Double-talk is the hard case for echo cancellation: the moment when both the near and far parties speak at once. An echo canceller works by adapting a filter to model the echo of the far party's voice, but during double-talk the microphone also contains the near party's genuine speech — and if the canceller mistakes that for echo and adapts to remove it, the near voice gets chewed up or clipped. So cancellers include a double-talk detector that freezes adaptation when both are speaking, preserving the near voice while still suppressing echo. Handling double-talk gracefully, without dropouts or half-duplex 'walkie-talkie' behaviour, is one of the main marks of a good AEC.
