I'm doing a puzzle that gets me to call a phone number which produces a load of beeping sounds. Putting this into audacity as a spectrogram produces this:


Zoomed in it looks like this:


This looks like on/off keying. The problem is that I don't know how to take this binary signal and convert it into actual binary (other than doing it by hand but yikes.)

So far GNU Radio seems to be promising but I'm not to sure on how to achieve what I'm looking for with it.

If someone could point me towards some info on where to get started with this that'd be a great help.

edit - Here's the



Zoomed in

zoomed in

Zoomed out

zoomed out

The overall recording is also low quality - the original is taken from calling a phone number which transmits this sound - getting a high quality recording of that call would also prove helpful.

https://drive.google.com/file/d/1l2RaZiVVYkF_SXvRsTH8lsvnq3GYKEWQ/view?usp=sharing - google drive link to the wav file

  • 4
    $\begingroup$ That looks like binary FSK to me. Also: you're likely to get more help if you can narrow down your question. Right now, you're either asking for a one-semester course on digital communications, or for someone to do most of the work for you. $\endgroup$
    – MBaz
    Commented Sep 23, 2021 at 0:20
  • $\begingroup$ @MBaz I see 4-5 frequencies in the first plot? $\endgroup$
    – endolith
    Commented Sep 23, 2021 at 12:19
  • $\begingroup$ I'd agree, this is FSK, but not binary FSK; in any case, it clearly isn't on-off-keying. (the signal is never "off") $\endgroup$ Commented Sep 15, 2023 at 13:02

2 Answers 2


What you are looking for is a format called SSTV, you can recognize it usually by its distinctive tone at the start, like in your audio file. I haven't seen people decode it manually, in my (very limited) experience, but some programs you can use to decode are MMSSTV, and there's also a CLI tool, if that's your thing. MMSSTV isn't the most easy to use so I'll give you a quick guide. Take your .wav file, and open it in some audio editing program, like audacity. Change the "project rate" (in audacity) to 11025. Save as .wav and then change the file extension to .mmv. Lastly, open MMSSTV and click File > Play sound from the file... and then open your .mmv file. then you image will be drawn on the screen and you're done.

  • $\begingroup$ Would you care to add some more information as to what SSTV is, how it could possibly help, and what is it that makes it better compared to other methods (such as an M-ary hypothesis test, filter matching, and other decision-making algorithms)? The way the answer is written right now provides no information about a solution apart from a link to some algorithm in GitHub. $\endgroup$
    – ZaellixA
    Commented Sep 15, 2023 at 10:56
  • $\begingroup$ thiu really seems completely unrelated to the problem at hand, sorry :( $\endgroup$ Commented Sep 15, 2023 at 14:16

Ask yourself this question: what happens if you multiply a sine wave with another sine wave sample-by-sample? If you now sum the result over the period of one of them what do you get? How does this change if the two sine waves have the same frequency?

  • $\begingroup$ I am not a Signal Processing expert to be able to infer the meaning of your question, but to me it doesn't seem like this is an answer to the question posted here. Would you care to elaborate and add some more information? $\endgroup$
    – ZaellixA
    Commented Jun 21, 2022 at 9:52

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.