Dreamcast sync issues w/ firmware 0.77a

[BuckoA51]

Update again – Traced previous problem to bad HDMI connection. Now had the Dreamcast running over 1hr on a 1.6 and no issues.

[pelvicthrustman]

I’ve done some more testing and realized I made a mistake – I forgot to restore the DTV 480p setting after initially setting it (I realize that’s what profiles are for, I just forgot after turning the OSSC on and off, new owner mistake). This led me to discover the reason I didn’t notice the problem on the first day I got the OSSC. The problem occurs only when 480p in Sampler is set to either Auto or VESA. The device heat seems to have no effect on signal stability in DTV mode, while changing the sampler setting instantly destabilized the signal after a long operating period. I ran the Dreamcast for 90 minutes @ 480p in DTV mode with no issues, flipping sampler back to VESA or Auto after operating for this long caused the LED to flash red and the LCD to show the strange 525p/525i/1050i output you can see in the pictures from my previous post.
Setting sampler back to DTV 480p returned the signal stability immediately.

@BuckoA51 I’m sorry I didn’t realize the above sooner, did you test the OSSC 1.6 in DTV mode?

I understand that DTV 480p mode is the recommended setting for the Dreamcast but I assume Auto/VESA still shouldn’t exhibit this kind of problem, right?

[BuckoA51]

Yes DTV 480p but I’ll re-run the test in 640×480 mode as you say shouldn’t be happening in any mode.

[James]

Just to update, my problem turned out to be the VGA box and not the ossc. No blackouts whatsoever ever on a 1.5 OSSC with 0.78 firmware.

[BuckoA51]

Thanks, haven’t had time to re-test yet but your report helps narrow things of course.

[faxesystem]

Hi,

I just got the time to play around with my OSSC 1.6 now. I already updated to firmware 0.79a. I’m using the OSSC on a BenQ RL2455 connected via a very good, thick and shielded HDMI cable. Right now I’m using my Dreamcast with a Blaze DC VGA adapter.

Here is what I’m getting.

Passthrough DTV 480 – Picture but losing sync and it get’s worse the longer I play
Passthrough VESA 640×480 – Picture without dropouts
2x480p DTV 480 – Picture without dropouts
2x480p VESA 640×480 – Picture without dropouts

Also when using the sample auto mode it’s choosing VESA 640×480 instead of DTV 480.

I guess it’s some incompatibility with my monitor but maybe there is another solution? I really like to use passthrough + dtv480 +manual scanline on 2D games. Also 3D games look better in passthrough since it’s a bit more blurred.

Thank you for you help.

[marqs]

I believe these issues with DC are related to the strict hsync/vsync alignment requirement of the digitizer chip. Alignment might be different between various VGA modules (if sync signals are not directly passed thru). Secondly, v1.5 and older OSSC PCBs have different R12 resistor (220ohm on v1.6, 330ohm on others) which also affects the alignment. Most certain fix is to use a sync combiner circuit and feed the signal to AV1, but you could also change R12 (e.g. back to 330ohm if you have v1.6 board) if you want to keep using VGA.

[BuckoA51]

All my tests are on a 1.5 OSSC and it’s working fine here with Dreamcast. Interlace via AV3 can be a bit hit and miss but that’s a separate issue. Strange…

[faxesystem]

@marqs would it be completely safe to change the resistor? I have all the tools to do that at work. So if there are no downsides in doing that I would try that before buying a Toro. And why were the resistors changed after all? 😀

Greetings,
faxe

[marqs]

Changing the resistor back to 330ohm is likely to degrade compatibility with interlaced signals, but you can work around that with “AV3 interlace fix” option on the menu.

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