Posted by Richard Crowley on 11/24/06 14:03

"Alx" wrote ...
> I have designed a board for using with a Digital Video Recorder (board
> level) for adding a Video Switcher and a Video DA to it.
> I've got some Video interference in all Video OUT's.
> Here's a link to see a captured Video that show the interference :>
> http://www.zapimage.net/web/BRUITS_VIDEO.wmv
> Here's a link to see my circuit diagram :
> http://www.zapimage.net/web/Circuit_Diagram.jpg
> Do I have to add a Genlock or someting else to resolve this problem?

Traditional troubleshooting (or "fault-finding" as they
say in England) technique calls for examining the signal
at each point in the path to see if that section is the cause
of the problem.

Under what conditions do you see this problem?
Is this what you see from a live signal at your input?
Or is this what you see from your video recorder board?
If this is the video from a live input, does the problem go
away if you disconnect the #2 input of the MUX?
Does the problem go away if you disconnect the power
from the video recorder board?
Do you see the same problem at the input of the DA?
At the output of the 2-channel MUX?
At each input of the 2-channel MUX?

My first guess is that you are seeing leakage from the
output of your video recorder board which is not running
synchronously (ie. it is not playing back in genlock with
the input). This would not be a problem (and you would
not need genlock) if you didn't have excessive leakage in
the MUX switch.

Note that switches made for video use have special circuits
which "short out" the unused input to avoid this kind of
leakage. Suggest investigating using special video MUX
cips rather than general-purpose switch chips.

[Back to original message]