nt end. When mute is activated, audio is removed from the information being transmitted to far-end sites (it is not generally removed from a taped recording of the proceedings). Figure 13-5. Video Phone (courtesy of Tandberg). Modern audio systems for video communication should include full-duplex digital audio, echo cancellation (with ability to turn this feature off), voice-activated automatic gain control, automatic noise suppression, an audio mixer, integrated tonal speaker test, real-time audio level meter for local and far-end microphones, microphone and VCR input audio mixing with VCR talk-over. Group Systems Echo Cancellation Echoes have always posed a sound quality problem in conferencing applications. Echo elimination is critically important in true roll-about applications because one cannot depend upon room treatment alone to address the problem. It is important to understand how a given system eliminates echoes. There are two methods: echo suppression and echo cancellation. Echo suppression attenuates microphones and loudspeakers on an alternate basis to prevent feedback. However, it causes clipping. Echo cancellation provides for smooth, natural exchanges by modeling an echo and subtracting it from an incoming signal. Echo cancellation systems must train themselves to room and network environments to operate effectively. Some units emit a burst of white noise to profile the room (sometimes called whooshing the room), which can be annoying to distant-end participants. Others use the voices of the meeting participants to train acoustic line- side echo cancellers. Some units automatically adapt to changing room and network conditions without losing stability. Still others become unstable when the unit is moved or when an object is sitting too close to the microphone. Increases or decreases in speaker volume may degrade performance, SO it is good for prospective buyers to experiment. When comparing echo cancellation equipment, it is important to compare convergence time, tail length, and acoustic echo re