Acoustic Echo Cancellation (Aec) - Extron electronics DMP 128 User Manual

Acoustic Echo Cancellation (AEC)

The DMP 128 C models provide one acoustic echo canceller processor for each of the
first eight mic/line inputs. A single reference can be selected for each AEC from a list of the
twelve line inputs.
About AEC
Echo occurs when audio from a talker in the far end is received and amplified into the near
end listener's room, with that sound then being picked up by microphones in the near end
acoustic space and sent back to the far end. The amount of signal sent back to the far end
talker can be substantial, and with the added transmission delay, the result is an echo effect
that seriously compromises communication in a teleconference or videoconference.
The Acoustic Echo Cancellation processor removes the potential echo signal at the near
end mic channel by comparing it to the received signal from the far end, designated as the
"reference," and then creating an adaptive filter to cancel the potential echo before it is sent
back to the far end.
AEC Setup
Successful operation of the AEC processing block is a function of proper gain structure and
selection of the AEC reference (see
provides an overview of the two elements.
Proper gain structure involves the relationship between the signal at the selected reference
and the signal at the mic input, within the context of proper levels for the reference and mic
inputs independently. The mic input gain setting is naturally optimized for the voice level of
the talker in that room; therefore the amount of signal from the far end that is picked up by
the mic is dependent on how much that far end signal is being amplified in the near end
room and the distance from the mic to the speakers.
The reference signal is the signal received from the far end, which is ultimately sent to a
sound reinforcement system within the near end room. The output of the video codec might
be connected to any of inputs 9 through 12.
In the AEC dialog, a reference can be chosen from any channel in one of three signal chains:
• Input Channels
• Virtual Return Channels
• Output Channels
Extron recommends using an input channel as the reference. An output channel or a virtual
channel can also be used as a reference; however, doing so adds a little more delay to the
signal being referenced.
Using an output or virtual channel reference allows for the combining of input channels to a
single reference, for example, in a conferencing setup where both a telephone and a video
codec are used. In this case, both the telephone and video codec input channels can be
routed to an output or virtual return, with that output or virtual return then chosen as the
reference.
When using an output channel as a reference, the reference point is post volume control;
therefore, changes to the listening volume in the room affects the AEC gain structure (see
AEC Dialog, below). If you have an output channel on the DMP 128 that is not being used,
you can isolate the reference channel from the channel being used for volume control by
routing reference signals to the unused output channel.
Optimizing Audio Levels
DMP 128 • Software Control
on page 101). This section
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