DC and Saturn sync signals

[TuZhao]

I'm playing my JAP Saturn fine on this cool Hantarex 20" arcade monitor of mine. Now I thought it would be nice to play my PAL DC on it, too.
So I hacked a DC RGB cable to my needings, following the same pinout I'm using for the Saturn. But the DC display is wrong: I see the correct colors, but the image is constantly moving from top-left to bottom right of the screen in a very fast way, with a noticeable horizontal distortion.
I thought the DC and Saturn had the same sync signal, ready to be feeded into an arcade monitor. The Hantarex MTC9000 manual says "VFreq=47-70 Hz" so it should not be a 60 vs. 50 Hz issue.
Do I need the sync separator circuit? The good thing about the Saturn is that we don't need that.

[Tom61]

The composite sync signal isn't quite right on the DC for arcade monitor use, at least in my experience. Also, check your cable, as it may be using composite video on the sync pin of the scart. If it is using the composite video, run that through an LM1881 circuit tio get sync.

If your cable doesn't use composite video, I'd recommend getting a VGA box and hacking it to output 15kHz (simple as cutting a wire in most cases).

[NFG]

There are two 15kHz RGB modes with the dreamcase, you might try the other mode. You'll have to hack the DC end of your cable to move the ground wire from pin 6 to 7 (or vice versa) to see if it helps.  Note that not all games support both modes.  It's weird that way.

[Darklegion]

Make sure that the cable is connected to the csync pin and not the composite video pin.The composite video pin is disabled during rgb mode....and as other people have said I doubt it would be able to strip the sync signal from it anyway.You could also try combining horizontal and vertical sync together(or running them seperately if the monitor supports it).These pins are active in rgb mode not just vga mode.

[TuZhao]

Thank you all for the quick answers!
I didn't think about this "double ground / sync signals" thing. Now I'll take a look at my SCART cable to see which ground I'm using for the monitor.
The VGA box mod would work only for VGA-compatibile games, wouldn't it?

[TuZhao]

The ground pin I'm using for the arcade monitor is the pin 7.
And the sync signal comes from the pin 10 - according to the pinout, this should be the composite sync (which is good for the MTC9000 whose manual states "Horizontal or composite sync").
Should I try to use the pin 6 for sync instead?

[Tom61]

The VGA box mod would work only for VGA-compatibile games, wouldn't it?

A hacked VGA box is seen the same as a RGB cable, and has the same compatability

Monitor side ground isn't what Lawrence is talking about, he's talking about shorting a certian pin to ground to change the mode of output of the Dreamcast. As far as your monitor, any ground should work.

There are two 15kHz RGB modes with the dreamcase, you might try the other mode. You'll have to hack the DC end of your cable to move the ground wire from pin 6 to 7 (or vice versa) to see if it helps. Note that not all games support both modes. It's weird that way.

First I've heard of this, could you make a note of this on the Dreamcast A/V pinout page? Might be helpful to some.

[TuZhao]

The thing I don't understand is how come the DC sees differences between a standard TV and the arcade monitor. It looks like the DC doesn't even output the sync signal (disconnecting the sync pin gave the same distorted display), so why the TV display is just fine?
I disconnected the +5V pin - could it mean something?