.
> Buffer size: 8.13ms
> Exact sample rate is: 1008000.009613 Hz
> Sampling at 1008000 S/s.
> Output at 24000 Hz.
> Now connected to IGate server noam.aprs2.net (
> Check server status here
> [ig] # aprsc 2.1.4-g408ed49<0x0d><0x0a>
> [ig] # logresp KO6TH-8 verified, server T2BC<0x0d><0x0a>
> [ig] KO6TH-8>APDW10:!3857.53N\12106.64W&
> and nothing after that...  Even with my TH-D74 in the same room, on EL
> power so as to not crush the input.
> _______________________________________________
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> aprssig at lists.tapr.org
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