Re: caller video window "splotchy"

John Ingham (johningh@tafe.sa.edu.au)
Mon, 21 Jul 1997 11:29:07 +0930


Lori recently said...

>I've just started using CU-See Me, and the caller window isn't working
>right. At first the picture is good, but over time it degrades into a
>patchwork of images as if the whole window isn't updated. It looks like
>a puzzle whose pieces aren't put together right. My own window looks
>great. The person I'm talking to has the same problem. I look
>splotchy, but his own window looks fine.
>
>I'm using...
> ZEOS 486-DX4-100
> Greyscale QuickCam
> SoundBlaster Pro
> Cardinal MVPV34ISP Modem (33.6 Kbps v.34)
> Windows 3.1
>

The effect you describe is to do with the way CU-SeeMe compresses the video
which is very similar to M-PEG. This type of "interframe" compression
builds on the picture already received by only sending updates to the video
rather than whole new frames. But if any packets are lost during
transmission (and this certainly happens over the internet), then the
received picture gets progressively more and more scrambled. The most
important thing to understand is that although the effect is noticable ONLY
to the receiver, the "fix" must be done at the transmitting end. Also,
when connected to a reflector, different receivers will experience
different levels of degradation depending on their path from the reflector.

Unfortunately, correcting the problem differs from version to version of
CU-SeeMe. The place to look is in the Video Preferences section under
Codec Settings. If you see a slider control marked something like "Good
Connection - Bad Connection", choose "Bad Connection"; in my experience
"Good Connection" means a LAN or Intranet not a dial-up modem to an ISP.
Alternatively, if you see a control marked something like "Smeared 'I'
Frames" enter a very small single digit number.

The effect of these settings is to force CU-SeeMe to re-send the whole
picture more often, and not just wait for a change in the subject. However
this will also result in a lower transmitted frame rate because this lowers
the compression efficiency. But I'd rather watch a slowly changing clear
picture any day than moving mush!

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/
=======================================================