Videotelephony comprises the technologies for the reception and transmission of audio-video signals by users at different locations, for communication between people in real-time.
At the dawn of the technology, videotelephony also included image phones which would exchange still images between units every few seconds over conventional POTS-type telephone lines, essentially the same as slow scan TV systems.
Currently videotelephony is particularly useful to the deaf and speech-impaired who can use them with sign language and also with a video relay service, and well as to those with mobility issues or those who are located in distant places and are in need of telemedical or tele-educational services.
It is also used in commercial and corporate settings to facilitate meetings and conferences, typically between parties that already have established relationships.
The concept of videotelephony was first popularized in the late 1880s in both the United States and Europe, although the sciences to permit its very earliest trials would take nearly a half century to be discovered. These evolved from studies and experimentation in the fields of electrical telegraphy, telephony, radio and television.
The development of video and television transmission technologies in the United States and the United Kingdom started in the latter half of the 1920s, spurred notably by AT&T, occurred in part to serve as an adjunct to the use of the telephone. A number of organizations believed that videotelephony would be superior to plain voice telecommunication. However video technology was to be deployed in analog television broadcasting long before it could become practical in videotelephony.
Videotelephony can be categorized by its functionality, that is to its intended purpose, and also by its method of transmissions.
Videophones were the earliest form of videotelephony, dating back to initial tests in 1927 by AT&T. During the late 1930s the post offices of several European governments established public videophone services for person-to-person communications utilizing dual cable circuit telephone transmission technology. In the present day standalone videophones and UMTS video-enabled mobile phones are usually used on a person-to-person basis.
Videoconferencing saw its earliest use with AT&T's Picturephone service in the early 1970s. Transmissions were analog over short distances, but converted to digital forms for longer calls, again using telephone transmission technology. Popular corporate videoconferencing systems in the present day have migrated almost exclusively to digital ISDN and IP transmission modes due to the need to convey the very large amounts of data generated by their cameras and microphones. These systems are often intended for use in conference mode, that is by many people in several different locations, all of whom can be viewed by every participant at each location.
Telepresence systems are a newer, more advanced subset of videoconferencing systems, meant to allow higher degrees of video and audio fidelity. Such high end systems are typically deployed in corporate settings.
Mobile collaboration systems are another recent development, combining the use of video, audio, and on-screen drawing capabilities using newest generation hand-held electronic devices broadcasting over secure networks, enabling multi-party conferencing in real-time, independent of location.
Personal computer based web cameras are an often modest form of videotelephony, usually used for point-to-point videophone calls.
Each of the systems has its own advantages and disadvantages, including video quality, capital cost, degrees of sophistication, transmission capacity requirements, and cost of use.