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

  • 3
    $\begingroup$ 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
    Sep 23, 2021 at 0:20
  • $\begingroup$ @MBaz I see 4-5 frequencies in the first plot? $\endgroup$
    – endolith
    Sep 23, 2021 at 12:19

2 Answers 2


I know I'm posting this answer way after you made the question, but what you're looking for is SSTV, it's an old thing that outputs to an image, I used https://github.com/colaclanth/sstv to decode it from a .wav file format, and thanks for giving me a source for my own figuring this out ig.


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
    Jun 21, 2022 at 9:52

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