Video Telephone

a form of communication in which subscribers see as well as hear each other and can show illustrations, photographs, and texts and obtain graphic in-formation and data from a digital computer.

The video telephone apparatus consists of telephone apparatus, a video unit whose receiver contains a kinescope and whose transmitter contains a television tube (vidicon), and a control panel with operating control members. The video telephone apparatus is placed on a subscriber’s work-table or in a telephone booth. The images are transmitted over two communications channels, one for each direction. Dialing a subscriber’s number and talking with him is done in the same way as in telephone conversation. The image, as in television, is broken up into lines and frames. The number of lines used is between 200 and 400 (most frequently 220-280), and the number of frames is between 25 and 30 per second (with alternate-line scanning). The width of the frequency spectrum for the original signal varies between 200 and 1,200 kHz. The frequency band of the communications channel can be narrower at the cost of reduced physiological and statistical redundancy in the transmitted information.

The beginning of the development of video telephones dates to the 1930’s. Systems with different parameters were tested in various countries. In Germany a video telephone was put into service between Berlin and Leipzig on Mar. 1, 1936; later, Nuremberg (1937) and Munich (1938) were added. The images were transmitted on wide-band cables and occupied a frequency band of 500 kHz. In October 1961, video telephone communication was set up from Moscow to Leningrad and Kiev on existing television channels of the intercity communications lines. In later years public video telephones also appeared in Tallin, Vilnius, Kaunas, L’vov, Kazan, Tashkent, and other cities. Since the 1960’s, work has been under way in the USSR, the USA, Japan, the Federal Republic of Germany, and other countries to develop video telephone systems in which the images are transmitted for great distances over group channels of long-distance communications systems utilizing frequency or time division of the channels and over short distances (several kilometers) on existing city telephone circuits or special communication lines.