Playing a bit with the RTL-SDR stick I h ... / 2013-05-27

2013-05-27 Playing a bit with the RTL-SDR stick I h ... 6 years ago
Playing a bit with the RTL-SDR stick I hoped to decode APRS traffic. APRS is Automatic Packet Reporting System, a standard for radio amateurs to broadcast digital messages reporting location, status, weather and lots of other things. Nowadays a lot of traffic is location information, but there is a lot more. Information is transmitted at 1200 bps AFSK Frequency shift keying - Wikipedia so it can easily be handled by equipment for FM phone transmissions.

Looking around for options for decoding APRS traffic with gnuradio I ran into SDR with Raspberry Pi and DVB-T receivers - JPMeijers.com where it was described to skip a lot of steps and use the rtl_fm utility from the rtl-sdr source and multimon to do the decoding.

Reception of the signal via the NOXON DAB+ USB dongle is very noisy but playing with gain settings helped in eventually getting something decoded. Signal via the Wouxun transciever sounds a lot better. So maybe an audio cable from the Wouxun to a sound-card is in order to help with decoding this and other digital signals.
$ rtl_fm -f 144800000 -s 22050 -o 4 -g 100 -C - | tee >(nc -l 8080) | multimon -a AFSK1200 -t raw /dev/stdin 
multimod  (C) 1996/1997 by Tom Sailer HB9JNX/AE4WA
available demodulators: POCSAG512 POCSAG1200 POCSAG2400 EAS AFSK1200 AFSK2400 AFSK2400_2 HAPN4800 FSK9600 DTMF ZVEI SCOPE
Enabled demodulators: AFSK1200
Found 1 device(s):
  0:  NOXON, DAB Stick, SN: 0

Using device 0: Terratec NOXON DAB/DAB+ USB dongle (rev 1)
Found Fitipower FC0013 tuner
Oversampling input by: 12x.
Oversampling output by: 4x.
Buffer size: 7.74ms
Tuned to 145064600 Hz.
Sampling at 1058400 Hz.
Output at 22050 Hz.
Exact sample rate is: 1058400.010094 Hz
Tuner gain set to 100.00 dB.
AFSK1200: fm PE1REO-9 to APERXQ-0 via PI1APU-0 UI  pid=F0
!5203.76N/00504.56E>095/052Info QRZ.COM
And indeed: PE1REO shows up on aprs.fi.

The tee command is to check audio with
$ nc thompson 8080 | play -t raw -r 22050 -e signed-integer -b 16 -c 1 -V1 -
The -g parameter in rtl_fm controls the gain, I had to search for a value which gave me a reasonable signal, at -g 100 about half of the databursts were decoded. It helps to hear the audio that rtl_fm is decoding to get an idea of reception.

It is truly amazing that this all can be done in software, with really minimal hardware.

My general idea is to be able to decode other digital amateur radio signals such as the telemetry from amateur radio balloon flights.

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