Judder With Horizontal Scrolling Over VGA PC CRT Monitor

[willinliv]

Hi,

Apologies if this has come up already. I've been trying to resolve an issue with stuttering of the image with Apogee games when scrolling horzontally, for example Keen Dreams. I have run through a number of steps to try and remove the issue. Initially I was using a Dell LCD screen, but thinking the issue was the limited 60Hz refresh rate I got hold of a couple of old PC CRT monitors: a Gateway2000 monitor, and a iiyama Vision Master Pro 411, but these still have the issue.

I believe I had configured the mister ini correctly to give them the best chance of optimal performance:

Connected via the VGA port on the I/O board
Config.ini with vga_scaler=0 for ao486 (v 22.09.14)
OSD setting with Vsync=variable

With this setup I am getting in game:
320x240 15,74KHz 70.1Hz
1280x1024 108Mhz 60Hz

However the screen judder still appears and I am left scratching my head. 70Hz is what I was expecting so I wonder what causes this. I tried swapping the screens but both had the same issue. I have some original hardware and I tried the same screens on that and they did not have the issue. I have also tried changing forced_scandoubler, vscale_mode (I thought these might only apply to the scaled HDMI anyway).

I had read online somewhere that the Apogee games had perculiar refresh rates so I don't know if this might be the case. I notice that Crystal Caves is horrendous until you enable the alternative video card support. I am also watching this thread with interest viewtopic.php?t=5396 and also https://github.com/MiSTer-devel/ao486_MiSTer/issues/10 . Has anybody got any other ideas or is this just how it is?

Thanks, Will

[FoxbatStargazer]

The top number is the "native" resolution. For keen dreams it really should be 320x200 @ 70.1hz. 240p is weird if that isn't a typo.
The bottom number is the scaled resolution, which is what goes out to even over the vga port in the ao486 core. (i.e. it's like vga_scaler=1 is forced on.) Note that it is 60hz, which smells like vsync_adjust=0, which means the monitor is displaying 60hz instead of 70. You need vsync_adjust=1 or 2 to get the refresh rate to match.

Still, I don't think that accounts for the weird hitching behavior, which is more like the screen going back a step before going forwards. I remember seeing this on Keen4 with one of the "fast scrolling" settings within the game's own menu that you can toggle to fix the issue. Don't know if Keen Dreams has something similar but it seems to be a similar engine so perhaps.

[akeley]

I had a quick look and yeah, there's an option called "SVGA compatibility" in game that causes this massive judder (Keen 4).

Other than that setting vsync_adjust=2 won't hurt, though I assumed you're using the settings from that thread you linked, and they have it in already?

Even with all these settings there is still a slight jitter when scrolling - but this just might be an in-engine thing (I don't have my real PC atm to compare).

[willinliv]

Thanks guys for this. I didn't know that the second line comes into play still, (i.e. VGA scaler forced on) but had noticed some weird things: more testing and learning needed by me! I'll take a look at keen 4 and vsync adjust 2. Cheers

[thorr]

The judder in that video doesn't look at all like a modeline / scaler issue. It looks more like a software issue. There are different VGA BIOS's available (Trident). Perhaps try running with a different VGA BIOS, or maybe try it on PCXT if that core can handle the game.

[willinliv]

Hi thinks again guys for your help with this. After a brief hiatus I have come back to it, trying again with a CRT, this time for ease plugged in via a HDMI>VGA dongle

FoxbatStargazer - thanks again for the info, just to confirm in case useful for others that I had made a type and resolution was indeed 320x200 15.74KHz 70.1Hz. Since tried with vsync_adjust set to 1 or 2 and the refresh rate of 70Hz now matches on both lines ('native' and output). vsync 2 gives a lot of blank screens for me inbetween video modes so 1 it is. However, like you suggested this did not solve the judder. In Keen 4 I see those settings, and I don't seem to have an issue on K4 anyway, but I have experimented. Could not see anything similar in Keen Dreams, ao maybe that's the way it is. I can live!

Thanks akeley and thorr - tried out with a few different bios including Trident and ET4000W32 and still much the same, might not have done correctly though. Also not tried PCXT which I'd not known existed but interesting to give it a try - like you say might struggle with Keen Dreams but will give it a shot maybe with the EGA Keens.

Even with this small judder, coming back to it after a break AO486 does look great on a CRT! I just need a new desk!

Thanks again, Will