Dreamcast Snow Screen

[SegaFanboy]

Hello,
I have some huge problems with my new OSSC. It works fine with Mega Drive, Snes, Gamecube and N64, but I don’t get the Dreamcast with VGA running. I use the recommended settings from the wiki: DTV 480p and H.active 640, to get the right aspect ratio.

I can play for 15 minutes, then I get a black screen, then a snow screen and then it goes out of sync. If I switch the OSSC off and wait 1 minute, I can play for another 15 minutes. It makes no difference if I use passthrough or Linex2.

My TV is a Samsung UE55JU7500 and it seems to like the OSSC, I can even run the Snes with LineX5. I made a small video, sorry for the quality:

Please help me.

[paulb_nl]

There is a thread here about Dreamcast sync issues: https://videogameperfection.com/forums/topic/dreamcast-sync-issues-w-firmware-0-77a/

[SegaFanboy]

Unfortunately this isn’t the same problem as I have. In the thread the setting DTV 480p fixes the problem. I’ve done some testing and realiced, that DTV 480p causes the sync issues. It runs perfect with “Auto” and “Vesa”. But with this settings the picture is too narrow and it’s not centered. This is why I bought the OSSC, because besides the Framemeister it’s the only device, which outputs the right aspect ratio with the Dreamcast.

I tried it on my PC monitor and another TV, always the same. On the monitor I didn’t have the snow screen, it went right out of sync after 30 minutes. Everything is fine when I put “480p in sampler” to “auto”.

[nmalinoski]

Which cable/device are you using to get VGA out of your Dreamcast?

[marqs]

Which board revision you have? I think all DC VGA sync issue are related to an issue in digitizer chip which has strict requirement for hsync/vsync alignment. Resistor R12 (which controls vsync delay) was changed from 330ohm to 220ohm for v1.6 PCBs since that improved compatibility of interlaced signals via VGA. However, that might have had a negative effect on some progressive sources as previous issue reports have all been for v1.6 PCB.

[SRRAE]

Does this happen when using other TVs?
I:ve played dreamcast for hours via the VGA and not had an issue.

[SegaFanboy]

Thanks for the answers. I’m using this transparent VGA box, a few years ago ebay was full of them:
https://www.estarland.com/product-description/SegaDreamcast/Dreamcast-VGA-Box-Converter/14472
Never made any problems, I played hundreds of hours with it.

I have a 1.6 PCB, it arrived last week. So it’s probably a hardware problem, that sounds bad…

[paulb_nl]

Can you try firmware 0.76? In the other thread it was reported that it started from firmware 0.77. Also try to change some sync settings for example H-PLL Pre Coast 3 and Post Coast 3.

[SegaFanboy]

The coast settings don’t help, but with firmware 0.76 everything is fine. The changelog of 0.77 says
-Sync processing rewritten, fixing interlace detection issues etc.
Seems that something went wrong there. Let’s hope that will be fixed, because I don’t have sound on 0.76 (downloaded the audio version).

[marqs]

Does OSSC report lost sync on its LCD screen and status LED?

[SegaFanboy]

No, it doesn’t. LED is green and the screen info doesn’t change. Even if the picture on my Tv or monitor is completely gone.

[paulb_nl]

When you press the info button on your remote does the OSSC stay on the info screen? In the past when I had slightly unstable sync with a SNES 1-CHIP the OSSC didn’t lose sync (green led) but the output lines would vary(could see it by pressing the info button repeatedly) so the display could not sync. In recent firmwares the OSSC will exit out to the main screen if the output lines change.

[SegaFanboy]

Ok, I made some testing totay (0.79). I was wrong about the red light, sometimes it flashes. Most of the time it stays green though. When I press info it goes back to main screen.

I was able to record the flashing. The resolution changes from 525p to some weird 500i and above 100Hz. On 0:30 I press “info”, it goes immediately back to the main screen.

[marqs]

Such behaviour indicates that the digitizer chip cannot get proper sync lock, or just reports incorrect parameters to processing logic. I’m not sure why that doesn’t happen with 0.76, but you could try freezing mode change detection to rule out the latter option? That can be done simply by e.g. pressing profile hotkey button while sync is still working, but without selecting a profile. The result whether that prevents subsequent sync loss or not should give me some hints where the problem might lie.

[SegaFanboy]

Thank you very much, but this also doesn’t help. The problem must be somewhere else. I tried everything and now I give up on this, there is just no way to get it running on 0.79. I ordered a Toro, probably the problem goes away if I send the 480p through RGB. Or probably we will see a fix for it soon.

Thank you all for the help, this is a really great community.

[Author]

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