Optimizing Teleconference Audio System Designs
Have you designed a large system for video or audio teleconferencing that looked straightforward and simple on paper, but turned out to be a configuration nightmare after it was wired up? Or perhaps you've even had to re-design the entire system to make it work properly? If you answered yes to either of these questions, help is on the way.
With that background, we can create a mix-minus loudspeaker zone feed. DSP 1 uses its matrix to feed mics 9-24 from the linked audio bus to loudspeaker Zones A-D. DSP 1 also uses its matrix to directly feed mics 3-8 to Zone A, mics 1, 2 and 5-8 to Zone B, mics 1-4, 7 and 8 to Zone C, and mics 1-6 to Zone D. Each loudspeaker zone fed from DSP 1 now contains mic audio from all zones except its own. The discussion for DSP Units 2 and 3 is identical.Tip #4: For optimum echo cancellation results, avoid routing far-end audio through dynamic feedback controllers.
Dynamic feedback controllers are sometimes used when local speech reinforcement is required during an audio or video teleconference. Special consideration of audio paths is needed to achieve the best echo cancellation performance.
Fig. 6 on page 49 shows a common design using a dynamic feedback controller, which degrades echo cancellation performance in a conferencing situation. This is because echo cancellers compare audio returning from a room with the original reference mix. This comparison identifies what the room is doing to the referenced audio (far-end audio) in terms of acoustic absorption, delay, etc. The echo cancellers then make needed adjustments to adapt to changing room conditions. From the echo canceller's point of view, a dynamic feedback controller makes the room appear to be changing more than it really is, which degrades echo cancellation performance because the echo canceller is trying to adapt to a false acoustic “picture” of the room.
Fig. 7 on page 49 shows an optimized design for use of dynamic feedback controllers, which separates the signal paths for optimal echo cancellation performance. Audio from local microphones is routed to the dynamic feedback controller and then fed to the loudspeaker zone. This is normal for use with local reinforcement. The difference is that the audio received from the far end (and local program audio) doesn't pass through the dynamic feedback controller on its way to the loudspeakers. Therefore, the echo cancellers aren't presented with a false acoustic picture of the room's effect on far-end audio, which allows echo cancellers to converge more accurately and rapidly.
(Note: Fig. 7 illustrates use of dynamic feedback controllers with a single loudspeaker zone. When using multiple zones in a conferencing environment, each zone must have its own dynamic feedback controller.)Tip #5: Improve echo cancellation results by using an optimized process flow between a microphone and the mixer.
Drag-and-drop architectures are appealing because of their flexibility. However, with increased flexibility comes increased opportunity to make mistakes in process flows, especially when using echo cancellers required for conferencing applications.
Fig. 8 illustrates an optimal echo cancellation process flow for a mic input channel in a conferencing environment. The echo-canceller is strategically placed for best performance.
The first stage, gain, is straightforward, but notice that automatic gain control (AGC) functions aren't implemented here. Placing an AGC function prior to the echo canceller and noise canceller degrades their performance by giving a false view of the room.
The acoustic echo canceller (AEC) is next. It must be placed as close to the room as possible to accurately respond to real changes in room conditions.
Following the echo canceller is the noise canceller (NC). Like the AGC function, if this were placed prior to the echo canceller, the echo canceller would see a false acoustic picture of the room.