PAL video with no video input but using external vsync

[hesv]

Hi.
Using code from the project “Text and Graphics Overlay” and inspired by the “Video experimenter shield” i would like to generate a PAL video signal using no real video input but synchronized to an external VSYNC signal. I hope you followed…I have made a home-brew prototype shield. See schematic.
Some background first to understand why this is desired for me. I have a computer with a video grabber board that is originally supposed to be connected to a CCD camera that takes H-sync and V-sync from the video grabber board and returns a video signal. I want to simulate a video signal from this camera. It is vital that the sync signals are master in this system. So..
I want to produce a video signal using tvout library with my mock-up HW with Vsync as the only input. I have taken the “Text and Graphics Overlay” sketch and uploaded it to my Arduino Uno board. If I remove the line initOverlay() which is done in the setup everything looks fine. I have a nice overlay on a black background. If i re-insert the initOverlay() line and apply my Vsync then i don’t get any sync signal at D9. It’s constant high level. Meanwhile it seems to be something at the video output at D7. I have checked the LM1881 specification and i think i conform to the signals this device should produce under these circumstances. I have no need for this device and hence not included it in my schematic. I have checked that the interrupt routine is working as well at the sketch. I have studied the other threads for this shield but i have not found anything that is applicable for me. Can someone give me any hint on what to test or to do to get any progress i would surely appreciate it! Thanks in advance!

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