Posted by Tom A. on 4, 2000 at 12:33 AM:
I still get a vertical jitter on my 29" arcade monitor that is in my JAMMA cabinet. I am using a TNT2 Ultra Video card with ArcadeOS and standard MAME under win98 DOS (bootgui=0). When I first turn on the cabinet/monitor, the picture is good for 30 to 60 minutes. After that time, the picture tends to jitter a bit vertically...basically it will get a little smaller and than back to normal size in a fraction of a second. It seems that after some time, say an hour or two, the problem lessens. This may not be the case, perhaps I just haven't noticed it, I'm not sure but it sure appears that it doesn't do it after an extended period of time.
I first thought it was something wrong with the monitor but I plugged in a Street Fighter II board and watched it for a couple of hours. No jitter whatsoever.
I use the fingerboard approach to utilize the JAMMA harness. I have a common ground for the R,G and B grounds (VGA pins 5,6 and 7) as well as the the sync ground (VGA pin 10) and the ground (VGA pin 5).
I have the horiz. sync (VGA pin 13) wired together with the vert. sync (VGA pin 14) to make the composite sync (JAMMA pin P) that the JAMMA harness requires.
Here's the meat of my question...I was looking at the schematic and I noticed that my monitor PCB has a connector for V.Sync but that it doesn't come from the JAMMA harness. In other words, the monitor PCB connector consists of Red (JAMMA pin 12), Green (JAMMA pin N), Blue (JAMMA pin 13), H.V. Sync (JAMMA pin P) and the last pin is V.Sync but it doesn't come from the JAMMA harness.
I checked the connector and there is a wire lead to this last pin. So, I am assuming that the monitor pcb takes the composite sync that a JAMMA pcb passes to the JAMMA harness and splits off the V.Sync signal and sends it to this pin while sending the composite sync to the H.V. Sync which I am assuming to be the composite sync as it is JAMMA pin P.
What I was wondering was, could I NOT twist the VGA pins 13 and 14 (horizontal sync and vertical sync respectively) and send the VGA horizontal sync (VGA pin 13) to the composite sync pin on the monitor PCB (JAMMA pin P) and the vertical sync (VGA pin 14) to the monitor pcb's V.sync pin?
Is my logic correct in assuming that this would eliminate the combining of the once separate horizontal sync and vertical sync from the VGA card only to have it separated again? Could this possibly give me a more stable sync that would possibly eliminate my jitter?
I'm most likely going to try this but before I do, I would like to know if my logic is way off as I am just making this up as I am going, I have no technical experience with electronics, I'm just winging it. ;-)
Am I making this more complicated than it really is? Could it be my video card instead that perhaps doesn't put out a stable sync that would account for the occasional jitter?
Any comments are appreciated!
-Tom A.