How to Choose an Acoustic Echo Canceller Application Note Polycom Installed Voice Business Group September 2004
Introduction Acoustic echo cancellers (AEC) greatly enhance the audio quality of a multipoint hands-free communications system. They allow conferences to progress more smoothly and naturally, keep the participants more comfortable, and prevent listener fatigue. An AEC solution that is poorly designed or inappropriate for the location will not provide these benefits and can even degrade audio quality significantly.
Near End Far End Near speech XMT Nonlinear Processing Near speech + echoes + distortion Mic Distortion Echoes AEC RCV Far speech Speaker Far speech Figure 1: Illustration of the effects of AEC operation and room acoustics on the transmitted speech. The far speech that travels through the receive path is not modified as it passes through the AEC.
frequently, if at all. With acoustic echo cancellation, the echo path is very complex (dozens or hundreds of reflections), lasts 100-200 ms, and can vary continuously during a conversation as people move around the room. Acoustic echo cancellers are therefore much more complicated devices. While line echo cancellers may have smaller price tags, they can’t perform under the conditions that acoustic echo cancellers can handle.
Form Factor The form factor of the solution is very important because it determines how useful it is in an application. The performance of the product may not matter if its form factor makes it inconvenient or impossible to use in the desired application. Of course, there can usually be some flexibility in choosing a form factor.
Step 2: Eliminate the products that don’t meet G.167 or the tail length needs of the application. It is relatively easy to determine how well an AEC cancels echoes. Most AEC products are based on the same algorithm: the adaptive LMS digital filter. This is a very well-defined algorithm that has been used for years. Since this process is well established, it is fairly easy to determine whether a manufacturer has done an adequate job of implementing it.
Room Acoustics G.167 testing is performed in real rooms. If the product meets the Because of differences in room requirements in these rooms, it is compliant. A device that is acoustics, a device may be G.167 G.167 compliant in one room, however, might not be compliant in compliant in one room and not another. This is because the acoustics of all rooms are different. another! This flexibility allows manufacturers to test their products in the types of rooms they were designed for and claim compliance.
The equation assumes that both the microphone and the speaker are mounted on the same wall (which is the worst case in terms of the number of reflections that will be cancelled). In that case, N must be an odd integer because the even reflections travel away from the microphone. For example, consider a 10x20x30 foot conference room with very reflective surfaces that requires 5 echoes to be cancelled. In such a room, a tail length of 6 * 30 / 1125 = 160 ms would be needed.
Step 3: Judge audio quality and state machine performance by comparative listening. Ideally, the AEC will let speech signals pass through it unharmed. This is actually the most difficult task in both designing and measuring the performance of an acoustic echo canceller. The real difficulty is in determining how the AEC sounds during double-talk, and whether it actually harms the sound quality by inaccurately determining whether it is in a double-talk state.
The two most critical states of the echo canceller are receive and double-talk. The receive state is the only opportunity for the echo canceller to converge correctly. It is also the time when the echoes are most noticeable because they are not masked by speech from the other side. During the receive state, the echo canceller must converge rapidly and apply nonlinear processing to further reduce the echo. If the state machine does not detect a receive state correctly, echoes remain audible.
acoustics have such a large impact. If this cannot be arranged, at least consider the operating environment differences in each case in the final decision. If possible, listen to the room acoustics with the echo canceller disabled so the effects of the different rooms can be compared. Seven Things to Listen For • • • • • • • Residual Echo. If there is excessive residual echo, the sound may have a hollow, distant quality or there may even be distinct audible echoes.
A Short Glossary of AEC Related Terms Near end The end of the connection where the echo canceller is located. This is where the echoes take place. Also, see far end. Far end The end of the connection opposite the echo canceller. The user on this end could be using a handset or another hands-free system. This is where the performance of the echo canceller are most noticeable. Also, see near end. Double-talk The state of the echo canceller when there is speech on both ends of the connection.
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