Polycom SoundStructure C16 Design Manual page 724

addition, echoes can overload communication circuits, resulting in a feedback condition called howling. If,
by the combination of loudspeaker volume and microphone sensitivity and positioning, the echoes are
louder than the originating speech, the teleconferencing equipment, or the network itself, can overload.
An acoustic echo canceller in the local room will remove the echo of the remote talkers' voice so it will not
be sent back to the remote talkers. Just as an acoustic echo canceller is used in the local room to prevent
echoes to be sent to the remote participants, the remote site would have a similar audio conferencing
solution to prevent the local talker's audio from echoing back from the remote room as shown in the following
figure.
Local Room
The sophisticated part of an acoustic echo canceller consist of an adaptive filter that is nearly continuously
adapting to adjust to changes in the echoes in the room caused by changes in the local room such as
participants moving around. The following figure illustrates the block diagram of an AEC system and shows
the adaptive filter at its center. It is common to refer to the signals associated with an echo canceller as
follows: the Near In signal consists of the local microphone audio (both local talker and the echo of the
remote talker), the Near Out is the signal that will be played into the local loudspeaker so that the local
participants can hear the remote participants. The Far In is the signal sent from the remote side and Far Out
is the local talker audio with the acoustic echo-removed that is sent to the remote participants.
Near Out
Near In
The performance of the AEC is often improved with the addition of non-linear processing (sometimes
referred to as center-clipping) applied after the adaptive filter on the Far Out signal. The non-linear
processing will remove low-level artifacts that result from imperfections of the adaptive filter, that is, when it
is not completely adapted. Acoustic echo cancellers typically will have a user adjustable control for the
amount of non-linear processing, allowing the processing to be more aggressive (larger amount of
suppression) or less aggressive (less suppression). More aggressive suppression will reduce the
occurrence of residual acoustic echoes to the remote site, but possibly at the expense of introducing some
clipping of the first syllable of local talkers audio during transitions from the remote people talking to the local
people talking. Aggressive suppression may also vary the level of the transmit audio signal based on
whether both the local and remote talkers are speaking at the same time. With less aggressive suppression
Polycom, Inc.
AEC
Filter
Remote Room
Remote Talker
Far In
Far Out
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