Acoustic Echo Cancellation

Will Fraser

Acoustic Echo Cancellation

What is acoustic echo and why do I need to cancel it? Acoustic echo occurs in a conferencing system when the far-side speech played in the loudspeakers is picked up by microphones in the room and is transmitted back to the far side. This transmitted signal is a delayed version of the original, which causes the echo.

The received far-side signal does not transfer directly from the speaker to the microphone, but is subject to the artifacts of the room. This may include differing signal paths causing reverb, frequency filtering and attenuation. These effects are the transfer function of the room. The transfer function of the room is also dynamic, as objects in the room move or the microphone moves position.

To correctly subtract the required signal, the AEC therefore needs to simulate the dynamic room transfer function. It can then apply that transfer function to the received signal and correctly subtract the modified original signal.

Each Soundweb London AEC card consists of 4 AEC input channels.

Each channel offers the following features:
•Independent 20Hz - 8kHz algorithm
•Individual AEC references
•Automatic Gain Control (AGC)
•Noise Cancellation (NC)
•Adaptive (Speech Passing) Non-Linear Processing (NLP)
•Extremely fast convergence rates of 49dB/s

NOTE: AEC Input cards can only be used in BLU-800, BLU-320, BLU-160 or BLU-120 devices configured for 48kHz operation.

AEC Definitions
Before we continue let’s review the definitions for some common terms used when discussing AEC.

Convergence Rate
Measures the speed of the linear processing component of the AEC algorithm and does not include the non-linear processing or suppression (NLP) as dictated by industry standards. This means this is a measure of how fast the algorithm can recognize and remove echo from the signal path.

Double Talk
Both far and near side speech are present.

Echo Return Loss (ERL)
This is a measure of the coupling between the AEC reference signal and the AEC input signal.

Echo Return Loss Enhancement (ERLE)
This shows the loss through the linear AEC algorithm (not including the non-linear processing.)

Far Side (Reference)
This is the remote side of the conference which will be heard from the near-side speakers.

Gain Structure
Proper gain structure will provide an adequate signal to noise ratio and reasonable headroom for an input signal.

Near Side (Local)
This is the local side of the conference where the echo canceller is located.

Non-Linear Processing (NLP)
The non-linear processing increases the power of the echo cancellation for difficult acoustic environments.

Noise Cancelation (NC)
Noise cancellation removes ambient noise from the AEC signal (e.g. computer fan noise).

Voice Activity Detection (VAD)
Detects whether the audio is speech or silence/background noise.

More....

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