Whistles can be a fun way to control a system, the DSP needed to do this is really simple, and whistles really punch through the noise floor like nothing else!

Whistles can be a fun way to control a system, the DSP needed to do this is really simple, and whistles really punch through the noise floor like nothing else! I've always used discrete change in pitch as the "symbol", so commands are encoded as (for example) UP UP DOWN UP, so this only depends on the relative pitch change between short whistles.