VIDEOPHONE

also called Video Telephone, device that simultaneously transmits and receives both audio and video signals over telephone lines. Such a device consists of a telephone; a display unit with a television picture tube, camera tube, and loudspeaker; a control unit with a microphone; and associated circuitry. The utilization of television as an adjunct to telephone service has been considered since the earliest demonstration of television itself. The first practical televisiontelephone service was established in 1936 by the German Post Office. Limited public service was provided over coaxial cables between Berlin, Leipzig, Nrnburg, and Hamburg until 1940. The American Telephone and Telegraph Company developed a modern videophone system in the late 1950s and began providing intercity service between Chicago, New York, and Washington, D.C., on a test basis in 1964. During the early 1970s this service, known as Picturephone, was offered to regular subscribers of the Bell System in the same cities. A similar system acquired more widespread use in Japan. High cost has limited the use of videophones. One of the main difficulties is the expense of transmitting video signals, which requires a minimum bandwidth of one megahertz. This is equivalent to the demands of more than 300 regular long-distance telephone calls. The development of new techniques of reducing transmission bandwidth and the use of improved fibre-optic telephone cables (assemblies of thin glass rods) may help to make videophone service more economical.