Jittery overlay/artifacts

[Alan]

Hi Michael,

Thanks for developing such a useful arduino interface! The basic overlay capabilities seem great but I have encountered a serious problem for my project.

The symptoms my system is experiencing appear to be identical to those experienced by pinkstond which he explained here -> http://nootropicdesign.com/forum/viewtopic.php?f=26&t=2433, there will either be noisy looking artifacts obscuring about 1/3 of the display while the overlay text is fine, or there are no artifacts, but the overlay text is offset vertically a bit, jumps horizontally to the right every second or so, and also jitters up and down a pixel or two. It seems random which of these two symptoms occurs, I can hit the reset button on the shield and it sometimes switches to the other issue.

You seem to have concluded that this may have been due to pinkstond’s use of interrupts with the onewire interfering with the video rendering. The thing is that I get these issues even when I upload an unmodified copy of the video overlay demo code from your site, or the gps overlay sample also.

Do you think it could be some sort of compatibility issue? I am running this on linux (lubuntu,) via the UNO board
but have’nt had any other problems with arduino on this system. I have arduino 1.0 and avr-cc installed and an unaltered version of the enhanced tvout library as per your instructions.

pinkstond’s picture:
[attachment=0:3uoy0256]6448136903_03265069eb.jpg[/attachment:3uoy0256]

thanks again
-Alan

[Michael]

Hmm. It’s hard to image what would cause this problem with the unmodified simple overlay demo sketch. I have had no reports of this problem before. It really does seem like you’re using interrupts or using too much memory, but in an unmodified sketch this would not be the case.

Have you adjusted the small 100K pot with a screwdriver? This can help tune the timing a bit.

[Alan]

Fine tuning the small potentiometer has definitely helped a bit with the text stability, but doesn’t stop the jittering, jumping, or artifacts. Detuning it far clockwise causes the overlay to prefer it’s rightmost position and also become warped a bit.

I actually found a magical fix to all the issues, which I’ll have to keep working with this week to make sure it’s stable, but maybe you can tell me why this works. If I put a 10 microF capacitor connecting pins 8 and 9 on your shield, all the jittering goes away and the overlay displays correctly on the video. (another maybe interesting fact: the overlay also displays correctly in “sync mode,” but without the video of course)

I’m pretty new at this and have trouble trusting my diagnosis, but this seems to be a board issue, or maybe a power source issue possibly. The capacitor bypassing the UNO seems to filter out whatever voltage fluctuation was being problematic.

Maybe you can tell me a better way of doing this filtering if the bypass capacitor is a bad practice or something.

Thanks again!
-Alan

[Michael]

I must say that I cannot explain why a bypass cap across 8 and 9 would clear up the jitter. I will keep thinking…

[Author]

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