Echo Return Loss - Polycom SoundStructure C16 Design Manual

While the delay in the network will increase the perception of echoes, it does not affect the performance of
the acoustic echo canceller in either the local or remote rooms. The acoustic echo canceller is concerned
only with the audio as it enters the room and reflected in the room and picked up again by the local
microphones. It doesn't matter to the echo canceller whether the signal from the remote talkers takes 1
millisecond or 10 hours to be received - the local echo canceller is only concerned with the audio once it
reaches the acoustic echo canceller and picked up by the local microphones. It is only the perception of
residual echoes that is influenced by the transmission delay. For instance an echo that is heard with 5 msec
of delay will not be objectionable at all, while the same echo heard with 500 msec delay will render
interactive communications impossible.

Echo Return Loss

An echo canceller's performance is measured by how well it can reduce the echo signal that is present at
the microphone. The natural reduction of the echo signal due to the physical separation of the loudspeakers
from the microphones is commonly referred to as echo return loss (ERL) and is measured in dB. As shown
in the following figure ERL is the ratio of 10 Log(A/B) where A is the signal that is sent to the loudspeaker
amplifier, and B is the signal picked up by the microphone. It is common for echo cancellers to have a
minimum required ERL for proper operation - exceeding the required ERL will slow or prevent the acoustic
echo canceller from properly converging. Typical values for ERL are 0 - 10 dB with 0 dB characterizing a
more robust echo canceller than one that requires 10 dB of ERL for proper operation.
While the ERL reduces the amount of echo present at the microphones, there will still be a significant
amount of echo that will be sent to the remote site if there is no further processing on the signal. The
enhancement of the echo return loss due to the presence of an acoustic echo canceller is referred to as the
echo return loss enhancement or ERLE. In the following figure, the ERLE 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.
Polycom, Inc. 727