Re: Relationship between fps and Kbps

John Ingham (johningh@tafe.sa.edu.au)
Thu, 2 Jan 1997 12:28:35 +1030


Adrian asked in part...
>>>Does anyone know why there can be high Kbps being sent between two people
>>>on DC connection yet the fps is prctically nothing? What is the
>>>relationship? <snip>

To which Mark replied...
>>In my experience, this is a function of your processor and video speed,
>>especially the processor speed. <snip>

Adrian then commented...
>I don't think that processor speed is the whole story because the processor
>demand shouldn't be varying so much yet I can be grabbing 13fps for a short
>while then 0.0 then 2.1 then 0.1 for a lot of the time. And as I said
>before what am I sending at a good rate of Kbps if the frame grabbing is so
>low?

Si then suggested an experiment using "cudoodle".

It is my understanding that the Transmission rate you achieve is a
combination of the settings you make in your own copy of CU-SeeMe and of
information received from the Receive CU-SeeMe concerning its capabilities.

If you are on a 28,800 modem but contacting someone on a LAN who has their
CU-SeeMe Receive Rate Cap set to 200Kbps, your system will behave
differently from when you are contacting another modem connected person who
has set their Receive Rate Cap to 20Kbps! And if the contact is via a
Reflector your transmit Kbps rate will be different again. Notice that I'm
hedging here when I say "different" because I'm not prepared to quote
actual figures, but I know that the other system's capabilities must be
taken into consideration.

Here's another experiment, though one which doesn't required "cudoodle".
Put on a very plain, unpatterned, unstriped shirt (or blouse) and try to
sit in front of a plain, evenly-lit background. Connect directly with a
friend and ensure that both systems are set for the same Rx and Tx Caps.
Sit *very still* and observe your Tx data rate, both fps and Kbps. At
first the Kbps is high and the fps is low, but after several seconds the
Kbps drops back and the fps goes up. Try *very small movements* such as
winking or opening and closing your mouth, all the time being sure to sit
very still. The Kbps and fps should not change much.

Now sway from side to side gently. You will notice that the Kbps jumps up
and the fps drops back. The greater the area of picture which changes from
frame to frame, the greater the Kbps and the lower the fps.

Now change your shirt (blouse) to a highly patterned or striped one. You
will now find that it is much more difficult to sit still to optimise your
fps rate. Even small body movements cause the Kbps rate to soar (with a
consequent drop in fps).

The conclusion is that CU-SeeMe has been optimised for minimum changes in
picture from frame to frame. The the greater the changes, the more data
which must be transmitted to describe those changes and the lower the fps
rate.

This post is already long enough so I will stop here and continue my
remarks on another.

John Ingham
Supervisor, Technical Operations, CALS (Centre for Applied Learning Systems)
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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