How does an echo canceller work?
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Conceptually, an echo canceller in a system keeps a copy of a part of an outgoing signal for a certain amount of time. This copy is updated as the outgoing signal leaves the system. An echo signal (similar to the original signal, yet lower in amplitude) may return to the system. If the echo signal returns to the system within a certain amount of time, it is correlated with the original signal that was transmitted and then removed if there is a significant enough correlation.
This was first published in October 2004