Re: Relationship between fps and Kbps (long)

John Ingham (
Thu, 2 Jan 1997 14:27:52 +1030

In my experience most problems with picture/sound breakup while attempting
to Videoconference using CU-SeeMe are due to lost packets caused by trying
to run a higher Kbps than the circuit can stand. Give yourself a decent
chance of success by setting the Receive and Transmit caps LOW in the first
instance. I'd rather have a low fps than picture shatter or broken sound.

But often the effects of YOUR settings are not obvious to YOU but only to
the other party(s) in your conference. After some 5 years experience of
videoconferencing, I guess the most useful advise I can give is, *He who
does NOT see(hear) a problem is the most likely CAUSE of that problem*!

Here is an experiment that I suggest that all users of CU-SeeMe undertake -
indeed it should be a pre-requisite before trying to contact any other
user! It will enable you to gain an appreciation of how you are
seen(heard) by others.

Connect to "Self" - it's the first "Connect To" or "Call" item under the
"Conference" menu. After a moment a second video window will open slightly
delayed relative to the Local Video window. This will show you as others
see you which can be very different from that seen in the Local Video
window (depending on the Codec you have selected in the "Audio/Video"

First let's assume you are running the White Pine Enhanced CU-SeeMe in
monochrome, for which you have selected the "CU-SeeMe Grey" codec, or using
the Cornell CU-SeeMe for which you'll need a monochrome video or Connectix

Once connected to "Self", you will immediately notice that the incoming
screen has a number of blank "tiles" which may or may not fill over a
period of time. If you move your hand around you can fill these up or
simply wait until they do so automatically as a "back-ground task" by
CU-SeeMe. But by using CU-SeeMe's default settings you'll find that you'll
have you wait a long time!

So open the "Communications" Preferences and play with the several settings
there. "Refresh Interval" determines how long it takes to fill up all
those blank tiles. The lower the number the sooner those blank tiles are
filled in, but the greater the load put on to the transmission as a
"background task". I use "30" but that is a personal judgement.

"Change Tolerance" adjusts CU-SeeMe's sensitivity to change in the Local
Video scene; if set too sensitive, CU-SeeMe will trigger on camera noise
and be constantly sending entire new frames. This will result in a high
Kbps rate and a low fps rate. If set not sensitive enough CU-SeeMe will
not notice small changes at all. I use a setting of 40 but that is
dependant on things like camera noise which is a factor of lighting so
should not be taken as gospel.

Now play with the Kbps settings in the Transmission and Reception
Preferences. You will notice that the Reception settings make more
difference than the Transmission settings. This is because the Receiver
part of CU-SeeMe constantly advises the Transmitter of the receive error
rate and the Transmitter part of CU-SeeMe throttles its Kbps rate
accordingly. (I understand that if too large an error rate is reported to
a reflector, it will not pass on any Audio at all, which explains why some
people have reported not being able to receive any audio. Indeed if you
are trying to receive broken audio at any time, close the receive window -
this signals the transmitter not to send any video packets and the audio
will suddenly come through clearly!)

If you are running the White Pine Enhanced CU-SeeMe with a color video or
Connectix camera, here is another experiment. Using the appropriate
digitiser and the WhitePine Color codec, connect to "Self" as before. This
time you should not see any blank tiles but a clean picture. The quality
can be set in the WhitePine Codec preferences window but if you are using a
28,800 modem anything more than 50% will give a very low fps.
Unfortunately at 50% the quality of the picture itself is significantly
poorer than the CU-SeeMe Grey codec - but it is in color and should have
higher fps! (You can use the WhitePine Color codec with a monochrome
Connectix camera if you wish).

Play with the various settings as before (click "Options" in the Codec
window) try checking the "Noise Filter" and the "Lossless" buttons. Notice
that they have little effect on the Local Video window but have a profound
effect on the "Self" window. Try to find a trade-off between picture
quality, and movement (fps). Remember that this will be how you are seen
by others!

You may find that after some time the picture begins to degrade - it gets
confused looking, like a half-done jig-saw puzzle. This is an analogous
problem to the blank tiles encountered with the Cu-SeeMe Grey Codec. The
setting for "Smeared I Frames" has an effect similer to "Refresh Interval"
and it determines how often a complete new picture is sent to overcome
accumulated errors due to lost packets. I use a setting of "40" but again
this is a personal setting.

I hope that all this makes some sort of sense once you have tried it for

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
Web Site: