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[Top]Desktop Video AGOCG Report
Such systems works by inputting the video image from the camera or other source and passing it to the framegrabber. Frames are then compressed in the computer before they are sent. Audio may also be compressed at this stage, but many systems rely on a telephone being used in conjunction with the videoconferencing equipment. On reception of the image, it is decompressed. Then the video circuitry in the receiving machine, such as the video card, displays the image from the remote system.
Most systems use an ISDN connection since it provides a cheap, high speed (64 Kbps) digital phone line, without digital information from the computer having to be encoded into analogue data using a modem. As ISDN lines are digital, they provide a more reliable way of transferring data. An expensive alternative is the X.25 standard packet switched stream wide area network. Internally within a company, an Ethernet network could be used, but with this the frame rate may not be consistent.7
Real time videoconferencing is less demanding of the host system than videomail because the video typically passes through the machine without taking up CPU resources or system-bus bandwidth. Because it is live, the data neither comes from nor is saved to a hard drive; users who want to save any videoconference are able to do so onto a VCR.
One of the earliest desktop video conferencing systems was the CLI Cameo Personal Videoconferencing System for the Macintosh, announced in January 1992. This uses compression technology developed jointly by CLI and AT&T. Cameo is designed to work over ISDN lines only and employs a scaled-up version of the algorithm that AT&T uses in its analogue based consumer videophone. The system transmits 15 frames per second of video, (about half the rate of TV video) and requires an external phone to transmit audio.
More recently, PictureTel, in association with IBM and Lotus, has announced the PC based PictureTel Live PCS 100.
Although the first videoconferencing systems appeared in the 1970s, the technology is still bogged down by conflicting standards and constricted electronic pathways. Before desktop videoconferencing can become effortless and ubiquitous the problems of inter-connectivity and bandwidth have to be solved. (See the earlier section on compression techniques.)
Assuming a full screen image of 640 x 480 pixels in true colour (24 bits per pixel) and NTSC standard 30 fps for full motion video, it requires a transmission rate of almost 27 Mbps for an uncompressed picture the quality of a TV broadcast, without sound and in one direction only. This will therefore require an increase in bandwidth or considerable progress in compression techniques.
The quality of the picture displayed depends on a number of factors including the frame rate. Some systems can now provide up to 30 frames per second, but at a compromised quality. Ten frames per second is considered acceptable.
The idea of seeing and speaking to someone who is not in the same room is now a reality in a primitive form with video phones for domestic use. A true video conferencing system should provide a full meeting metaphor with facilities for many people to interact visually, exchange documents and work with shared resources. Nowadays a personal computer is sufficiently powerful to provide the backbone for a desktop videoconferencing system. Recent advances in technology have led to cheap and fast hardware to implement compression algorithms. Added to this there is the increasing availability of ISDN lines that cost little more than a normal telephone line. These three factors mean that desktop videoconferencing can become an affordable reality.
There are a number of software applications which can now enable videoconferencing. A review article comparing six such applications can be found in MacUser issue of 20 August 1993, pages 61 65.
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