Iirc there was some small algorithm, or perhaps even a piece of hardware, that should trigger when music is playing so that the phone isn't active all the time.

Iirc there was some small algorithm, or perhaps even a piece of hardware, that should trigger when music is playing so that the phone isn't active all the time. From there, I guess they could use any old detection algorithm; for me, the magic was in this super-energy-efficient bit of the chain, though I never read up on the details (if they ever provided any)