My friend sent my yet another mysterious recording... He told me he was inspired by PicoCTF 2019 and made his own transmissions. I've looked at it, and it seems to be really compact and efficient. Only 31 bps!!
PSK31 is a highly-efficient data mode that lets you work long distances, even when you can barely hear the signal. PSK31 stands for Phase Shift Keying 31 baud (or 31 bits per second/bps). Unlike RTTY (radio teletype) the characters are formed by changing the phase of the sound wave, not by using different tones.
sudo add-apt-repository ppa:kamalmostafa/fldigi sudo apt-get update sudo apt-get install fldigi