VGA > Scart horizontal sync (?) issue

[Minwah]

I've got a PC with an ATI 9550 AGP card running Soft15khz (default modelines). I've got this hooked up to a 21" Sony TV via Scart (RGB) and it looks good...but every so often part of the image in the top quarter of the screen will twitch to the right momentarily. This is a problem in all video modes including interlaced ones.

I am sure the TV is ok as I've tried it with other sources with no problem. Also, I can bring up the service mode and the OSD is perfect while the Scart image in the background twitches. Likewise I am sure the PC/video card is ok...I have tried different combination and the same thing happens with this TV. Finally I tried to rule out the VGA > Scart cable by trying an 'UMSA' adaptor - same problem.

So, I think the TV does not like something about the sync signal. I have the horizontal and vertical syncs from VGA connected together for the Scart composite sync. Vertically the sync is perfect so I assume the horizontal sync is not quite right for the TV.

I'm not really sure what to do about this...but have a few ideas:

1) Connect a 75 ohm resistor on the sync line. I don't know if this will help but have found a few sites suggest this when making a VGA > Scart cable.

2) Configure the graphics card to use composite sync in Catalyst control center and separate my horizontal/vertical syncs in the cable.

3) Try some custom modelines.

I can't really see 1 & 2 fixing the problem as it seems to me to be to do with sync timing, which I don't think these things would alter. As for 3, should I use trial and error and varying horizontal refresh frequencies and see if it makes any difference?

If anyone has any suggestions it would be much appreciated!

[SailorSat]

Does it happen with all resolutions?
If so, try #1.
If not, try #3.

As the UMSA doesn't help, you can rule out #2, as the UMSA generates correct CSYNC.

[Minwah]

Does it happen with all resolutions?
If so, try #1.
If not, try #3.

As the UMSA doesn't help, you can rule out #2, as the UMSA generates correct CSYNC.

It does happen in all resolutions I have tried. I will give the resistor a try first then.

Many thanks for your help!

[Paradroid]

every so often part of the image in the top quarter of the screen will twitch to the right momentarily

As SailorSat mentioned, if you're using the UMSA to combine sync, then you know your CSYNC signal is good.

I would try installing Arcade_OSD and experiment with tweaking modeline timings to see if the image becomes more stable.

[Minwah]

It does happen in all resolutions I have tried. I will give the resistor a try first then.

No joy with the resistor. I think it's slightly worse actually, the rest of the screen has a bit of a shimmer to it.

[Minwah]

I would try installing Arcade_OSD and experiment with tweaking modeline timings to see if the image becomes more stable.

Thanks, I didn't know about that - it looks really useful. I will give it a go...

[haynor666]

On my Sony 29 '' upper part of screen is sometimes slightly off to right side, On Toshiba 21 everything is fine so this might be issue with TV. As Paradroid suggested tweaking resolutions with ArcadeOSD could help.

[Minwah]

I tried Arcade OSD to tweak my modelines, varying the horizontal refresh rate etc...and no joy - just always has the 'twitching' problem.

So, I've given up on the TV for this use and am picking up another one to try later...