Polycom C16 - Manual (Page 478)

Design Guide for the Polycom SoundStructure C16, C12, C8, and SR12
B - 16
would be the ratio of 10 Log (B/C) which, due to the acoustic echo canceller,
should be a larger number than the ERL. Typical values for ERLE are 15 - 25
dB.
The non-linear suppression employed by acoustic echo cancellers is usually
not included in the ERLE measurement as the suppression will make the ERLE
appear much higher.
For some manufacturers of echo cancellers, the term ERL is replaced with an
equivalent term of conferencing room gain. The conferencing room gain is the
negative of the ERL, or defined as 10 Log (B/A). Lower room gain
(equivalently higher ERL) in this case will improve the performance of the
echo canceller.
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
AEC
Lo
c
al R
o
om
Local Room
Am
p
B
A
C