G-Sync on FreeSync Monitor with less than 2.4:1 range

Talk about NVIDIA G-SYNC, a variable refresh rate (VRR) technology. G-SYNC eliminates stutters, tearing, and reduces input lag. List of G-SYNC Monitors.
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Der_KHAN
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Joined: 16 Jan 2019, 09:20

G-Sync on FreeSync Monitor with less than 2.4:1 range

Post by Der_KHAN » 16 Jan 2019, 09:48

Hi everyone,

now that G-Sync can be enabled on FreeSync monitors I want to set it up on my LG 38UC99-W. Sadly this monitor is only supposed to use FreeSync between 52 and 75Hz. I can enable G-Sync in the nvidia control panel and between about 58 and 75 fps everything seems very smooth. Anything below shows massive stuttering so far. I have also enabled V-Sync in the control panel and disabled it in-game. I'm only interested in getting rid of the stutter for now.

My question is: what happens here if I drop below the minimum FreeSync range of 52Hz? A display with a range of 2.4:1 would be able to just scanout a frame twice, effectively doubling the framerate and refresh rate. My monitor, however, can't double anything between 37.5 and 52fps, because that would exceed the maximum refresh rate of 75Hz.

So will V-Sync kick in between 37.5 and 52fps? And will G-Sync activate again below 37.5fps?

In the past I have been using Adaptive Sync at 60Hz, so that I wouldn't get capped at 30fps if my pc wasn't able to run a game at 60+fps with V-Sync enabled. Would I be better off if I turned off V-Sync in the control panel and just used a FPS limiter in order to avoid the 30fps cap when my game drops below 52fps?

Could I possibly extend the lower limit of my FreeSync range with the Custom Resolution Utility, like AMD users can? On my old MacBook pro the Monitor will only work at 30Hz, so I know that it can handle lower refresh rates.
LG Electronics 38UC99-W, 37.5", 3840x1600, 30-125kHz (hor), 56-75Hz (vert), Adaptive Sync/​AMD FreeSync 52-75Hz
Gainward GeForce RTX 2080 Ti Phoenix GS (connected via DisplayPort)
NVIDIA Driver 417.71 WHQL (2019.1.15)
Windows 10 Enterprise 1809

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Chief Blur Buster
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Re: G-Sync on FreeSync Monitor with less than 2.4:1 range

Post by Chief Blur Buster » 16 Jan 2019, 19:55

Der_KHAN wrote:My question is: what happens here if I drop below the minimum FreeSync range of 52Hz? A display with a range of 2.4:1 would be able to just scanout a frame twice, effectively doubling the framerate and refresh rate. My monitor, however, can't double anything between 37.5 and 52fps, because that would exceed the maximum refresh rate of 75Hz.

So will V-Sync kick in between 37.5 and 52fps? And will G-Sync activate again below 37.5fps?
First, get familiar with Understanding Display Scan-Out Lag With High Speed Video, and realize that not all pixels refresh simultaneously. Now, after playing those high speed videos at the above link, come to this post, and study this graph:

Image

What happens is that when a refresh cycle occurs on a VRR monitor
1. Not all pixels refresh at the same time.
2. It takes (1/Max Hz)second for a monitor to finish refreshing.
3. Monitor is "busy" scanning out a refresh cycle for (1/Max Hz)second.
4. Therefore, your monitor is busy for 1/75th second once it's begun a refresh cycle (whether a fresh one or a repeat-refresh one)
5. Repeat refreshes occur automatically when your frametime exceeds (1/Min Hz)second -- In your case, 1/52sec.
6. If your monitor is below VRR range, your graphics drivers will automatically begin repeat-transmitting a refresh cycle no longer than 1/52sec.
7. During this time your monitor is automatically busy for 1/75sec right after that, which means it will "block" the next frame (ala VSYNC ON behaviour) until it's finished that repeat-refresh scanout pass.
8. That length of "blocking wait" creates a stutter + slight amount of lag. The wait length will depend on when the next frametime is between (min-vrr duration) thru (min-vrr duration + scanout time). For you, that's a time range of (1/52sec) thru (1/52sec + 1/75sec). That's a range of 19.2ms frametime through 32.5ms frametime for your specific monitor (1/52=0.0192sec) and (1/75=0.0325sec).
9. If the new frame arrives between those two frametime (e.g. exactly between 19.2ms and 32.5ms), your monitor will block the next frame (next refresh cycle) until 32.5ms after the last successfully-begun refresh cycle. Because the monitor is still unfinished scanning out the repeat-refresh.

Low framerate compensation requires a wider VRR range, more than 2:1, so that the monitor/drivers can intelligently repeat-refresh quickly enough not to collide with an attempted frame delivery while the monitor's still busy refreshing the previous frame. You need at least 2:1 for LFC (Low Frame Rate) compensation algorithms, preferably 2.4:1 because repeat-refreshing essentially doubles Hz, e.g. 24fps LFC is 48Hz on a display that has a "30Hz-144Hz" VRR range. Now if your range is not big enough, that means LFC is impossible, and you're permanently stuck with a narrow VRR range.
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       • Official List of Best Gaming Monitors
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Der_KHAN
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Re: G-Sync on FreeSync Monitor with less than 2.4:1 range

Post by Der_KHAN » 17 Jan 2019, 06:27

Thank you for your detailed reply with the linked article on display Scan-Out lag. I suppose my monitor is not a high end gaming type and only does full frame buffer processing instead of line buffer processing. I am not sure I completely understood the numbers, though. If I drop below 52fps, the monitor will go back up to 75Hz and behave like G-Sync is disabled. Did I get that right?

In the meantime, I have done a little more testing and changed a few things. I have set the G-Sync mode to fullscreen only instead of full screen and windowed, updated the monitor software/firmware from 3.06 to 3.11, disabled the overdrive function under "Game Adjust">"Response Time" (which didn't really seem to change anything) and most importantly, I used Custom Resolution Utility 1.4.1 to set the lower refresh rate limit to 30Hz (and then restarted the video driver with restart64.exe).

Both with G-Sync+V-SyncOff and G-Sync+V-SyncOn gaming at 55fps or 50fps seems a lot smoother now (V-Sync set in nvidia control panel). I should also mention that ocasionally the screen flickers black when starting a game or dropping the framerate significantly with an fps limiter. This has also ocurred before the recent changes I have made.

Now I can't be 100% sure about this, because I changed several things at once, but at first glance it looks like CRU was indeed able to change the lower limit of the vertical refresh rate. Is there maybe some tool that I can use to verify which refresh rate the monitor is actually currently running?
Attachments
CRU_LG-38UC99-W.jpg
CRU_LG-38UC99-W.jpg (66.58 KiB) Viewed 758 times
LG Electronics 38UC99-W, 37.5", 3840x1600, 30-125kHz (hor), 56-75Hz (vert), Adaptive Sync/​AMD FreeSync 52-75Hz
Gainward GeForce RTX 2080 Ti Phoenix GS (connected via DisplayPort)
NVIDIA Driver 417.71 WHQL (2019.1.15)
Windows 10 Enterprise 1809

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