OSSC Loses Sync Randomly with 15KHz JAMMA from Konami System573

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    I’ll try to explain my setup, and it is definitely possible there’s something wrong outside of the OSSC. However, reading the wiki there’s a few things that stand out to me and I wanted to see if anyone could help.

    I am attempting to capture input from DDR 5thMIX on a System573. I have an RGB splitter that I’ve built myself based on the THS7374 and SN74HCT32D for handling sync buffering of the SYNC signal. The splitter input is attached to the video lines going directly to the cabinet monitor (RGB, sync and gnd), and I was able to successfully run 31KHz video from a newer board for several hours with no problems on the OSSC. I bring this up to say that I am fairly confident in the connections between the cabinet and the OSSC. Because I have RGBS with TTL sync, I am running video to the OSSC using a VGA cable on AV3. I have RGB on pins 1-3, pins 5, 6-8 and 10 connected to GND, sync connected on 13 and the rest of the pins floating. Shield is not connected to digital ground.

    The OSSC picks up and retains sync most of the time on System573, even when switching between 240p and 525i (old DDR loves to do this for text-heavy sections and in-game display). When the sync is lost, it does not appear correlated to any bright flashes or resolution switches. Sometimes it will hold for 5-10 minutes, and sometimes it will lose sync every second or so. I’ve hooked up an oscilloscope to the sync line on pin 13 and verified that it is 5V TTL and the shape of the signal does not appear to change when the OSSC loses sync.

    I noticed this line in the wiki “interlaced content via AV3 in RGBHV/RGBS mode requires a hack to work (TVP7002 issue).” but I can’t find any elaboration on this. Could this be an issue? I don’t think the sync LPF has anything to do with AV3 RGBHV/RGBS, right?

    This may be related, but I am not sure. In my debugging I ended up making a male<->male connector to MITM the signals easier and hooking only pins 1-3, 5 (hsync gnd) and 13 (sync) and this seemed to alleviate the issue. However, given that it is intermittent I can’t say *for sure* that it was just deciding not to act up while I had my eye strictly on it. It wouldn’t make sense for additional grounds to cause interference, right?

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