I've recently started looking into APRS, and can easily receive some APRS signals from my station - however, I can't seem to decode them using QTMM or any similar software.

Is this because they're too weak / noisy, or am I doing something else wrong? I presume it's picking up an APRS envelope but failing a checksum or something similar - if there's any way to just get partial packet data I'd be interested in that as well.

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    $\begingroup$ How clean is your audio setup? Is it possible the audio gain is too high? Try recording some music radio, or a clean sounding ham station, through your sound card, to WAV. Use Audacity or similar. Play it back and see how it sounds. Even the loudest sounds should not be distorted. $\endgroup$ – tomnexus Aug 8 '15 at 14:46
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    $\begingroup$ @tomnexus First thing I tried, first with a clean sounding APRS signal, then with some music, finally with a range of tones of different frequencies and amplitudes. All were fine (somewhat unfortunately, that would be an easy fix!) $\endgroup$ – berry120 Aug 8 '15 at 16:56
  • $\begingroup$ The audio file is no longer available. Is it possible that the OP either solved his problem or moved the file to another location in drive which would have broken the share reference. Could the OP please update? $\endgroup$ – Rowan Hawkins Mar 10 '16 at 4:00
  • $\begingroup$ Afraid I don't have the original clip, but have updated the answer. $\endgroup$ – berry120 Mar 10 '16 at 14:45
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    $\begingroup$ Thanks! I just moved the comments and all's neatly cleaned up. $\endgroup$ – Kevin Reid AG6YO Mar 10 '16 at 16:14

I never did manage to decode the signal reliably, which I believe to be a combination of two factors - the radio I was using was attenuating the upper frequency somewhat, and there was some additional interference in the area. APRS also uses AX25 which is hardly the most resilient protocol - at 1200baud, you get a single sine wave oscillation for each bit, so if a slight bit of noise gets in there it's easy to see how the packet can be unreadable - there's not much room for error on the physical layer. FWIW, I've also later read that QTMM isn't the best decoding tool on the block - I didn't try many others since QTMM was so simple and appeared to work for audio clips I found online, but that could be the next port of call if I were to revisit the project.


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