Avaya BCM Echo Cancelation Overview

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BCM Technical Tip

Release Date: 2011/05/13
Region: GLOBAL

BCM Echo Cancelation – Overview and Limitations

Purpose of this bulletin

The purpose of this bulletin is to describe how the echo cancellation works for IP sets in the BCM
product.

The document provides a description of the functional components of the echo canceller and
describes how those components interact together to minimize any echo that may occur during
conversation

Finally the bulletin identifies any call scenarios where echo may still occur even when the
canceller circuitry is in operation.

How does the echo canceller work?

TDM sets do not experience muting during double talk because they are not connected to echo
cancellation circuitry in the BCM. Such connections are not required because the TDM set is
directly connected to the BCM and no codecs are required for voice transmission. The TDM sets
in the attached diagram are connected by a time switch multiplexor directly to the PRI trunk
circuit. Any echo which may be introduced as part of the PRI connection is heard as side tone by
the TDM set since the connection is a direct one.

The nature of the VoIP protocol makes IP sets much more susceptible to echo than regular digital
sets. The CODECs which must be used (G.711, G.723, G.729 etc) have specific payload sizes
and as such introduce potential transmission delays which can manifest as echo. As such when
an IP set makes a call on a BCM echo cancellation circuitry is part of the connection by design.

The ECAN circuit has 2 major parts

The adaptive filter component
The NLP (non linear processor) component.

Adaptive Filter

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The adaptive filter can dynamically identify echo and subtract it from the receive path. The
process of identifying the echo and determining how much echo to remove is called "training" or
“convergence.” The training process has certain requirements in order to be successful.

When the adaptive filter is fully trained it can eliminate most but not the entire echo in the receive
direction.

Looking at the "Echo cancellation: Success case" slide, the red triangles in the transmit direction
represent the signal going out to the network. Some of that may be reflected back in the receive
path as echo. (The few red triangles seen in the receive signal of most blue triangles.) The
adaptive filter looks at the transmit signal and compares it to the receive signal it gets from the
network. The circuit is able to identify possible echo and minimize it. The resulting signals are
then sent to the NLP. Echo is still present but is much reduced as evidenced by the much smaller
red triangles.

The signal is sent to the NLP which passes it through to the codec and on to the IP set. The
difference between the received signal and the echo artifacts is now so small it is not discernible
to the caller.

Here are the requirements for successful training of the adaptive filter.

The adaptive filter can only successfully train in "single talk mode." In other words, where the

calling party from the BCM is speaking (transmit) and when there is no speech from the
called party (receive). This would be typical in a normal conversation. If the receive side is
transmitting a constant signal (such as a radio broadcast or music on hold as was used in
some of the lab tests) the adaptive filter cannot identify the echo component of the transmit

side and therefore cannot train properly.
Training is done on a per call basis.
It can take up to a maximum of 10 seconds to train the filter. If double talk occurs during the

training period the training period will be prolonged. The echo canceller algorithm constantly

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