[BUG] DOS VGA 70Hz mode not smooth – appears converted to 60Hz internally

[arncht]

Issue

When running DOS VGA mode at ~70Hz (69.77Hz detected), scrolling and motion are not smooth. Although the input is correctly detected as 69.77Hz and the monitor reports 70Hz output, the actual frame pacing behaves as if it is internally converted to 60Hz. The result is visible stutter/judder, identical to forcing output to fixed 60Hz.

Mode-X at 60Hz works perfectly smooth.

Test

DFP output mode: 1920x1080 (24-120Hz): Standard 1080p output supported by all modern HDTVs. [default]
HDMI VRR flag: Freesync
Framelock: On OR Framelock: Off (source Hz)

Tested with one DOS game and one demoscene demo.

[marqs85]

Which monitor/TV is in question, and does it behave differently if
a) Adaptive-LM mode is used instead
b) VRR is disabled?

[arncht]

Samsung Odyssey OLED G8 G85SB, 34"
a) same
b) no image without VRR

[marqs85]

I'm afraid the monitor is the culprit - perhaps it assumes maximum pixel clock in its VRR mode and does not work properly on native clock for 70Hz. That reminds I should port the monitor test firmware from OSSC Classic which would enable easily testing things like this.

[arncht]

Thanks for the quick feedback! One question:
How exactly does the OSSC Pro drive the refresh in VRR mode? Does it output the exact original rate (e.g. 69.77 Hz), or does it round to a standard value like 70.000 Hz?

I’m asking because maybe the issue comes from the monitor falling back to 60 Hz when it sees a non-standard refresh/pixel clock. If the output is always exact, perhaps having an option to round to the nearest VESA timing could improve compatibility with displays like the G8 OLED.

[arncht]

I also tried without VRR to see which resolutions/refresh rates the monitor accepts. Up to 1280×1024 it does accept 70 Hz input just fine, but unfortunately the scrolling is still stuttery – same issue as with VRR enabled.

[marqs85]

In framelock mode it outputs exactly at the same rate by definition. Enabling VRR flag does make no difference in timing, it's just metadata which gets altered. You could try 720x480p output mode with extremely long vertical blanking to emulate how 70Hz source using 120Hz VRR range from PC would be timed. Changing V.total from 525 to 900 should do it.

[arncht]

With V.total=900 I get a garbled/desynced image; with VRR enabled there is no picture at all.
It looks like the G8 rejects this non-standard timing. The VRR flag alone doesn’t help if the base timing isn’t accepted.

Would it be feasible to add an option to round the output refresh in VRR/framelock mode—e.g., drive a clean 70.000 Hz instead of the exact 69.770 Hz—and let VRR absorb the tiny drift? My hunch is that some displays (like the Samsung G8 OLED) behave better with standardized pixel clocks/refreshes and may otherwise fall back to internal 60 Hz handling when they see a non-standard rate. An optional “round to nearest standard (e.g., VESA/CVT-RB) refresh” toggle could improve compatibility.

[marqs85]

I doubt the tiny difference in refresh rate / pixel clock makes a difference for the monitor, but you could try it with framelock turned off and output set to 70.00Hz (in which case one frame every ~3sec would get duplicated).

The display might be expecting some metadata which is not provided by Pro, but there's very little openly available documentation on how Freesync metadata should be set up (and even less for VESA VRR).

[arncht]

Tried with output profile set to 70.00 Hz and VRR enabled.

Result: no visible improvement in scrolling smoothness compared to exact 69.77 Hz.

Observation: monitor still reports 70 fps, but motion looks identical to 60 Hz fallback, so I suspect the display internally reverts to a fixed 60 Hz mode despite showing the nominal refresh.