called in this application) is used to distribute the output of multiple video appliances simultaneously to every monitor or every room throughout a building. In a typical installation, all the video sources are located on an equipment rack, and are connected as inputs to the matrix switch. Where central control of the matrix is practical, a typical rack-mount matrix switch offers front-panel buttons to allow manual connection of inputs to outputs. An example of such a usage might be a sports bar, where numerous programs are displayed simultaneously. Ordinarily, a sports bar would install a separate desk top box for each display for which independent control is desired. The matrix switch enables the operator to route signals at will, so that only enough set top boxes are needed to cover the total number of unique programs to be viewed, while making it easier to control sound from any program in the overall sound system. Such switches are used in high-end home theater applications. Video sources typically shared include set-top receivers or DVD changers; the same concept applies to audio. The outputs are wired to televisions in individual rooms. The matrix switch is controlled via an Ethernet or RS-232 connection by a whole-house automation controller, such as those made by AMX, Crestron, or Control4, which provides the user interface that enables the user in each room to select which appliance to watch. The actual user interface varies by system brand, and might include a combination of on-screen menus, touch-screens, and handheld remote controls. The system is necessary to enable the user to select the program they wish to watch from the same room they will watch it from, otherwise it would be necessary for them to walk to the equipment rack. The special crossbar switches used in distributing satellite TV signals are called multiswitches. Implementations Historically, a crossbar switch consisted of metal bars associated with each input and output, together with some means of controlling movable contacts at each cross-point. The first switches used metal pins or plugs to bridge a vertical and horizontal bar. In the later part of the 20th century, the use of mechanical crossbar switches declined and the term described any rectangular array of switches in general. Modern crossbar switches are usually implemented with semiconductor technology. An important emerging class of optical crossbars is implemented with microelectromechanical systems (MEMS) technology. Mechanical A type of mid-19th-century telegraph exchange