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.



2220 Comments For “G-SYNC 101”

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Clueless
Member
Clueless

I read the article multiple times now and I feel like I didn’t understand anything.

I have a GSync compatible Display (LG 32GP850-B), set to 165Hz. GSync fullscreen is enabled. VSync ist set to on. FPS Limit is set to 162.

When I open the test on page 1 (G-SYNC Simulation of Variable Refresh Rate) and test Quebec or Stars for example the result on top (GSync) looks ok, the result at the bottom however (VSync on) does not. I see microstuttering and a blur here and there. But the moment I disable VSync at the simulation it looks ok again.

How does this behaviour in a windowed browser window (that sounds weird) fit your recommendation of GSync on + VSync on? Thank in advance.

HopperTron
Member
HopperTron

This is an amazingly detailed site and thanks for it!

I’m curious about advice here with respect to G-Sync + V-Sync + LLMU (or in-game framerate limiting) vs. advice I’ve read elsewhere — mainly for competitive shooters — around _not_ using G-Sync and instead running as high FPS as possible. The theory is that since input is calculated per frame, you get lower input lag with higher FPS at the expense of tearing, which kinda makes sense in the abstract. But what I’ve read in these articles suggests that slowing down the frames sent to the monitor via G-Sync does not necessarily affect input lag — possibly because internally the render loop is still calculating input, it’s just not showing up on the screen? So maybe what people think is lower latency is actually the ability to be aiming during a partial screen refresh?

I’d be curious as to your thoughts. Going from a 60Hz monitor to 144Hz has been absolutely amazing and good graphical fidelity is lovely but I mostly play online shooters, so if there is truly an advantage to forgoing image quality for very very high FPS then it might be worth making various profiles.

Bounty
Member
Bounty

Hello!

Is there a difference between Low Latency mode “On” and “Off”? Does it have any impact on Gsync at all?

HashiruOtoko
Member
HashiruOtoko

Any idea why my LG 27GL850 OSD shows Elden Ring in borderless windowed as running at 60Hz?

HashiruOtoko
Member
HashiruOtoko

Looks like it acts as more of an inaccurate FPS monitor, despite saying Hz, and that’s for both places in OSD that have a readout.

mano4261
Member
mano4261

Hello, If I am using g sync and v sync and an FPS cap… is there any advantage in using g sync and v sync and nvidia reflex? I am asking if it is better to use in game fps cap or nvidia reflex to cap my fps.

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