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I finally got around to pulling out an old Toshiba laptop with CGA output to test against. It works fine, at 15.69kHz and 69.91Hz. Looking at the output signal through the scope produces this: https://imgur.com/a/LFWUP7o
Comparing with the WP-1’s output, the vsync pulse is much shorter, only four scanlines (191us) in contrast to the WP-1’s 15 (921us). Everything else looks pretty much the same. I think I have the equipment (a PSoC dev board) to reduce the pulse width, so I’ll give it a try, but surely the OSSC only really cares about leading/trailing edges, depending on the sync polarity?
…although I haven’t found any sync polarity setting in the OSSC menus, so maybe it’s trying to autodetect and the very long pulse means it can’t decide whether it’s positive or negative polarity?