DIY computer radio

FM radio from your computer. It is kind of built-in to the rasppi GPIO clock output. Proper antenna design.

Networking over audio, or even low-bandwidth radio, is a whole fascinating enthusiast community on the internet. Airchat is a low-bandwidth acoustic chat wossname targeting FM-type broadcasts. Initial reviews were negative. fldigi is possibly the leading generic data transfer tool here. Typical modes are not fast; you might be able to type faster. soundmodem is another suite of audio transmission tools; the maintainer has vanished. An example of usage shows what can happen here. Packet radio is built on soundmodem.