Re: Captivator Pro - Answers

Chuck Elliot (
Thu, 04 May 1995 17:45:23 -0400

>Date: Thu, 04 May 1995 15:56:36 +0100
>From: (M. Carleer)
>Subject: Re: Captivator Pro - Answers
>Let me explain exactly wht's happening:
> - In Overlay mode, the dig card never sends any data to the PC. Instead,
>the output of your video card is passed through the dig card; with the help
>of the signals coming through the VESA or the feature connector, it is the
>dig card itself that decides when and where on screen to replace the video
>signal by the TV digitized signal. In fact, if you could by any means stop
>the dig card from working and still start VfW, you would simply see a window
>of uniform colour (usually magenta by default) where the TV picture usually
>appears in Overlay mode. Now the dig card detects this colour and replaces
>it with the digitized picture. Yhis procedure is well know in TV studio to
>show people speaking in front of a picture or a data panel which in fact is
>not present in the studio. The improtant thing is: no digitized TV picture
>is ever transmitted to the PC in this mode, and if your dig card is only
>capable of Overlay, there is absolutely no way in which to recover this data.
> - In Preview mode, it is the responsibility of the program to go and fetch
>the digitized info out of the video memory on the dig card. This means that
>this memory must be mapped into the PC's main memory. And this is why these
>cards usually impose a limit on the quantity of memory that can be present
>in your PC: the dig card is an ISA-bus card with a limit of 16Mb of
>addressable memory (this is an ISA limitation), so the video memory must be
>mapped in these 16Mb.
> You might think that with overlay cards, you would never be able to grab
>and store a digitized picture. This of course is not true. The thing is,
>these cards usually send the data from the dig video memory through a
>compressing chip which then sends the compressed data directly to disk. One
>such card is the Video Blaster RT300. Such a card does not work with CUSeeMe
>because it's memory is not mapped to the PC main memory, instead the output
>of the memory is sent directly to an Intel Indeo compressor chip which in
>turn sends the compressed data serially to the hard disk. There is no other
>way to retrieve the compressed pictures than to read it from the disk, and
>CUSeeMe would then have to decompress it, which is too time consuming.
> Hope this clears up things a bit. I didn't send this to the list because
>I started saying you are messy, but feel free to do so in all or in part.
> Cheers,
> Michel
> Michel Carleer
>Laboratoire de Chimie Physique Moleculaire Phone : +32-2-650.24.25
>Universite Libre de Bruxelles CPi-160/09 Fax : +32-2-650.42.32
>50 Av F.D. Roosevelt e-mail:
>B-1050 Bruxelles
Chuck Elliot
Sheffield Hallam University UK
tel. 0742 53315: fax. 0742 533161