 for example, by an electron beam in a cathode ray tube or by ultra-violet radiation in a fluorescent lamp. photo diode A basic element that responds to light energy in a solid-state imaging system. It generates an electric current that is proportional to the intensity of the light falling on it. picture-in-picture PIP. A video display mode in which a small video image is superimposed on a quadrant or smaller area of a video screen. This small image or "window" can be opened in order to display a second video input on a monitor. This is a particularly valuable feature in a single monitor videoconferencing system; it allows the near-end to view themselves in a small window while simultaneously seeing the far end. Picturephone AT&T's video-telephone that was introduced at the 1964 World's Fair in Flushing Meadow, New York. The device incorporated a camera that was mounted on top of a 5.25" X 4.75" screen. Since audio signals were transmitted separately from video signals, the system required a transmission bandwidth of 6.3 Mbps and, therefore, could not use the PSTN. It never became popular. pixel The smallest element of a raster display. A picture cell with specific color and/or brightness. Point of presence. Typically the closest location of a carrier to a subscriber and the point at which that subscriber will be supplied with service from that carrier. A multi-wire input/output to a computer or other electronic component. The number of ports determines the number of simultaneous users, the number of peripheral devices one can simultaneously connect with, or a combination of both. Plain Old Telephone Service. Analog narrowband telecommunications service designed for transmitting voice calls. prediction A type of motion compensation in which a compressed frame of video is reconstructed by a receiving codec by Glossary of Terms comparing it to a preceding frame of video and making assumptions about what it should be. Presence Ability to identify the location of a communication recipient and the type of device that 