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History Analog Video Transmission Videotelephony - AccessScience from McGraw-Hill Education http://www.accessscience.com/content/videotelephony/732100 (http://www.accessscience.com/) Article by: Bleiweis, John J. JJB Associates, Great Falls, Virginia. Publication year: 2014 DOI: http://dx.doi.org/10.1036/1097-8542.732100 (http://dx.doi.org/10.1036/1097-8542.732100) Content History Infrastructure Bibliography Analog video transmission Video telephone hardware Additional Readings Digital video transmission Video telephone software A means of simultaneous, two-way communication comprising both audio and video elements. Participants in a video telephone call can both see and hear each other in real time. Videotelephony is a subset of teleconferencing, broadly defined as the various ways and means by which people communicate with one another over some distance. Initially conceived as an extension to the telephone, videotelephony is now possible using computers with network connections. In addition to general personal use, there are specific professional applications, such as criminal justice, health care delivery, and surveillance that can greatly benefit from videotelephony. See also: Teleconferencing (/content/teleconferencing/680075) History Although basic research on the technology of videotelephony dates back to 1925, the first public demonstration of the concept was by American Telephone and Telegraph Corporation (AT&T) at the 1964 New York World's Fair. The device was called Picturephone, and the high cost of the analog circuits required to support it made it very expensive and thus unsuitable for the market place. The 1970s brought the first attempts at digitization of transmissions. The video telephones comprised four parts: a standard touch-tone telephone, a small screen with a camera and loudspeaker, a control pad with user controls and a microphone, and a service unit (coder-decoder, or CODEC) that converted the analog signals to digital signals for transmission and vice versa for reception and display. This system required a wideband infrastructure (telephone circuits) with a capacity of 6.3 megabits/s. See also: Analog-to-digital converter (/content/analog-to-digital-converter/031800); Digital-to-analog converter (/content/digital-to-analog-converter/195210) During the 1980s companies attempted to market videoconferencing for both public and private use, for example, setting up nationwide networks of videoconference rooms. Analog video transmission The primary limiting factor in videotelephony development has been the requirement for switched broadband circuits. The public telephone infrastructure was developed for narrow-band voice communication only. Television network broadcasts and television cable distribution systems are wideband in nature, but they are not switched between users and are only one-way. Transmission requirements can be reduced considerably by utilizing smaller screens with less spatial resolution, by reducing image refresh rates, and by delivering black-and-white images only. However, the requirements are still far beyond the capacity of ordinary telephone lines. Due to these limitations, true analog videotelephony has never really developed. 1 of 5 12/1/2015 9:46 AM Videotelephony - AccessScience from McGraw-Hill Education http://www.accessscience.com/content/videotelephony/732100 Therefore, considerable attention has been devoted to digitizing and compressing video signals to make their transmission more economical. See also: Cable television system (/content/cable-television-system/757246); Telephone service (/content/telephone-service/681000); Television networks (/content/television-networks/682400) Digital video transmission Pure digitization of video signals is normally done using pulse code modulation (PCM), which requires sampling and encoding to produce a transmission rate of 45 megabits/s. The video telephones from the 1970s used the differential pulse-code modulation (DPCM) compression technique, which reduced the required transmission rate to 6.3 megabits/s. This technique transmits only the differences between successive samples. Another compression technique popularized in the 1980s is the discrete cosine transform (DCT), which takes advantage of the fact that several adjacent points within an image essentially are the same. A third, widely used compression algorithm, called motion compensation, is based on the fact that moving objects keep the same form. Thus an image of an object must be transmitted only once, and its subsequent movement requires transmission of only its new position and orientation. See also: Data compression (/content/data-compression /757264); Pulse modulation (/content/pulse-modulation/556900) During the 1980s the International Telecommunications Union (ITU) agreed to use a standard for video coding and decoding called H.261. This standard uses a combination of the three compression algorithms and specifies standard image formats. Using the standard, good-quality video images may be communicated at transmission rates as low as 64 kilobits/s. A slightly different version of this standard is also common, called H.263. Several audio standards are in use: G.723, G.711, G.722, and G.728. Two general, higher-level videotelephony/videoconferencing standards developed by the ITU are commonly adhered to: H.320 for ISDN videotelephony and H.323 for IP videotelephony. Infrastructure Considerable improvements in telephone, computer, and television infrastructures have been made over the years. Telephone connections over copper cable have been replaced by optical fiber, the capacity of connections to the home has been increased and standardized, and more efficient technologies to use transmission capacity are available. Cable television connections have become interactive. Computer/data communication infrastructures, such as the Internet, World Wide Web, and high-speed, wide-band links such as digital subscriber line (DSL) and cable modems have become more widespread. These developments have converged to make video communication possible at affordable rates. See also: Internet (/content/internet/757467); Modem (/content/modem/429050); Optical communications (/content/optical- communications/471800); World Wide Web (/content/world-wide-web/757621) The basis of communication over the Internet is the Internet Protocol (IP), which allows for data in packets to be routed to different addresses using the Transmission Control Protocol (TCP). The data in the packets may be voice, video, data, graphics, and so forth. Thus, multimedia exchanges may take place between two or more parties. The exchanges can be over the public Internet or over private corporate intranets. Some companies have started replacing their traditional telephone private branch exchanges (PBXs) with IP-based PBXs, which would theoretically allow everyone in the company to have a video or multimedia telephone—totally separate from a computer. This could provide, for example, unified messaging—the ability to store voice mail as e-mail, answer telephone calls with e-mails and vice versa. See also: Electronic mail (/content /electronic-mail/225550); Private branch exchange (/content/private-branch-exchange/546050) Video telephone hardware Small residential video telephones, computer-based desktop video telephones, and small videoconferencing setups have been introduced to fulfill diverse needs. One such commercially available residential videophone is about as big as a typical 2 of 5 12/1/2015 9:46 AM Videotelephony - AccessScience from McGraw-Hill Education http://www.accessscience.com/content/videotelephony/732100 office desk telephone with a small flip-up screen that has an eyeball camera above it. Although it will work with several standards, this phone is primarily designed for use over Integrated Services Digital Network (ISDN) lines in which a residence gets three circuits; one circuit is used for control and the other two for voice and video. See also: Integrated services digital network (ISDN) (/content/integrated-services-digital-network-isdn/347850) An example of a computer-based desktop videophone consists of a PCI (Peripheral Component Interconnect) video/audio CODEC board to add to a personal computer, a composite color camera, audio peripherals, and visual collaboration software. Video telephone software Videotelephony software has been developed and made widely available that permits real-time collaboration and conferencing, including multipoint and point-to-point conferencing. Multipoint means, for example, that three people in three different locations could have a video telephone conference call in which each could see and hear the others. In addition to the basic audio and video capabilities, such software provides several other features such as a whiteboard, background file transfer, program sharing, and remote desktop sharing. John Bleiweis Bibliography E. M. Dickson and R. Bowers, The Video Telephone, 1974 S. J. Emmott and D. Travis (eds.), Information Superhighways: Multimedia Users and Futures, 1995 K. Finn, A. J. Sellen, and S. B. Wilbur, Video-Mediated Communication (Computers, Cognition and Work), April 1997 A. M. Noll, Anatomy of a failure: Picturephone revisited, Telecomm. Policy, 16:307–316, 1992 DOI: 10.1016/0308-5961(92)90039-R (http://dx.doi.org/10.1016/0308-5961(92)90039-R) R. Schaphorst, Videoconferencing and Videotelephony: technology and Standards, Artech House Telecommunications Library, January 1997 Additional Readings J. Alvear and R. Yari, You've got a video call… on your desktop, Techsearch NetGuide,
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