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    By: Richard BaraniukAs a part of collection:"Adaptive Filters"

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Adaptive Echo Cancellation

Module by: Douglas L. Jones. E-mail the author

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An adaptive echo canceller is a specific type of adaptive interference canceller that removes echos. Adaptive echo cancellers are found in all modern telephone systems.

Figure 1
Figure 1 (fig1AdaptiveEcho.png)
The hybrid is supposed to split the opposite-going waves, but typically achieves only about 15dB of suppression. This signal will eventually reach the other end and be coupled back, with a long delay, to the original source, which gives a very annoying echo.
Figure 2
Echo canceller
Echo canceller (fig2AdaptiveEcho.png)
Because the input to the adaptive echo canceller contains only the signal from the far end that will echo off of the hybrid, it cancels the echo while passing the near-end signal as desired.

Narrowband interference canceller

Figure 3
Figure 3 (fig3AdaptiveEcho.png)

A sinusoid is predictable ΔΔ samples ahead, whereas s k s k may not be, so the sinusoid can be cancelled using the adaptive system in the Figure. This is another special case of the adaptive interference canceller in which the noise reference input is a delayed version of the primary (signal plus noise) input. Note that ΔΔ must be large enough so that s k s k and s k - Δ s k - Δ are uncorrelated, or some of the signal will be cancelled as well!

Exercise 1

How would you construct an "adaptive line enhancer" that preserves the sinusoids but cancels the uncorrelated noise?

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