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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
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