This is a relayed message from Telephonics in response to your post.
There are basically two forms of teleconferecing. Point to point where only two parties are interconnected. Obviously, the other form is what we term multi-point. In thes mode a number of people participate in a conference. To provide multipoint conference capability one needs to connect each PC (as you said to a central point) to a switch. Ideally, you want a non-blocking matrix type switch. In telephony, its switch functions like a telephone central office switch and provides connections as ordered by the tlelphone instruments which are connected to it. As you know, you can dial a single number abd be connected to a single party or you can call the audio conference operator and get a multi-party audio conference established. In video its the same. The PC must be connected to the switch. This connection can be twisted wire telephone, lan cable, isdn, or any other form of digital transmision. However, the cheapest cost of installation is through use of the spare twisted wire pairs already in buildings. As I've indicated, analog video can be transmitted over UTP and be exactly comparable to what we are used to viewing on our home TV screen. This is full motion, full color, 30 frames per second with audio and video in sync. (This means the audio is in step with the llip movements of the speaker appearing in the video. So as not to beat this syubject any more-there are devices available from a variety of sources that permit the transmission of NTSC (analog) avideo over UTP for various distances. The low end is 300 feet and the high end is 4000 feet. Note there are devices that offer capability within this band.
Now for the switch-here is where the real distinction between desk top video systems can most clearly be defined. The number of input and output ports is the criteria. One requiires a port for each PC you wish to connect. So the more ports, the larger the number of end users (PC's) that can be connected. Secondly, the switch must abe capable of permitting multiple multi-pint video conferences to take place simultaneously. For example, if I had a 24 x 24 switch, I would expect to be able to have 2-6 party conferences (each using 6 ports for a total of twelve), an 8 party comference useing 8 other ports, and a 4 party conference (using 4 other ports). These connections use a total of 24 ports. Therefore, the larger the switching matrix,the more video comunications can be provided without running into busy signals. There is nothing wrong with systems that employ small sized switches.. They work. For example, the SEXI system at the library of congress, switching provides 14-16 ports. I have reservations as to the acceptability of this limited switching function to some of the grandiose plans for expansion of the service throughout Capital Hill. It will take a far larger switch to satisfy this community. I do not know what SEXI is doing in this area.
The third consideration in video transmission is what we call bandwidth compression. That is to say the use of algorithms which operate on the video signal to compress the picture elements transmitted so that lesser bandwidth can be used thereby reducing the transmission cost. There are many techniques being used today. For the future there are a body of folks whoo favor an approach developed by DR. Barnsley. He refers to his aglorithmic approach as 'fractals'. The second and competing technology is referred to as 'wavelets'. Both techniques have their supporters but the military have settled on wavelets as their choice. Implementation of either technology requires computor power. The more power, the less bandwidth to transmit is necessary. We can employ both these technologies today by utilizing very powerful work stations-not PC's. The real growth in the desktop arena will come when additional power can be cheaply added to a PC and will permit fractals or wavelets to run on these machines. There are today, companies who are well along in the development of these powerful accelerator boards and the unique processing architectures that will greatly improve the desktop world of videoconferencing. When??!!
Wish I knew, but it is coming. As an example, within the military, there are specialized image and video transmission systems operating at 60 frames per second and with resolution (quality of the image) far beyond anything in the commercial world. But as one participant in this BB said, we all know the military doesn't care about cost. But in these examples, the only way to meet the operational needs of the miltary is to provide the ultimate and, of course, it's expensive.
Hope this helps you grasp the essential elements which are:
1. The end user equipment-the PC
2. the transmission link to the switch
3. the connection from the switch to the conference participants. The participants may be within the campus limits, or may be a mizture of within and without that geographical arena. Where outside is required, it is emminently feasibvle to provide for their inclusion by utilizing codec's. These coder/decoder/ accept output which is both analog and digital, video and audio. In the case if an analog input, the coder recognizes the need and first digitizes the analog signal, then compresses it, just as it does to an incoming digital signal and send it on its long distance journey through whatever wide band digital transmission [ath it is connected to. At the far end, just the reverse takes place. Can one get 30 fps over these long links? Answer...definitely yes but you get what you pay for in codecs and bandwodth just like anything else.
Telephonics.
Relayer's note: Whew! |
