Dreamcast issue - 60Hz on PAL flickering, sync issue?

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MLConian
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Joined: Mon Apr 05, 2021 2:29 pm

Dreamcast issue - 60Hz on PAL flickering, sync issue?

Post by MLConian »

So I recently bought a Dreamcast (22 years late to the party, yes) and I'm now noticing some strange things that I hadn't heard of before. Most notable is the banding at the top of the screen when playing 60Hz games on a PAL Dreamcast.
Whenever I play either an imported NTSC game, set a PAL game to 60Hz or play a PAL game in full screen as opposed to letterboxed (Skies of Arcadia does this, for instance - I don't know if it runs at 60Hz or not, but it just boots into full screen), I get a flickering band at the top of the screen. It's most notable on blues. I managed to capture a video of it, playing Sonic Adventure 2 NTSC-U on my PAL Dreamcast.

Before coming here and trying to solve this problem I've found the following:
  • - I've tried this on 2 PAL Dreamcast consoles, both have the issue. I've also read about quite a number of people having this problem, but I'm not sure if ALL PAL Dreamcast consoles have this.
  • - The flickering band is only visible on CRT, I think the deinterlacing my flatscreens apply also filters out the flickering.
  • - I've tried both Dreamcast consoles on both my CRT tvs. I use these CRTs with OG Xbox and GameCube running full screen PAL60 games without any issue, so it's definitely coming from the Dreamcast units.
  • - Both Dreamcasts have been hooked up through composite and S-video cables, with not difference. I've also found people on Reddit stating they're using the RGB cables from Packapunch with the same result. The issue seems to be gone when using VGA, but as some of the very reasons I bought a Dreamcast don't support this, I'd really like to get the other video output options working properly (Skies of Arcadia).
Because people who were using the RGB cables even had issues, I started comparing all video cables and found one thing that composite, S-video and RGB have in common, that is not present on VGA cables: pin 10. All cables, except for the VGA cables, use pin 10 on the Dreamcast AV out for CSYNC, whereas the VGA cables use 2 different pins for separate HSYNC and VSYNC. According to the SEGA Europe service manuals I found, the Dreamcast GPU generates HSYNC and VSYNC which is then combined by a chip to create CSYNC. However, this chip seems to also be linked to the video system/region of the console (in the service manual, there's a label with "NTSC/PAL" pointing toward that chip). This leads me to believe that in PAL consoles, this chip is tuned to 50Hz games and it simply can't keep up with the demand of 60Hz, leading to the flickering band. The VGA mode completely bypasses this chip, so that would explain why the problem isn't present in VGA mode even on PAL consoles.

This is all just purely theoretical spitballing however, as I don't have a clue whether my theory holds any merit, and no idea how to go about testing this. Maybe someone with a PAL Dreamcast and a sync combiner could test whether 60Hz give any problems over RGB/composite/S-video - I think the Toro box takes HSYNC and VSYNC from the board and combines it for the RGB out?

I'd really like to be able to hook up my Dreamcast to my CRT and play in 60Hz without issues or additional hardware, but I'm a complete novice when it comes to this. I've only recently unearthed my old consoles, started collecting and appreciating the value of a good CRT (I'm really glad I never threw mine out), but I've never dealt with any video processing whatsoever. I'm willing to learn though!
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