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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.

Again, the advantages of connecting this way are fewer linked audio busses and simple management of the far-end device mix-minus cross-feeds.

Tip #3: Use the mix-minus feature of linked DSP audio busses to simplify mix-minus loudspeaker zone management.

Large audio and video teleconferencing spaces typically use local speech reinforcement to enable all participants to hear talkers in the same room.

Mix-minus feeds to loudspeaker zones are used to allow more gain before feedback in local speech reinforcement applications. For example, all microphones physically located under a specific overhead loudspeaker zone will be routed to all loudspeaker zones except the one directly above them. Therefore, local speech audio coming from the overhead loudspeakers of a specific zone contains a mix of all speech audio minus the audio from the microphones located in that loudspeaker zone.

Some DSP boxes have a mix-minus feature implemented on their linked audio busses. This is a very powerful feature, but users must understand that the mix-minus feature of linked DSP audio busses is not the same thing as the mix-minus feeds to loudspeaker zones. Mix-minus operation of linked audio busses is defined by the DSP unit's design, whereas mix-minus feeds to loudspeaker zones are created by the system designer using the DSP's matrix mixer/router. The following example illustrates how to use both of these mix-minus functions to simplify zoned speech reinforcement systems.

The layout in Fig. 4 is a basic mix-minus speech zone design, but it uses more linked audio busses than are necessary. In fact, it will require at least eight linked audio busses. To group microphones located in loudspeaker Zone E and send them to the first DSP unit will require the use of an audio bus. Similarly, mics located in loudspeaker Zone F will need to be grouped and placed on a separate audio bus. The same requirement exists for mics located in Zones G-L.

In Fig. 4, mics located in loudspeaker Zones A-D don't need to be placed on external busses because they're connected to the DSP unit feeding all the loudspeaker zones. They can be routed directly to the required zones through the matrix of the DSP unit they're connected to. This is the key to using the mix-minus property of linked DSP audio busses.

By using DSP boxes that have a mix-minus feature on their linked audio busses, a simple re-grouping provides the same end result with only one linked audio bus instead of eight (see Fig. 5). While not every design falls as neatly into place as this one, the same concept can be used to simplify most designs.

Fig. 5 illustrates optimized connections for a mix-minus operation. Note that microphones located in a specific loudspeaker zone are connected to the DSP unit that feeds the same loudspeaker zone. For example, mics 9 and 10 located in loudspeaker Zone E are now connected to the DSP that feeds the power amp for Zone E loudspeakers.

It's important to understand the basics of a linked DSP audio bus mix-minus operation before applying it to a mix-minus zoned loudspeaker system.

Assume that all three DSP units illustrated in Fig. 5 place all their mics (1-24) onto the same linked audio bus. Mix-minus bus operation means that each DSP unit sees all mics placed on the single bus except the mics it placed on that bus. In other words, any given DSP unit looking at a specific bus will see a mix of all the audio on that bus minus the audio it placed on that bus. DSP Unit 1 will see mics 9-24 but not mics 1-8. DSP Unit 2 will see mics 1-8 and 17-24 but not mics 9-16. DSP Unit 3 will see mics 1-16 but not 17-24.

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