CGA/VGA-Scaler / LM1881 generates white noise

[ninn]

Hi fellow modders, hackers and games!

I got a problem.


the problem:
under certain circumstances, my scaler setup will displays white noise in the displayed picture.

I created a sketch for you. It is a pretty good lookalike what I am seeing,
and I can not make a conclusion of it. Hope you can help me. Let's have a look at what I am experiencing onscreen:






As you can see, the displayed picture looks like it is invaded by little black dots. At first, those white pixels are  randomly distributed, but they tend to be a lot more visible in the letterbox-area of the picture (cause the letterbox is black, good contrast). At second sight, they tend to appear at fixed positions, forming 2,3 paralell vertical lines running down the screen. They seem to be attracted to black, but maybe they are just better visible there. The white dots are single pixels,  not collections of pixels.

my setup:

I am using a well known combo.

RGB-Console ->
-> LM1881 ->
-> CGA/VGA-Scaler (RGBHV)
-> monitor



The Setup works great for MegaDrive or PS2 via RGB. No dots visible with them.
The Setup will generate the dots when replacing the console with the SetTopBox, which outputs RGB too.

Whatever I connect, the picture is in sync.



the solution:

Still seeking for it.
What is generating those dots?
Do you got an idea?




Kind regards,
ninn