Acoustic echo cancellation removes the far party's voice — played out of your loudspeaker and picked up again by your microphone — from your outgoing signal, so they don't hear themselves echoing back. It works by taking the loudspeaker signal as a reference and using an adaptive filter to model the room's echo path, then subtracting that estimate from the mic input. The hard parts are double-talk (both people speaking at once, where the canceller must not chew into the near voice) and nonlinear or drifting echo from cheap speakers, which a residual-echo suppressor mops up. AEC is the first stage of every conferencing pipeline; without it, speakerphone calls become howling feedback loops.
