Re: question about settings cu-seeme shareware and firewall

John Ingham (johningh@tafe.sa.edu.au)
Thu, 28 Aug 1997 17:55:22 +0930


Nico Pot recently asked...

>I have a question about cuseeme from Cornell.
>Version 0.92b2 for windows.
>Under VIEW, PREFERENCES, VIDEO you have the CHANGE TOLERANCE and REFRESH
>INTERVAL.
>CHANGE TOLERANCE is at 40
>REFRESH INTERVAL is 100
>
>What does this two things do, and what are good values.

As you will know, the grey-scale CU-SeeMe transmits its picture made up of
little blocks of video. Part of the compression algorithm used assumes the
receiver already has the basic picture in store so only the *changes* due
to subject movement need to be transmitted. The transmitting system
determines which part of the picture contains movement by comparing the
current frame with the previous one and transmitting only the differences.

The "Change Tolerance" setting determines how big a difference is required
to be classed as requiring to be retransmitted. An an example, suppose you
had a very noisy camera; your system would try to retransmit the whole of
every frame because there would always apear to be movement across the
whole screen. So you set "Change Tolerance" to minimise the amount of data
being sent needlessly, but no so far as to be insensitive to real movement
in the scene - how can you tell?

One good way is to connect to "Self" and watch the received picture as you
make the adjustment. Too far one way will slow down the frame rate, too
far the other way will cause "picture shatter" as you move or perhaps no
movement at all.

Due to the nature of the data transport used by CU-SeeMe, the internet does
not necessarily pass on every single block of data, so errors in the
received picture can slowly grow. To overcome this, CU-SeeMe randomly
resends data on *all* picture blocks regardless of whether they have
changed or not. The "Refresh Interval" sets how long it takes to
completely rebuild as picture. If this interval is set too low, what
should be a "background" task takes up too much of the available data
reducing the compression efficiency and slowing down the frame rate. Again
this can be observed by connecting to "Self".

When connected point-to-point, or when you join a reflector, CU-SeeMe
transmits your entire first frame so the other receivers should get your
complete picture quite quickly. However, video from others who were
already on the reflector before you, may take some time to build up on your
screen depending on their "Refresh Interval" setting.

I hope this helps your understanding of what is going on with these two
controls.

John Ingham
Supervisor, Technical Operations, TLC (Tele-Learning Consortium)
Adelaide Institute of TAFE (Training and Further Education)
GPO Box 1872 Adelaide South Australia 5001
Ph: +61 8 8207 8550 Fax: +61 8 8207 8552
Email: johningh@tafe.sa.edu.au
Web Site: http://www.tafe.sa.edu.au/video-conf/
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