rotocols that govern how a communications device uses a local area network (LAN). The IEEE's 802 standards 802.3 through 802.12 specify access methods for LANs and MANs. acoustic echo canceller An AEC is used to eliminate return echoes in an acoustically-coupled tele-meeting. AEC's are used in full-duplex audio arrangements in which all microphones are active at all times. This situation causes an increase in ambient noise that an AEC is designed to mediate. acoustics The qualities of an enclosed space that define how sound is transmitted, its clarity and how the original signal will be distorted. active video lines The lines that convey information in a television signal, e.g., all except for those that occur in the horizontal and vertical blanking intervals. additive color Direct light that is visible directly from the source: the sun, light bulbs, video monitors. The wavelengths of direct light can be viewed in three primary colors, red, greed and blue (RGB). Combinations of these three frequencies (for that is what colors are) result in most perceivable color variations. additive primaries By definition, three primary colors result when light is viewed directly as opposed to being reflected: red, green and blue (RGB). According to the tri-stimulus theory of color perception, blending some mixture of these three lights can adequately approximate all other colors. This theory is harnessed in color television and video communications. addressable The ability of a device to receive communications over a network whereby the unique destination of the device can be specified. Typically, an address is a set of numbers (such as a telephone number) that allows a message to be intercepted and interpreted for Glossary of Terms purposes of an application. ADPCM CCITT Recommendation G.721. Adaptive Differential Pulse Code Modulation. A technique for converting an analog signal into digital form. It is based on standard sampling at 8 kHz and generates a 32 Kbps output signal. ADPCM was extended in G.726, which replaces both