Konsolkongen wrote:I agree. I have two different MD1 boards, and I've never been able to completely remove the jailbars from either of them. I've even tried bypassing the CXA1145 using ArcadeTV's board, but that didn't make much difference, and for some reason they made column 7 and 8 of the 240p test suite colour pattern look indistinguishable, so I removed them again.
If you're using the OSSC the phase value can have a noticeable difference in how bad the jailbars show up, especially in 256x240p optimal mode, where the jailbars appear much more noticeable for some reason.
I was actually thinking about the bypass caps on the VDP last night. Did you increase the value of the ones already on the board? (C16, C17 and C18 if I remember correctly). Or did you add some in parallel? And did you see a difference doing that?
1000000% this. The ArcadeTV bypass board also made columns 7 and 8 of the colour test pattern on the 240p test suite indistinguishable, and the reason for that is ArcadeTV's board is missing components to show the colours at the correct levels, making them appear too dark. You need to add a 5k6 ohm resistor on each colour line
before they enter any of the components on the bypass board, and attach them all to 5V to pull them up. This will show the colours at the correct levels. I think they are missing on ArcadeTV's board because I believe his original intention was not to isolate the colour lines, and simply connect the board in parallel, because the pull up resistors are already present on the motherboard. The Mega Drive's 5V line is notoriously noisy, however, so I added my own resistors and connected them to the 5V supply on the bypass board, which taps its supply directly from the 7805 to avoid noise.
I didn't bother removing the other bypass caps, I just added another 100nf cap on each in parallel to the existing ones. It has made a difference in my opinion - correct me if I'm wrong but although there are decoupling caps already present on the analogue lines, I couldn't find them on the digital VCC and GND, so perhaps those are the ones that made the most difference.
I totally agree with you on the OSSC front, you can completely hide the jailbars in 256x224 optimised mode, but you need to have the picture slightly out of phase to do so. Because the optimal sampling rate of the Mega Drive for 320x224 optimised mode is 427.5 and therefore cannot be selected on the OSSC, you tend to get jailbars more on one side of the screen than the other, depending on whether you opted for 427 or 428, and your phase adjustment.