Gebruiksaanwijzing /service van het product OMNIVS500U van de fabrikant Polycom
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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 fati gue.
AEC Nonlinear Processing Mic Speaker Echoes Near speech Far speech Far speech Near speech + echoes + distortion Far End Near End XMT RCV Distortion Figure 1: Illustration of the effects of AEC operation and room acoustics on the transmitted speech. The far speech that trav els 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 conti nuously during a conversation as people move around the room. Acoustic echo cancellers are therefore much more complicated devices.
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 fo rm factor makes it inconvenient or impossible to use in the desired application.
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.
Room Acoustics G.167 testing is performed in real rooms. If the product meets the requirements in these rooms, it is compliant. A device that is G.167 compliant in one room, however, might not be compliant in another. This is because the acoustics of all rooms are different.
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.
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 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.
acoustics have such a large impact. If this cannot be a rranged, 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.
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.
Technical Support For support on the Vortex product line, call toll-free (USA/Canada) 888-248-4143, then select option 1, then option 3. For exclusive Integrator and Consultant focused suppo rt (through our PASS program), dial 1.408.474.2048; this number will get you help on video and Vortex products.
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