Post by themanfromsaturn on Feb 27, 2019 18:20:41 GMT
I've modified my Japanese model 2 system with a 50/60Hz toggle switch and a region switch.
I know that most PAL games will run OK in 60Hz mode, but for completeness and to support those games that might have issues, I included the ability to toggle between 50/60Hz. I did this by lifting pin 79 on the VDP2 chip and the switching appears to be working.
I'm using one of those cheapo Euro RGB SCART cables off of eBay (the ones with the awful shielding) and it works fine in 60Hz mode.
However, when I switch the Saturn to output at 50Hz, which I understand is basically NTSC-50, the TV seems to continuously lose the image, i.e. it goes black, then the image reappears every few seconds. The same TV has no issue with my UK PAL Saturn running at stock 50Hz using the same cheapo SCART cable.
I thought that given I'm using an RGB SCART cable that the fact that the Saturn is outputting NTSC-50 would not be a problem (unlike s-video or composite where I understandably get a monochrome image due to the different colour encoding).
Can anyone shed any light on why this might be happening and what I'd need to do (re-wire the cable, mod the Saturn?) to get a stable image in 50Hz mode? Could it be the TV (I don't have another one to test with right now though)?
I imagine it is because those "cheapo Euro RGB SCART" cables use the +9V on PAL units to get the RGB switching voltage, but that pin is c-sync on JP machines. Simply put, they are designed for PAL units, which have a different a/v pinout.
A less likely issue is that a NTSC unit modded for 50hz doesn't output exactly 50Hz but a slightly off signal. However I doubt this would give you the symptoms described above, this usually just gives you some slight tearing on digital TVs (and no problems at all on CRTs).
Post by themanfromsaturn on Feb 28, 2019 8:29:07 GMT
I checked the cheapo SCART cable pin connections against the Saturn's AV port. It seems that pin 1 on the AV port (the raw sync) is not actually wired up in the SCART cable.
The cable takes the 5V available on pin 4 of the AV port and sends this to pin 8 and pin 16 (via a resistor) on the SCART connector. The 5V isn't enough to coerce the TV (it's a modern LCD TV) into 4:3 aspect ratio, but it's enough to switch it to RGB mode.
I guess it could be something about the quality of the cable, but the same cable works fine on my PAL Saturn which is modded to work at 50/60Hz, so I'm starting to wonder whether it is something about the sync on composite video that the TV presumably uses from pin 20 on the SCART. Perhaps the TV is not so happy with the sync it gets from the Japanese Saturn at 50Hz?
I wanted to rule out the cable quality by using my original Sega PAL SCART cable, but I'd need to mod the Japanese Saturn to send +9V or maybe +5V to pin 1 of the AV port instead of sync, right?
Post by themanfromsaturn on Mar 7, 2019 21:18:25 GMT
Given the Japanese Saturn would actually output NTSC-50 (or thereabouts) in 50Hz mode, is there any way of making it output PAL-50 by hardware modification? If so, what changes would be required to achieve this?
I know it would probably be pretty pointless to do such a modification, but I'm curious
If your machine uses the Hitachi 315-5746 PLL (IC20), you can change X1 to a 17.734475 MHz crystal and swap JP1/JP2 around, to turn a NTSC machine into PAL.
If it uses a Cypress PLL, then you can't mod it, not without making your own PLL and programming it to the proper inputs/outputs (there are at least two).
I'd just keep two separate machines, or a PAL machine with a 50/60 switch is you are intent on only having one machine. Or just keep a JP machine in 60hz mode and toss the PAL titles; I can't name one PAL exclusive, PAL optimized title, that is worth playing.
Post by themanfromsaturn on Mar 14, 2019 9:51:25 GMT
Thanks for the info. I think I'll keep it as it is and as you suggest play the NTSC titles
Going back to the issue I was having with the unstable image when running the Japanese console in 50Hz mode, I am now pretty sure it was the TV.
I recently acquired another LCD TV for testing and it works fine... most of the time. There are occasions when the image repeatedly flickers or is offset vertically, but switching the TV input away from SCART and back again, or power cycling the TV will normally after one or two attempts get it to produce a stable image.
It isn't perfect, but it does handle it, whereas the other TV didn't. Maybe this will be useful for anyone else observing the same issue.