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I'm trying to demodulate an AFSK signal. My device uses 1200 baud rate and 1200/600Hz as mark/space. I tried using almost every software for demodulation, but nothing worked. Most recently, I tried QTMM AFSK1200 Decoder.

Here is my signal plot and the audio recording. I received this signal in FM mode with my radio which is connected through usb port with the PC.

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  • $\begingroup$ In order for us to be able to help you, we need more info about the problems you are having and your setup. $\endgroup$
    – W8AWT
    Mar 12, 2015 at 15:15
  • $\begingroup$ @KD8NXH Your edit broke the links. You have to include the references to the URL-footnotes, not just square brackets. Also, when improving links, please link the actual descriptive text (noun phrase), not generic “here” or “click here”. $\endgroup$
    – Kevin Reid AG6YO ♦
    Mar 14, 2015 at 1:16
  • $\begingroup$ AFSK 1200baud with 1200/600Hz can be anything. What you also need to know is the protocol. Are you sure what you are trying to decode is HDLC with AX.25 framing???? $\endgroup$
    – user4479
    Apr 2, 2015 at 17:14

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That does not sound like 1200 baud AFSK, which is probably why you can't demodulate it with your software.

Here is 1200 baud AFSK as used in amateur radio:

Note the first of these is compressed (.ogg), so probably can not be demodulated. The 2nd, however, is not compressed, so you can feed it to your software as a test.

Notice how these have a narrower frequency range. They sound vaguely vocal. The squawk at the start of each packet, which is used for synchronization, is also very distinctive. The squawk consists of an alternation between the two tones at 1200 baud.

Your sample has a much wider frequency range. It has a "windy" quality to it. There's no audibly discernible squawk at the the beginning.

KB9UKD has many more samples of other digital modulations.

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