Good advice Ashley.

The quick and dirty way to do what David wants is to put an
ATI 'All-in-Wonder' card into his machine ($180C). The AIW cards
digitize
the incoming video and put it into a variable sized window (including
full screen, even at 1280x1024) for display on your monitor. It also
has a TV out capability (S and composite), so you'd set
your screen to 640x480 resolution
(to match that of your incoming video) and then make the video your
desktop, with whatever graphics you want on top.

They have a SDK for their API so writing your own C app to drive
the card is not too hard.

The AIW cards do ALL the work on-board (by default they use Magenta
to key in the video) so the video stream happily continues even when
your box locks up.

Now if you really want to MIX the signals (as in variable transparency),
the ATI card won't do it since it's strickly a background keyer.

You might also want to look as some of the older Elsa cards.
Their 'Gloria Synergy' card can output any single desktop window
as video, so concievably you could get your video into your system
via any video digitizer, then use the Elsa to output that window,
with graphics overlay, as video.

And then there is the DPS product line of 'personal time base
correctors'. They would allow you to retime your incoming
video to that of your computer, where you can then build a
simple analog mixer to combine them.

Do remember that the bandwidth of NTSC/PAl video is a fraction
of what you have for your desktop (200 line pairs vs the 400 of
a common 800x600 screen) so your graphics have to be constructed
-specifically- for video output.

For more discussion, contact me off-list. I do a fair bit of
computer video in my 'other life'.

Ashley Roll wrote:
>
> Hi David,
>
> I'm no expert in this, but from what I know your going to have a few
> problems with a "mixer".
>
> You would have to ensure that the two video signals are synchronised (called
> "locked" in the video industry I think - you use equipment that supports
> external "gen-lock") so that when you mix the signals you get the correct
> bit of it and the timing isn't disturbed causing the image to rip apart..
> Basically this is difficult. If your very lucky you video source may have a
> gen-lock input so you could derive a lock signal from your computer video to
> lock them together..
>
> How complex are the graphics? On screen displays are normally done by
> arranging a switch in the video signal and switches between the real signal
> and a specific colour level (voltage) at the right time in the video
> frame/field/scan line. You could possibly use a PIC or SX to do this :) Look
> at http://www.piclist.com/techref/scenix/lib/io/dev/video/index.htm for some
> ideas.
>
> These actually generate the entire signal, but you would just switch between
> the live video and your set voltage representing a colour. (if you get
> really tricky that voltage could be generated from your PIC to give you
> several colours)
>
> There are also on-screen display chips that do this all for you..
>
> The "big boys" now all edit digitally, you digitise the video for your
> entire movie (at movie resolution of at least 4096x3072 for "low res") to a
> big disk array, mess with it with some expensive software and send it our
> again.. it is possible to do this is real time, but not cheap.. not cheap at
> all..
>
> Some "home" video capture cards should let you do what you want also which
> would be a lot cheaper :) You should be able to have them overlay video into
> a "keyed" area of your display and you can take the TV out and record it.
> All the "gen-locking" is done by digitising the video and copying pixels
> normally.
>
> Good Luck.
> Ash.
>
> > I am trying to find a circuit or device that would allow me
> > to mix baseband
> > video from my computer (Already has TV Out) with a 2nd video
> > source.  I am
> > trying to do  Graphics On Screen Display (with existing video in
> > background), so Keyed Overlay support is needed.
> >
> > I have found a few scan converters that take VGA and offer
> > Keyed Overlay
> > onto a video source, but tend to be too expensive ($500 - $3000).
> >
> > I am hoping to find a simple circuit I can build (good at
> > building, bad at
> > designing), or even a cheap (<=$200) unit I could purchase,
> > that would do
> > good quality keyed video mixing. (No need for broadcast quality).
> >
> > Any help would be appreciated.
> >
> > David.
>
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