G-SYNC 101: G-SYNC vs. V-SYNC OFF w/FPS Limit


At the Mercy of the Scanout

Now that the FPS limit required for G-SYNC to avoid V-SYNC-level input lag has been established, how does G-SYNC + V-SYNC and G-SYNC + V-SYNC “Off” compare to V-SYNC OFF at the same framerate?

Blur Buster's G-SYNC 101: Input Latency & Optimal Settings
Blur Buster's G-SYNC 101: Input Latency & Optimal Settings
Blur Buster's G-SYNC 101: Input Latency & Optimal Settings
Blur Buster's G-SYNC 101: Input Latency & Optimal Settings
Blur Buster's G-SYNC 101: Input Latency & Optimal Settings
Blur Buster's G-SYNC 101: Input Latency & Optimal Settings

The results show a consistent difference between the three methods across most refresh rates (240Hz is nearly equalized in any scenario), with V-SYNC OFF (G-SYNC + V-SYNC “Off,” to a lesser degree) appearing to have a slight edge over G-SYNC + V-SYNC. Why? The answer is tearing…

With any vertical synchronization method, the delivery speed of a single, tear-free frame (barring unrelated frame delay caused by many other factors) is ultimately limited by the scanout. As mentioned in G-SYNC 101: Range, The “scanout” is the total time it takes a single frame to be physically drawn, pixel by pixel, left to right, top to bottom on-screen.

With a fixed refresh rate display, both the refresh rate and scanout remain fixed at their maximum, regardless of framerate. With G-SYNC, the refresh rate is matched to the framerate, and while the scanout speed remains fixed, the refresh rate controls how many times the scanout is repeated per second (60 times at 60 FPS/60Hz, 45 times at 45 fps/45Hz, etc), along with the duration of the vertical blanking interval (the span between the previous and next frame scan), where G-SYNC calculates and performs all overdrive and synchronization adjustments from frame to frame.

The scanout speed itself, both on a fixed refresh rate and variable refresh rate display, is dictated by the current maximum refresh rate of the display:

Blur Buster's G-SYNC 101: Scanout Speed DiagramAs the diagram shows, the higher the refresh rate of the display, the faster the scanout speed becomes. This also explains why V-SYNC OFF’s input lag advantage, especially at the same framerate as G-SYNC, is reduced as the refresh rate increases; single frame delivery becomes faster, and V-SYNC OFF has less of an opportunity to defeat the scanout.

V-SYNC OFF can defeat the scanout by starting the scan of the next frame(s) within the previous frame’s scanout anywhere on screen, and at any given time:

Blur Buster's G-SYNC 101: Input Lag & Optimal Settings

This results in simultaneous delivery of more than one frame scan in a single scanout (tearing), but also a reduction in input lag; the amount of which is dictated by the positioning and number of tearline(s), which is further dictated by the refresh rate/sustained framerate ratio (more on this later).

As noted in G-SYNC 101: Range, G-SYNC + VSYNC “Off” (a.k.a. Adaptive G-SYNC) can have a slight input lag reduction over G-SYNC + V-SYNC as well, since it will opt for tearing instead of aligning the next frame scan to the next scanout when sudden frametime variances occur.

To eliminate tearing, G-SYNC + VSYNC is limited to completing a single frame scan per scanout, and it must follow the scanout from top to bottom, without exception. On paper, this can give the impression that G-SYNC + V-SYNC has an increase in latency over the other two methods. However, the delivery of a single, complete frame with G-SYNC + V-SYNC is actually the lowest possible, or neutral speed, and the advantage seen with V-SYNC OFF is the negative reduction in delivery speed, due to its ability to defeat the scanout.

Bottom-line, within its range, G-SYNC + V-SYNC delivers single, tear-free frames to the display the fastest the scanout allows; any faster, and tearing would be introduced.



3625 Comments For “G-SYNC 101”

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anthony3192
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anthony3192

I’m looking for a dry and precise answer, if I set the reflex active, with gsync and vsync from the ncp, how should I keep the low latency mode active from the ncp? I keep it on ultra. I ask you this because for games that don’t have reflex cameras I’m forced to set the fps cap to obtain the lowest possible latency? Or is it also okay to enable gsync+vsync+ultra low latency? Another question, I read that you wrote that ultra low latency mode could have worse performance for 1%L, how do you recommend setting it? How did you set it all up? I would like a permanent setting so I can forget about it.

hentfreaka
Member
hentfreaka

Just to be sure: does it still not matter if i choose to Limit my fps via Nvidia (576.88) or RTSS?

Are both performing the same?

PS: I sometimes read Nvidia App might also cause frame time spikes. Any thoughts on that? I mainly use it for RTX HDR.

Krazzeekane
Member
Krazzeekane

I can’t find any mention of what to do with the ‘Triple Buffering’ option in NVCP/NVPI. I know to disable any in-game triple buffering options, but bo I set it the NVCP option to on or off? I read it only works for OpenGL games which I do play sometimes as I like older games. Just wasn’t sure if it would make any difference one way or the other, and I can’t find anything recent on about it.

I use a g-sync capable display (LG-27GR83Q-B), and have followed all the other options you listed and everything is running great. Just unsure about the Triple Buffering option.

tomatojuice
Member
tomatojuice

Why do you recommend disabling in game vsync in favour of Nvidia’s?

RS_Cypher
Member
RS_Cypher

Hi

Thank you very much for this write-up. Very informative!

I am however experiencing weird behaviour when using G-Sync/V-Sync in conjunction with any of framerate-limiting (be it in-game, through external limiters or even Reflex).

Whenever I use limiters, the refresh rate of my TV (LG G2) starts to fluctuate wildly, leading to VRR flicker. Any idea what might cause this? Would this be something caused by a firmware update on the TV?

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