Causes Of Perceived Echo; Double Talk; Background Noise - Mitel MiVoice Business Voice Quality And Troubleshooting Manual

Voice Quality Troubleshooting Guide
Echo Cancellers are Unidirectional
Another important point to note about echo cancellers is that they are unidirectional. They
cancel echo in only one direction. In most scenarios, cancelling echo in one direction is all that
is needed. However, in some special cases, it may be desirable to cancel reflections in both
directions. In such a scenario, two echo cancellers would be needed, one for each direction.
Non-Linear Processing – NLP
Non-linear processing (NLP) is used by echo cancellers to suppress any residual echo that the
adaptive filter does not cancel. As the name suggests, the NLP does not use linear processing
(which the adaptive filter uses). The adaptive filter used in an echo canceller cannot usually
fully cancel the echo, because there are usually some minor non-linearities in the echo path.
Some low-level residual echo almost always remains. The NLP suppresses or blocks the
remaining echo. There is a trade-off between using more or less aggressive NLP. When NLP
is less aggressive, it may fail to remove echo if a large amount of echo remains after the adaptive
echo cancellation filter. This issue may occur when there are large non-linearities, or when the
adaptive filter does not accurately mirror the echo in the signal. When NLP is too aggressive,
it can result in choppy audio if two parties are talking at the same time (double talk). The goal
is to prevent both echo and choppy audio, but sometimes, the solution is to establish a balance
between residual echo and choppiness.
Echo Cancellers in the Mitel Communications Platform
Echo cancellers are used in
• IP phones to cancel acoustic echo
• Line cards and analog option boards to cancel electrical (line) echo.

CAUSES OF PERCEIVED ECHO

There are several conditions that can cause users to believe that they are hearing echo on a call.

DOUBLE TALK

Double talk occurs when both parties in a conversion talk at the same time. During double talk,
the NLP (echo suppressor) is disabled. This is necessary to allow the other speaker to be heard.
However, this also means that residual echo is not blocked. If the echo canceller filters are
properly adapted, problems will not occur. However, double talk makes it difficult for the filters
to maintain proper adaptation and sometimes it can cause the echo canceller to lose adaptation.
In this case, the echo canceller quickly re-adapts when double talk is removed.
The type of double talk that occurs during a normal conversation should not degrade voice
quality. However, it is possible to create echo, clipping, or a combination of the two, if users
continuously talk over each other in an effort to stress the echo canceller. Echo that is artificially
generated in this fashion should not be considered a voice quality issue.

BACKGROUND NOISE

There have been instances where a very loud background noise has caused people to believe
that they are experiencing echo; for example, in rooms with noisy machinery, call centers with
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