Avaya BCM Echo Cancelation Overview

Brand: Avaya

Model: BCM Echo Cancelation

Category: PC/workstation barebones

Type: Overview

COMPAS ID: 152113

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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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monitors the call quality and will try to adjust its parameters automatically if echo is detected

during the call.

The adaptive filter can lose its training if the transmit level is excessive resulting in distortion

in the received echo or if there are clock slips in the PRI circuit. (Note: it has been

recommended that the customer sites ensure that the loss plan setting for the PRI circuit be

changed from "ISDN" from "PSTN" since the transmit level is lower.) This could also result if

the calling party is speaking very loudly into the phone.

NLP (Non Linear Processor)

The NLP looks at the ratio of receive signal to transmit signal, and decides if muting of the receive

path should occur. Under normal circumstances the NLP would only mute the receive circuit

when the caller is speaking but the destination is not. This removes any residual echo from the

circuit for the calling party.

When both parties are speaking at the same time and the adaptive filter is fully trained the NLP

will pass the signal to the codec and on to the IP set since the amount of echo in the signal is

minimal.

The NLP can switch additional loss into the path to eliminate any residual echo. If the timing of

the echo directly overlaps the normal receive signal a small amount of echo may find its way to

the calling party (overlap of red and blue triangles in the diagram) but it is inaudible to the caller

due to the much louder receive signal. See Figure 1.

However, if the adaptive filter is unable to train for some reason, then no echo cancellation occurs

and all echo is sent to the NLP. The NLP determines that the necessary rations are exceeded

and mutes the circuit.

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If the echo (the red triangles) is level is similar to the actual receive signal in amplitude, echo

cancellation becomes especially difficult to achieve without also muting the receive signal.

The NLP can be turned off through Element Manager but one is strongly advised not to do so

since the calling party will be subjected to full echo while speaking.

Double Talk Scenarios

In BCM 5.0 and BCM 6.0, Avaya has preformed some changes to the ECAN behaviour during

Double Talk situations, i.e. when both sides of the calls talk at the same time.

In previous BCM releases, the NLP algorithm has been very aggressive in muting in Double Talk

situations since it was detecting echo.

Avaya has changed the ECAN behaviour to be less aggressive with muting when Double Talk is

detected. Since muting action is taken based on the amount of echo present in the call, adjusting

the algorithm to mute less, and results in the possibility of echo during double talk situations.

Conclusion: ECAN capabilities and limitations

With the design of the echo canceller some echo can be expected during the following situations.

During the initial training period at the beginning of a call. 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. Callers are advised not to interrupt each other during

conversation during the convergence period thus causing doubletalk.

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The adaptive filter can lose its training if the transmit level is excessive resulting in

distortion in the received echo or if there are clock slips in the PRI circuit. In other words,

it is much harder for the echo canceller to detect and cancel echo when a voice signal is

loud or distorted. The distorted signal causes nonlinearity effects on echo canceller and

degrades its functionality.

Callers are advised to maintain a reasonable volume during conversations. If the

environment is noisy, then callers should make efforts to shield their phones from the

background noise and do not use the hands-free option. .

Since the NLP is less aggressive about muting during double talk, some echo is likely

when both people speak at the same time. Callers should avoid speaking at the same

time.

There is a 32 millisecond echo path delay design limitation of the ECAN circuitry. Echoes

which have a path delay of greater than will not be suppressed by the ECAN circuitry.

Users experiencing echo with a delay of more than 32 milliseconds should contact a

network assessment to determine the cause of the delay to the voice path.

The interface level setting for the PRI circuitry can have an influence on perceived echo.

Two settings are possible: ISDN or PSTN. Choosing the higher level on a site which has

echo will exacerbate any echo which is present.

Issued by:

Avaya SSD Tier 4 Support

Internet: http://www.avaya.com

Avaya BCM Echo Cancelation Overview

Brand: Avaya

Model: BCM Echo Cancelation

Category: PC/workstation barebones

Type: Overview

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Avaya BCM Echo Cancelation Overview

Avaya BCM Echo Cancelation Overview

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