turn loss enhancement (AERLE) ratings. Convergence time is the measure of how long it takes a canceller to model the acoustic characteristics of the room in which it is installed. The time, measured in milliseconds, should be as short as possible (less than 70 ms) because echoes will sneak through until the canceller completes its model of reality. Tail length is measured in milliseconds and corresponds to how long an echo canceller will pause after an original signal's echo has been modeled to wait for the return echo. The longer the tail length, the less acoustic treatment a room will require. AERLE, measured in decibels (dB), describes the maximum echo cancellation produced by the canceller. The higher the dB rating, the better equipped the system is to remove echo. Typical values range between 6 and 18 dB. Evaluators should disable the variable attenuator, or center clipper, for accurate measurement CAMERAS Any group systems should feature one or more color cameras. Cameras should provide about 65° field of view, about 25° tilt (up/down) range, about 100° pan (left/right) range, 265° field of view, 12x zoom (f-4.2 to 42mm), auto-focus, automatic white balance, presets, and far-end camera control. Nearly all are based on integrated circuits called charge-coupled devices (CCDs). A CCD camera focuses a scene sample not on an imaging tube (as in older cameras) but on a solid-state chip. A CCD is much smaller than an imaging tube. A one-megapixel CCD camera module, Video Communications: The Whole Picture including full camera functions and a mechanical shutter, measures about 7 mm, and uses pixels that measure less than 3 X 3 microns. The lens directs photons to strike the surface of the CCD chip, which is divided into individual elements (rather than the continuous photosensitive surface of an imaging tube). The surface of the CCD resembles a matrix of individual elements which, in turn, define the camera's spatial resolution capability. Each unique element samples the light that strikes it, converts that light t