Rattlegram is an app i've used with Ham for some time to send audio based messages over analog audio. It also works well in air-gapped noisy rooms too, due to t

Rattlegram is an app i've used with Ham for some time to send audio based messages over analog audio. It also works well in air-gapped noisy rooms too, due to the excellent efficiency of COFDM (coded orthogonal frequency division multiplexing). Of course not many situations are actually like that, so more of a fun experiment, but the Ham use is actually pretty useful.