Videophone
From Wikipedia, the free encyclopedia
A videophone is a telephone which is capable of both audio and video duplex transmission. It differs from videoconferencing in that it expects to serve individuals, not groups.
[edit] Early History
AT&T conducted experiments and demonstrations of a Picturephone product and service in the early 1960s. The demo unit was usually in a small oval cabinet on a swivel stand, intended to stand on a desk. Videophones, possibly AT&T units, were featured at the Telephone Association of Canada Pavilion (The 'Bell' Pavilion) at Expo 67, an International World's Fair held in Montreal, Canada in 1967. Several demonstration videophone units were available for the Fair-going public to try, who were permitted to make live video calls to recipient volunteers in the United States. Color was not employed. The equipment packaged a Plumbicon camera and a small CRT display in the cabinet. Video bandwidth was 1 MHz with about 200 scan lines. The equipment included a Speakerphone hands free telephone, with an added box to control picture transmission. Cable amplifiers were spaced about a mile apart (1.6 Kilometers) using ordinary twisted pair cables with built-in six-band adjustable equalization filters. For distances of more than a few miles, the signal was digitized at 2 MHz and 3 bits per sample DPCM, and transmitted on a T-2 carrier.
Picturephone was offered to the public in New York City and Washington DC in the early 1970s. The screen was larger than in the original demo units, approximately half a foot (15 cm) square in a roughly cubical cabinet. Picturephone booths were set up in Grand Central Station and elsewhere. With fanfare, Picturephones were installed in offices of Westinghouse and other progressive companies. Hundreds of technicians attended schools to learn to operate the Cable Equalizer Test Set and other equipment, and to install Picturephones. New wideband crossbar switches were designed and installed into 5XB switch offices, this being the most widespread of the relatively modern kinds. Unrelated difficulties at New York Telephone, however, slowed the effort there, and few customers signed up in either city, and the service faded away after a few years.
AT&T sold the VideoPhone 2500 to the general public in 1992 to 1995 with prices starting at US$1500 and later US$1000.[1] It was limited by connecting by analog phone lines at about 19 kbit/s; the video portion was 11,200 bit/s,[2] with a maximum frame rate of 10 frames per second, but typically much lower. The VideoPhone 2500 used proprietary protocols.
[edit] Protocols
The original Picturephone system used contemporary crossbar and multi-frequency operation. Lines and trunks were six wire, one pair each way for video and one pair two way for audio. MF address signaling on the audio pair was supplemented by a Video Supervisory Signal (VSS) looping around on the video quad to ensure continuity. More complex protocols were later adopted for conferencing.
[edit] Call Setup
Videoconferencing in the late 20th century was limited to the H.323 protocol (notably Cisco's SCCP implementation was an exception), however recently a shift towards SIP videophones has occurred. In accordance with the adoption of SIP telephony for home users, videotelephony is also slowly becoming available to home users from a number of hardware and software providers. Closed solutions such as Skype also now provide video.
Another protocol used by videophones is H.324. Videophones that work on regular phone lines typically use H.324, but the bandwidth is limited by the modem to around 33 kbit/s, limiting the video quality and framerate. A slightly modified version of H.324 called 3G-324M defined by 3GPP is also used by some cellphones that allow video calls, typically for use only in UMTS networks.
[edit] Video Compression
Main article: Video compression
The most commonly used video codecs are H.263 and H.264. Skype uses the proprietary protocol VP7.
[edit] Current use
Videotelephony is most prominently used in large corporate conferencing setups, and is supported by systems such as Cisco CallManager. Other companies such as Tandberg, Radvision, and Polycom also offer similar merchandise.
Today the principles, if not the precise mechanisms of a videophone are employed by many users world-wide in the form of webcam conferences using personal computers, with cheaply available webcams and microphones and free instant messenger programs. Thus an activity that was disappointing as a separate service found a niche as a minor feature of products intended for other purposes. A videophone can be created by using an old or inexpensive computer and dedicating it to run a video softphone.
In 2004 Telmex, the biggest telephone service provider in Mexico, introduced Videophone service over regular phone lines (apparently H.324). The service, as of March 2006, had not enjoyed widespread adoption. Telecom Italia supplies LG-Nortel videophones, which also appear to be used by Telmex.
[edit] Trivia
- In many science fiction movies and shows that take place in the future, videophones are used as a primary method of communication. Examples of videophones include the Picturephone from Pee-wee's Playhouse and 2001: A Space Odyssey and on the British cartoon DangerMouse, in which the title character regularly communicated with headquarters via videophones in both his home and his car.
- A device with the same functionality was used by the cartoon character Dick Tracy since the 1930s. Called the "2-way Wrist TV", the fictional detective often uses the phone to communicate with the police headquarters.
- AT&T VideoPhone 2500 prototypes are visible in the movie Gremlins 2: The New Batch.
- In Futurama, the videophone is often used in the space shuttle they travel in, usually on missions.
[edit] See also
[edit] External links
[edit] Dedicated videophones
[edit] Softclient videophones
- AIM Phoneline
- WigiWigi
- Kapanga Softphone SIP client for telecom applications
- Linphone An open source SIP videophone for linux and windows OS.
