Polycom OMNIVSINT500, OMNIVS500, OMNIVS500U User Manual

How to Choose an Acoustic Echo
Canceller

Application Note
Polycom Installed Voice

Business Group

September 2004

Introduction

Acoustic echo cancellers (AEC) greatly enhance the audio quality of a multipoint hands-free
communications system. They allow conferences to progress more smoothly and naturally, keep the
participants more comfortable, and prevent listener fatigue. An AEC solution that is poorly designed or
inappropriate for the location will not provide these benefits and can even degrade audio quality
significantly. There are many AEC solutions available, ranging in price from several dollars to several
thousands of dollars. There is also a broad range in quality and performance. This paper outlines some
guidelines for determining the performance needed in a given location, judging the performance of an AEC
solution based on specifications and listening tests, and ultimately finding the best AEC solution for any
application. Presented first, however, are guidelines for verifying that an acoustic echo canceller is
necessary in an application.

Why AEC?

Acoustic echo cancellation is useful in any hands-free telecommunications situation involving two or more
locations. Acoustic echo is most noticeable (and annoying) when delay is present in the transmission path.
This would happen primarily in long distance circuits, or systems utilizing speech compression (such as in
videoconferencing or digital cellular phones). Even though this echo might not be as annoying when there
is no delay (such as with short links between conference rooms in the same building or distance learning
over fiber-optic cable), room acoustics will still affect the sound and may hamper communication. Also,
howling can occur if the microphone is positioned too close to the speaker whether or not there is
transmission delay, and is eliminated by most acoustic echo cancellers.

Acoustic echo cancellers can be used in both narrow band (3.5 kHz) and wide-band (7 kHz) conferencing
systems. Narrow band applications include teleconferencing and low bit-rate video conferencing. Wide­band applications include high quality teleconferencing and videoconferencing, as well as distance
learning. Users of wide-band conferencing systems should be particularly interested in using acoustic echo
cancellation, as it would allow them to make the most out of the additional audio capabilities of their
systems.

The primary beneficiaries of an echo canceller are the people at the far (or remote) end of the transmission
path. The near (or local) echo canceller prevents the echo of the remote peoples’ voices from being
returned (i.e. echoed) to them through the audio system. People speaking on the same (local) end as the
AEC should not notice the AEC if it is doing its job properly. While the people on the far end receive the
benefit of better audio quality, it also enables the conversation to flow more smoothly, benefiting both
parties.

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Near speech

Near End

Far End

XMT

Nonlinear

Processing

Mic

Distortion

Echoes

Near speech +

echoes + distortion

AEC

RCV

Far speech

Speaker

Far speech

Figure 1: Illustration of the effects of AEC operation and room acoustics on the transmitted speech. The far speech that travels
through the receive path is not modified as it passes through the AEC. In an echo canceller that is poorly designed, there may be
residual echoes as well as distortion added to the near speech signal (these effects are described in detail later). This degrades
the speech that is transmitted, so that the poor audio quality is noticed on the far end.

Why not just use a speakerphone?

Speakerphones are half-duplex, which means that only one person can talk at a time. The speakerphone
determines which side is active (or louder) by comparing the signal levels on both sides. It turns off the
other side until the louder side is finished. Once one side has “captured” the circuit, most speakerphones
do not permit any sort of interruption. This inhibits the natural flow of conversation, especially if one party
is long-winded.

Acoustic Echo Cancellation vs. Line Echo Cancellation

Acoustic echo cancellation and line echo cancellation both address similar problems, and are often based
on the same technology. However, a line echo canceller generally cannot replace an acoustic echo
canceller, because acoustic echo cancellation is a more difficult problem. With line echo cancellation there
are generally only one or two reflections from
telephone hybrids or impedance mismatches in the
telephone line. These echoes are usually delayed
by less than 32 ms, and do not change very

With acoustic echo cancellation, the echo path
is complex and can vary continuously as people
move around the room.

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frequently, if at all. With acoustic echo cancellation, the echo path is very complex (dozens or hundreds of
reflections), lasts 100-200 ms, and can vary continuously during a conversation as people move around the
room. Acoustic echo cancellers are therefore much more complicated devices. While line echo cancellers
may have smaller price tags, they can’t perform under the conditions that acoustic echo cancellers can
handle.

Steps to Choosing An AEC

Now that the need for an acoustic echo canceller is recognized, the best AEC solution for the application
isdetermined based on the following four step selection process.

• Find AEC products with the features and form factor needed for the application. There may

be several acceptable form factors. Even if one seems particularly suited to the application, consider
all of them for a broad selection of price and performance.

• Eliminate products that don’t meet G.167 or the tail length requirements of the application.

Although these two factors are necessary, they are not sufficient. However, if an AEC solution does
not meet these requirements, it will most likely not sound very good at all, so don’t waste time
arranging listening tests. If it does meet these requirements, further testing and evaluation should be
done to ensure that it is appropriate for the application. When possible, find out the testing
environment as well as the results of the G.167 testing.

• Judge audio quality and state machine performance by comparative listening. A panel of

several people should listen to the different solutions (preferably the same people, under similar
conditions, during a short time span). They should listen for the common problems echo cancellers may
have, as well as overall quality.

• Choose the best solution. Weigh the performance, price, and convenience of each solution, and

choose the one that will work best in the application.

Step 1: Find AEC solutions with the features and form factor needed for the application.

Features

Certain features may be desirable for certain applications. For example, wide-bandwidth may be a
necessity for videoconferencing or high quality audio conferencing systems. For integrated systems, the
number and quality of microphones and speakers will be an issue. Automatic control of microphone and
speaker levels may be desirable. A graphical user interface (perhaps through a connection to a Windows
machine) may be needed. These kinds of features are too varied to be discussed in detail in this paper, but
will certainly be a consideration in the selection of an echo cancellation solution.

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