Multi Channel Vs. Single Channel Aec; Muting Microphones - Polycom SoundStructure C16 Design Manual

Multi Channel vs. Single Channel AEC

The highest quality audio conferencing solution occurs when each microphone has its own acoustic echo
canceller and noise canceller as shown on the right side of the following figure. It is clear that each
microphone will pick up different background noise levels and possibly types of noise based on their
proximity to different noise sources such as projectors, HVAC vents, lighting fixtures, etc. If each
microphone is processed independently, then only the noise that is affecting a particular microphone will be
removed from that microphone signal. This minimizes the likelihood of residual artifacts from the noise
processing on the underlying signal. If the microphones are first summed together and then the noise
processing is performed (as shown in the left side of the following figure), each local talkers' voice will be
processed by the same noise reduction algorithm to remove noise regardless of whether that noise was
incident on that particular microphone.
Similarly with respect to the acoustic echo canceller, it is also better to process the signals independently
as each microphone has its own acoustic view of the room and sees an acoustic echo that is different from
the other microphones. If multiple microphones are mixed into a single acoustic echo canceller, then as
each microphone becomes active with respect to its automatic microphone mixing algorithm, the acoustic
echo canceller must reconverge to the echo path from the microphone that becomes active. The
performance of the acoustic echo canceller will degrade as the number of open microphones that feed into
it is increased. Multiple microphones should only be fed into a single acoustic echo canceller if the echo
return loss is high such as in acoustically well treated room or when microphones are physically separated
from loudspeakers or their gating (becoming active or inactive) is closely controlled.
Similarly with respect to the acoustic echo canceller, it is also better to process the signals independently
as each microphone has its own acoustic view of the room and sees an acoustic echo that is different from
the other microphones. If multiple microphones are mixed into a single acoustic echo canceller, then as
each microphone becomes active with respect to its automatic microphone mixing algorithm, the acoustic
echo canceller must reconverge to the echo path from the microphone that becomes active. The
performance of the acoustic echo canceller will degrade as the number of open microphones that feed into
it is increased. Multiple microphones should only be fed into a single acoustic echo canceller if the echo
return loss is high such as in acoustically well treated room or when microphones are physically separated
from loudspeakers or their gating (becoming active or inactive) is closely controlled.

Muting Microphones

When muting microphones in an audio conferencing system, it is best to mute the microphones in the signal
chain after the AEC has processed the local microphone's audio signal. This allows the acoustic echo
canceller to continue to adapt to changes in the room, keeping the acoustic echo canceller converged even
though the microphone is "muted".
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