(or four cameras in a single site). Most often, the receiving codec "stacks" these discrete images on a single monitor, where each occupies one quadrant (in Hollywood Squares fashion). Video Communications: The Whole Picture Figure 13-7. Workgroup Application (courtesy of Tandberg). Continuous presence also works well in point-to-point applications in which a large group is seated around a table. In situations like this, a single camera cannot capture everyone in one image, SO two are used. One camera is focused on people on one side of the table; a second is aimed at those on the other side. Again, the two images are stacked for transmission and offered on a split screen basis (where one-half of the table is displayed on the top half of the monitor and the other is displayed below) or in a de-stack mode where two monitors are used to display the images. Continuous-presence requires double or quadruple the transmission bandwidth of single image applications. Hence, it works best at 384 Kbps and above (with 512 Kbps being the preferred minimum). OPERATOR'S INTERFACE The system operator's interface, also called the control unit, is critical because many people who would benefit most from video communication are not interested in learning how to use it. A group system control unit should be menu-driven and intuitive. Most allow the operator to dial calls using speed-call, switch between room and document cameras, activate the camera's pan, tilt and zoom features, control Group Systems audio including volume and muting, and access peripherals such as slides projection systems, fax machines, and VCRs. Some systems allow the operator to move cameras at the remote site. This feature is called far-end camera control and is valuable when people on the distant end have not been trained to use the system. Historically, a desktop-mounted device, the control unit now takes many forms. In the desktop category, there are control units that emulate a telephone, those that use an electronic template and stylus, and those that rel