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Figure 4: Echo Cancellation. The adaptive filter is first trained by passing white noise through the telephone channel, ensuring the far-end signal b(k)=0 , and thus performing an adaptive system identification on the echo path. After adaption at time if the error e(k) =0 then the adaptation is switched off and the signal at the earpiece is essentially : e(k) = Echo - PseudoEcho + b(k) ~= b(k) and the echo has been cancelled. Clearly the echo canceller has to be of sufficient impulse response to model the echo path, and the adaption time must be short enough to be an acceptable to the callers - these are the decisions of the DSP engineer. This type of echo control is also used in V32 modems where the microphones are replaced by binary data sources.