this protocol is used by some radioamateurs but is based on 1980s military comms. Normally it is transmitted via a HF radio link on lower or upper side bands, it only needs about 1 500 Hz audio bandwidth (half that of a telephone circuit).
I do not have a valid TX license so I recorded the audio on an old cassette recorder with its internal microphone
then played it back to another netbook using the internal microphone, with my hi fi loudspeakers playing trance loud in the background (to create deliberate interference noise on top of the signal). I was impressed that it still worked; and it appears this protocol could even be used for sending data as it can do the full ITA5 alphabet with upper and lower case…
