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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.

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.

By using these basic connection and process flow ideas, you can simplify general routing requirements in the room, make it easier to manage echo cancellation references, and get the best system echo cancellation performance when using dynamic feedback controllers in a conferencing environment. The following five design tips will help you optimize multi-unit audio DSP systems for video and audio teleconferencing applications using the internal audio data bus more efficiently.

Tip #1: Simplify echo cancellation reference management by connecting all far-end audio sources and local program audio to the same physical DSP unit.

Echo cancellation, the enabling technology for teleconferencing, is simply the process of preventing unwanted audio from passing through a microphone channel to the far end. For example, we want audio from talkers in the local room to pass to the far end, but we don't want that same audio coming from the far end to return to the far end because that's the “echo” we want to cancel. We also don't usually want local program audio to pass to the far end through an open microphone channel. Instead, it should be routed directly from the source to the far end.

To achieve echo cancellation, we must create a mix of the audio we don't want to pass to the far end. We call this the “reference mix” because echo cancellers on the microphone channels use it as a reference to create a signal with equal amplitude but opposite phased voltage. This signal is then applied at the right time to audio coming from the microphone to cancel out the audio that was in the reference mix (far end and local media). Audio not in the reference mix (local talkers' audio) is allowed to pass to the far end. (Important note: For best echo cancellation results, audio used in a reference mix should be post-process audio. In other words, the cancellation reference mix should be a sample of the signal being sent to the power amps.)

Fig. 1 illustrates a common audio flow design using multiple linked DSP units. Two specific inefficiencies exist in this design. First, it uses more audio paths than are necessary to create and distribute echo cancellation references. Second, it requires too many audio paths to create the mix-minus audio feeds for the far-end interfaces (telephone interface and video codec) and to route received far-end audio and transmitted local audio. These inefficiencies can lead to integration problems because audio paths are a finite resource. When used up, they require additional external wiring to make the system work properly.

Fig. 1 shows a system layout that is not optimized for cancellation reference management purposes because, again, it uses up unnecessary audio paths — a finite resource — to get all the needed cancellation reference sources together. In this design, audio from the telephone interface is connected to a different DSP box than the audio from the video codec, while program audio (non-speech audio sources that are also heard in the same room, such as a CD, VCR, TV, or DVD) is connected to a third DSP box.

If we choose to create the reference mix on the DSP box connected to the telephone interface, and if we choose to keep all audio separated, we tie up four busses just to move the codec L and R audio and the program L and R audio to the DSP with the telephone interface. Then we need to use another bus to pass the reference mix among the linked DSP units. We've now tied up five busses to create and distribute the reference mix. There's got to be a better way, right?

By doing a simple rearrangement of the physical connections, we can easily create a correct reference mix using only one bus instead of five.

Fig. 2 shows an audio flow design that is optimized for echo cancellation purposes. The far-end audio devices (telephone interface and video codec) and the local program audio are connected to the same physical DSP unit. This allows the reference mix to be created in that DSP unit. The reference mix is then placed on a single bus for distribution to all other linked DSP units.

Tip #2: Simplify management of audio feeds to the far end by connecting audio inputs and outputs of video codecs and telephone interfaces to the same DSP unit.

We can simplify feeds to far-end devices and external program switchers by arranging physical connections as illustrated in Fig. 3 on page 46. We don't have to jump onto any of the linked DSP units' busses to move local program audio to the far end or to create our mix-minus for the telephone interface/video codec cross-feeds. These can all be done on the DSP unit connected to these devices, without using linked audio busses.



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