Real time implementations utilize finite impulse responses (FIR) to cancel out far end echo signals. Though adequate in most cases, there is always a non-zero residual echo, which may be substantial in certain applications. Consider the systems depicted in Figure 1 below:
Figure 1: Single line AEC architecture
The received signals are of the form:
A typical normalized energy profile of a room impulse response is shown in Figure 2 showing that just about of the energy is contained in the first 20 milliseconds (~256 samples) . Thus, using an echo tail of 20m milliseconds will leave some substantial residual echo.
Figure 2: Sample room impulse response energy profile
The residual echo can be considered as a stationary interference with spectral domain cancellations being employed to reduce it.
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