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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