[AG-TECH] VIC problems at 25fps

Steven s.jack at compserv.gla.ac.uk
Fri Mar 12 04:54:25 CST 2004


 

Hello I have noticed a problem with vic on our machines here in the UK. The
problem is manifested in two ways.

1.	The GUI for the vic window on the video machine is unresponsive,
i.e. it is difficult to access GUI buttons, etc and the window often doesn't
redraw correctly.
2.	On the display machine video windows which have been expanded remove
themselves periodically. You can also notice that there has been some
re-ordering of the transmitted windows within the vic application.

It is as if the transmission is stopped and re-started. There are no join or
leave requests on the network during this time.

With some experimentation I found that altering the frame rate that you are
transmitting changes this behaviour quite dramatically. Changing the frame
rate below 25fps results in a responsive GUI on the video machine and the
windows the displayed video streams remain stable. Indeed moving the frame
rate up to 30 fps produces a similar fix, though the GUI is slightly
hesitant in responding. Note that with a PAL signal connected the actual
transmitted frame rate cannot achieve greater than 25 fps.

I also noted that if there is no signal connected then the displayed streams
are not interrupted. I also noted that this has little to do with the data
rates involved as, there is little difference from sending black and burst
signals to sending an 'off-air' TV signal resulting in data rate of
approximately 1.5Mbps.

 

In have repeated this on 2 different AG rooms and both behave identically,
though tests with the PIG on a windows XP machine using a web cam were not
able to demonstrate this effect.

 

A quick fix is to set the 4 transmissions to 24 fps, but I wonder if anyone
else is suffering from the same problems or has any insights.

This obviously will only affect those using PAL sources, though it may be
interesting for someone with an NTSC or indeed SECAM source to comment when
running at 25fps.

 

Steven

 

Steven Jack

Videoconferencing Manager

University of Glasgow

Computing Service

Glasgow G12 8QQ

UK

Tel +44(0)1413303828 Fax +44(0)1413303820

Email: s.jack at compserv.gla.ac.uk

 

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