Freesync stutters when close to 50hz

[WDeranged]

I have an ASUS ProArt PA278QV which is supposed to support a Freesync range of 48~75Hz. I've noticed that the closer I get to 50fps the more stutters I'm seeing. Anywhere between 60-75fps is smooth but once I start dropping below 60 the stutters start appearing.

My specific use case is a Commodore Amiga emulator running in Retroarch. It's a 50hz PAL machine so 50fps is required for smooth scrolling. Other emulators that require 60hz NTSC are perfectly smooth. Normal PC games are also smooth. Though I did an experiment and limited the framerate to 50fps, I saw the same stutters.

Frametime graphs are level. My PC isn't stressed either. Is it possible my monitor just doesn't have a good Freesync implementation? It seems the closer to 50hz I get the more stutters I see.

[RealNC]

Yeah, the monitor's freesync isn't very good. It isn't surprising since freesync requires a freesync range where the upper limit is more than double that of the minimum. Like 48-120 or 48-144. If you don't have that, you can expect more issues with low framerates. Normally what would happen here is that freesync would instead use 100Hz instead of 50Hz and double each frame to prevent stutters. But since your upper limit is 75, this doesn't work.

What you can do here is switch to 50Hz fixed refresh rate (if the monitor doesn't support that, you can try adding a custom resolution for it with CRU) and then use WinUAE and enable its beam racing lagless vsync, which was implemented in WinUAE 4.0.0:

https://www.winuae.net/2018/06/20/winuae-4-0-0/

If the monitor behaves well at 50Hz with no frameskips, this should give you perfect motion with zero input latency. It's best to do some research and find out the exact refresh rate of a PAL Amiga and then try and get as close to that as possible in your custom resolution.

[WDeranged]

Thanks for the reply. I'm definitely leaning towards my monitor not being able to handle riding so close to its lower limit. I can run 50hz fine without freesync, in WinUAE and Retroarch.

The only thing keeping me from using WinUAE is the lack of advanced shader support. Mega Bezel CRT shaders in retroarch are too good to live without.

[Chief Blur Buster]

Frametime graphs are level. My PC isn't stressed either. Is it possible my monitor just doesn't have a good Freesync implementation? It seems the closer to 50hz I get the more stutters I see.

If you want to fix low-framerate stutters, purchase a new monitor with a VRR maximum Hz about 2.5 times the VRR minimum Hz.

The average LFC stutter penalty is always the halfpoint of MaxHz, or rather 0.5/75sec = 6.7ms average stutter error with a stutter error range of [0..13.3ms]. Remember, 13.3ms stutter error translates into 13.3 pixeljump at 1000 pixels/sec motion, or more than 50 pixeljump at 4000 pixels/sec motion.

Even if your games only run at 75 frames per second, having 240Hz as the maximum Hz will:

(A) Reduce your LFC stutter penalty dramatically to generally undetectable levels
(0.5/240sec = 2ms stutter error for 50fps sample and hold is generally invisible);

(B) Reduce the latency of low frame rates, since your 50fps frames are refreshed in 1/240sec.

Why not try one of those new 240Hz OLEDs? Even mere browser scrolling looks ~4x clearer than 60Hz OLED, and 5-6x clearer than 60Hz LCD, so there's everyday-user ergonomic benefit too. Also, software-based BFI works better at 240Hz, which means you can reduce emulator motion blur by 75% by using 240Hz BFI on your 60fps content.

Frametime graphs are level. My PC isn't stressed either. Is it possible my monitor just doesn't have a good Freesync implementation? It seems the closer to 50hz I get the more stutters I see.

Some possible workarounds:

(A) Try use ToastyX CRU to manually range-edit your VRR LFC threshold lower. Force your monitor to refresh as slow as 40Hz.
Many 48-75Hz LCDs can refresh 40-75Hz, so you might be able to wiggle by.

(B) Try use microsecond accurate frame rate capper such as RTSS to framepace it accurately to stay above LFC threshold. In-software cappers may fluctate where some frames are 1/47sec and other frames 1/53sec. Those below-Hz frames will stutter badly, as an averaging-based framerate capper tries to framepace 50fps so close to the threshold, with some frames going below min Hz threshold. A more accurate capping algorithm could avoid this.

You're using WinUAE, there's something even better
When using WinUAE, try using 50Hz fixed-hz and its Lagless VSYNC mode. It has less lag than VRR. It uses actual beam racing that I helped Toni implement to raster-beamrace the realraster with the emulator raster Using Lagless VSYNC Algorithm. Use this beamraced sync mode as an alternative instead.

[Chief Blur Buster]

The only thing keeping me from using WinUAE is the lack of advanced shader support. Mega Bezel CRT shaders in retroarch are too good to live without.

If you want CRT filters on your retro boxes or CRT filters on a PC running WinUAE, I have another idea solution for you: Retrotink 4K
www.retrotink.com/post/retrotink-4k-blur-buster-approved?ref=area51

An external Retrotink 4K can accept an HDMI (from PC) or composite/svideo/rgb (from Amiga), and output to a modern monitor with CRT filters AND black frame insertion added to the signal. It's Blur Busters Approved as I worked to add simultaneous BFI+CRT filter support, ala TestUFO Variable Persistence BFI (view this link on 240Hz, don't view at 60Hz). I've tested it up to 720p 360Hz, but probably works with 480Hz at lower resolutions. 120Hz (up to 1440p) will reduce blur by 50%, and 240Hz (up to 1080p) will reduce blur by 75%.

Note: Blur Busters earns a small commission via RETROTINK links suffixed in "?ref=area51"