re (constant bandwidth is assigned no matter how much video is transmitted). Evaluating audio quality under different circumstances enables one to learn what works best. It is revealing to listen closely to audio quality during scene changes (e.g., someone moves around) and to compare that to the quality delivered when there is little or no motion. Group Systems Audio Channels All group-oriented systems offer at least two channels (stereo sound). Additional channels are now common as well. Some provide a center channel; others provide up to five channels (known as surround-sound). When a center channel is added to a group system video communication unit it tends to anchor the sound field. As meeting participants move around the room, the audio portion of the conference appears to remain fixed. When only two channels are used (left and right speaker), it is possible to detect which speaker is producing which sounds by moving between them. A central loudspeaker also provides better frequency response matching across the stereo sound field and helps to match the action between the picture and the associated sound. When only two channels are used, the prominent an angular deviation between the sound and its picture can be detected by some listeners. Microphones A microphone is a transducer that converts sound into an electrical signal. Microphones vary in how they manage the ratio between signal and noise, how they prevent distortion, and how they use companding (noise-reduction circuitry) to improve signal-to-noise ratios. Of course, there are many different types and quality levels available. Cutting corners with inexpensive microphones is not wise. Microphones do not comprise a big part of the overall project cost, but good ones can make an enormous difference in the perceived quality of a system. Suppliers and consultants can be helpful in identifying the ideal number and types of microphones for a given installation. Some microphones are built into the system control unit while others offer flexibility by being 