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Videoconferencing systems and methods of signal transmission (including networks)
Choice of System
Physically, videoconferencing systems can be broadly classified into 4 main groups:
- Desk-top (including PC-based systems)
- roll-about systems
- studio-based systems
- TV-type broadcast systems.
Choice of Methods of Signal Transmission
Systems can operate using a variety of different methods of transmitting the audio and video signals, e.g. network, satellite, microwave etc.
As a general rule, small scale systems will use either the public telephone network (usually ISDN) or a network such as SuperJANET. Studio-based systems will usually use SuperJANET. Again, lectures/demonstrations given by TV-type broadcast can use SuperJANET but can also be well suited to satellite transmission.
Depending on the purpose of the communication and the capability of the system, the communication will be point-to-point, multi-point or onto many distributed communication. In point-to-point communication, there are only 2 locations involved. There is normally full two-way audio and video communication. In multi-point communication, more than two locations are linked simultaneously and each location involved can be seen and heard by all the other locations. Which location is viewed at any time depends on how the video-switching is achieved. With most systems capable of multi-point conferencing, it is only possible to view one distant location at a time although all locations can be heard. There are two ways of switching the video viewed on the monitor:
In voice activated switching, the video is switched to the location of the person talking (or making a noise). In manual switching, one person controls which location is shown on the monitor. (In both cases, the location being shown to the others can see the last location that was switched to their monitor.) It is suggested that unless there are task-related reasons for choosing manual switching, voice activated switching should be used.
- voice activated (sound activated)
- manual (chairperson activated).
One to many communication involves one location broadcasting to a number of others. Usually, there will only be cameras at the main location. Depending on the configuration, the ‘receiving’ locations may or may not be able to send audio signals, e.g. in a lecture situation, students at ‘receiving’ sites may be able to ask questions of the lecturer at the main location.
The codec is a piece of equipment that encodes and decodes analogue/digital data to produce the final sound and picture. The codec can have a major influence on the quality of sound and picture. Consideration in the choice of codec are: interoperability, flexibility, cost, ease of upgrading, quality of transmission, supplier (other hardware preferences and terms, conditions etc).
The bandwidth used affects the speed with which the signals can travel and this also influences the quality of sound and picture. Generally, the higher the bandwidth, the better the quality although compression techniques have im proved considerably and it is possible these days to have quite acceptable quality with fairly low bandwidth. The purpose of the communication will dictate the parameters of acceptable quality. Where the resolution of the picture needs to be very high or where full motion is required, for example, it may be worth ensuring that high bandwidths can be used.
The kind of system required will depend largely on the purpose of the video conference. The following table shows the likely configurations according to the main types of use of videoconferencing described previously.
It is of course possible to mix configurations if there is sufficient compatibility of equipment. For example, if a seminar was taking place between two stu dios, it may at some point in the seminar be useful to bring in an expert from perhaps yet another institution. If he/she had a desk-top system that was com patible, they could join the seminar for a short while, thus maximising their use of time.
The ITU recommends a videoconference terminal must as a minimum include the following user interface modules (as well as a control module and a co dec.) At least - a camera, a screen monitor, a microphone, a loudspeaker, control keys and lighting controls (if not present in the screen).
The survey reported in Chapter 3 gives details of additional facilities that some institutions have found useful.
Virtual Environments Visualisation