Volume Control; Aec Troubleshooting Guidelines - Polycom SoundStructure C16 Design Manual

If the microphones are muted in the signal chain before the acoustic echo canceller, then while the
microphones are muted, the AEC will not be able to adapt as there will be no signal present. Once the
microphones are unmuted, the AEC may have to reconverge to any new echo paths. This may result in a
momentary acoustic echo to the remote site until the AEC reconverges.

Volume Control

As shown in following figure, if the gain on the loudspeaker amplifier is increased, the echo return loss will
decrease meaning that there will be a more stronger echo at the input of the microphones. This means that
the echo canceller will need to work harder to remove the echo. If the amplifier is turned up too much
(sometimes by as little as 6 or 10 dB), the acoustic echo canceller will not be able to operate properly. This
may result in persistent residual echo under low echo return loss (high echo) situations.
To properly adjust the volume of local room, one should adjust the sources feeding into the echo canceller
(the far in signals) or if that is not practical, at least ensure that the echo canceller reference is also adjusted
as the loudspeaker level is adjusted to match the adjustments of the amplifier. Under these conditions the
ERL will remain relatively fixed, allowing the echo canceller to continue operating properly.
Local Room
A
B C

AEC Troubleshooting Guidelines

In the event that a configured conferencing system has acoustic echoes that are heard by the remote
participants, the most important step in troubleshooting is to mute signal paths and determine when and if
the echo goes away. If the echo goes away when the local microphones are muted, then the local echo
canceller is causing the echo issue.
The most common reason for acoustic echo is that the echo return loss of the room is not high enough to
allow the acoustic echo canceller to properly adapt to the remote audio. This is usually solved by reviewing
the gain structure and turning down the amplifier and bringing up the signals that make up the echo
canceller reference.
The next most common source of echoes is that the echo canceller reference does not contain all the
remote audio sources, allowing one or more remote audio sources to be interpreted as local speech by the
echo canceller and consequently sent to the remote participants.
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