Also Chirp. I wrote an error correcting modem for them. Getting a simplex packet from A to B is the easy part. But any kind of bidirectional protocol (say for f

Also Chirp. I wrote an error correcting modem for them. Getting a simplex packet from A to B is the easy part. But any kind of bidirectional protocol (say for flow control) falls apart in an acoustic medium because duplex operations are thwarted by echo and phase. It's a long time since I played with the idea but I reckon you could do a lot more now with phones that have mic arrays and enough CPU power to do more serious DSP.