Benefits of G-Sync if FPS<refresh rate

[THX1138]

As per topic - if your FPS in games is actually lower than your monitor's refresh rate (say, you get 85 FPS but your monitor's refresh is 144hz), would G-sync give you any benefit?

I'm thinking no - since you wouldn't need to worry about tearing, and since you wouldn't be using v-sync, you don't need to worry about lag or stutter either.

Is this right?

[RealNC]

Lower FPS than refresh rate is actually the main reason G-Sync exists So if you get 85FPS on a 144Hz refresh rate, this is when G-Sync kicks in and becomes useful, since it lowers the refresh rate of the monitor from 144Hz to 85Hz so that it matches the FPS you get. Although it doesn't just match the refresh rate, it actually makes the monitor wait for the GPU to send the frame before drawing it (because otherwise you'd still get tearing even if FPS and refresh rate are the same; see below.)

And there is stutter with lower FPS than refresh rate even without V-Sync. The starting point on the screen where stutter begins to happen without V-Sync is moving across the screen (bottom to top or top to bottom) over time.

Note that tearing does not depend on FPS. Without V-Sync, you get tearing regardless of how many FPS you get. Even if FPS matches the refresh rate of the monitor, you still get tearing. What V-Sync does is not an FPS cap (that's just a side effect). What it does is synchronize the output of the graphics card to the vertical blanking signal of the monitor (meaning it synchronizes to the time where the monitor is about to draw a new image.) Without that synchronization, the monitor will draw a part of the current image and a part of the new image, resulting in a horizontal line (or sorts) that separates the two images. When FPS and Hz match, then that tearing line will have fixed position on the screen (with some jittering; not exactly fixed) rather than moving upwards/downwards, but it will still be there.

[Chief Blur Buster]

As per topic - if your FPS in games is actually lower than your monitor's refresh rate (say, you get 85 FPS but your monitor's refresh is 144hz), would G-sync give you any benefit?

G-SYNC means framerate equals Hz.
G-SYNC can change refresh rate over 100 times a second.

With G-SYNC:
At 30fps, your monitor is 30Hz.
At 35fps, your monitor is 35Hz.
At 40fps, your monitor is 40Hz.
At 45fps, your monitor is 45Hz.
At 50fps, your monitor is 50Hz.
At 55fps, your monitor is 55Hz.
etc.
etc.
etc.
At 130fps, your monitor is 130Hz.
At 135fps, your monitor is 135Hz.
At 140fps, your monitor is 140Hz.
At 144fps, your monitor is 144Hz.

And everything in between. 39.875fps means 39.875Hz. 113.9fps means 113.9Hz.

It's a Variable Refresh Rate technology.

You essentially get the "permanent perfect capped-out VSYNC ON look" at all times even during varying frame rate, without the input lag of VSYNC ON. Fluidity of everything always look as smooth as TestUFO animations even when the framerate is randomizing, because of G-SYNC's ability to eliminate stutters during framerate transitions. Small changes in framerates are no longer perceptible, large changes in framerate is much more subtle and less jarring.

For more information see the animations in G-SYNC Preview Part #1

[THX1138]

Oh, I always thought that FPS>refresh was the reason why G-sync was made.

My understanding is that tearing occurs because the GPU would send the next frame before the monitor scan for the previous frame is complete. But to use the following example where the FPS is 72 and the refresh is 144hz:

GPU frames --> Monitor scans

Frame 1 --> Scans 1 and 2
Frame 2 --> Scans 3 and 4
Frame 3 --> Scans 5 and 6

There should be no tearing in this example?

Also, because there are no "excess frames" generated by the GPU which are "discarded" because monitor refresh cannot keep up, there also should not be any stutter because you will not miss any frames.

I'm a newb at this, so please correct me if I'm wrong .

The reason why I'm asking is because I am thinking of switching to a BenQ XL2420Z (from an old Samsung Syncmaster 245T - terrible for gaming), and I mostly play BF4 with a GTX 780 but I never hit 144 FPS consistently (on ultra settings). But with news of the XL2420G releasing this quarter, I'm wondering whether I should hold out until that releases but I'll need some info on whether G-sync gives any benefit at all if the GPU doesn't draw 144 FPS. Or, I could just purchase the Z first and then, if G-sync gives some benefit, hope that a separate G-sync DIY upgrade kit will be released for the Z (I think the G is essentially the Z but with G-sync built in).

[RealNC]

My understanding is that tearing occurs because the GPU would send the next frame before the monitor scan for the previous frame is complete.

This happens regardless of framerate. If the GPU doesn't wait for the display to finish the current frame first before sending the next frame, tearing occurs.

But to use the following example where the FPS is 72 and the refresh is 144hz:

GPU frames --> Monitor scans

Frame 1 --> Scans 1 and 2
Frame 2 --> Scans 3 and 4
Frame 3 --> Scans 5 and 6

There should be no tearing in this example?

Only if V-Sync is on. Otherwise, it might look like this:

Frame 1 --> Second half of scan 1 up to the first half of 3
Frame 2 --> Second half of scan 3 up to the first half of 5
Frame 3 --> Second half of scan 5 up to the first half of 7

Result: tearing. This is the whole point of V-Sync; it makes sure that each scan displays exactly only one frame. Two frames cannot end up in the same scan. When more than one frame end up in the same scan, this causes tearing.

You can easily test this yourself, of course. Make sure V-Sync is disabled and cap the framerate of BF to 72 or 48 FPS (both map perfectly to 144Hz; 72FPS will scan each frame two times, 48FPS will scan each frame three times.) Now in the game, look quickly left and right. There will be tearing. If you don't have a 144Hz monitor but a normal 60Hz one, then you need to cap to 30FPS or 15FPS to get the same two/three scans per frame result.

(Heh, I suppose it just occured to be that 144Hz monitors are very nice for frame capping my games. With 60Hz, only a 30FPS cap would avoid this kind of stutter, but it's not smooth as it's... well, 30FPS. With 144Hz, I can cap to 48, which will look way smoother.)

Also, because there are no "excess frames" generated by the GPU which are "discarded" because monitor refresh cannot keep up, there also should not be any stutter because you will not miss any frames.

Only true with 72FPS on 144Hz, because 72 is a divisor of 144. With anything else than 72FPS that is not a divisor, there will be stutter.

Also, note that stutter is not the result of missing frames. There's never a missing frame. This kind of stutter happens because there's no even distribution of frames across monitor scans. With 72FPS on 144Hz there is an even distribution and thus every frame is scanned twice. This looks smooth. With 73FPS or higher, there isn't an even distribution, and therefore some frames can only be scanned once, not twice. Since some frames are scanned once and others twice, the result is stutter. The other way around is also true. With 71FPS or lower, some frames need to be scanned three times instead of two. This also stutters. And that's with V-Sync enabled. With V-Sync disabled, you get tearing on top of stutter, and the position on the screen where the stutter manifests is moving across the screen top to bottom or bottom to top.

G-Sync fixes that. With G-Sync, there's always an even distribution of frames across scans, and therefore the stutter that normally happens because of FPS/Hz mismatch is removed. Other kinds of stutter are not removed, of course. Playing a game at 20FPS for example is not going to look smooth, because that's a different kind of stutter which is not the result of FPS/Hz mismatch, meaning that 20FPS would look "stuttery" even on a 60Hz or 120Hz modes, even though there's a perfectly even distribution of frames across scans.

I'm wondering whether I should hold out until that releases but I'll need some info on whether G-sync gives any benefit at all if the GPU doesn't draw 144 FPS.

As stated earlier, G-Sync is there for when you can't get 144FPS constantly. If you always get 144FPS, G-Sync is not that useful anymore for stutter removal. It's still half as useful for its input lag reduction compared to V-Sync.

[Chief Blur Buster]

RealNC, very good explanation.

This kind of stutter happens because there's no even distribution of frames across monitor scans.

Yes, the aliasing effect between frame rate and refresh rate. This also happens with VSYNC OFF too, even at framerates greater than refreshrate.

Now, to help out, this is an animation what tearing looks like:



Tearing happens most often when you're turning/strafing very fast past vertical edges (e.g. wall edges) between high-contrast colors. Brief disjoints then show up.

VSYNC OFF can also contribute to microstutters, too: Also, VSYNC OFF also adds a degree of microstutter. For any given vector of trajectory on the screen, there's always an aliasing effect between the refreshrate & the framerate -- this can even create visible microstuttering even at 300 frames per second (more easily seen with LightBoost + 1000Hz mouse), that only disappear during VSYNC ON or during GSYNC. At 300 frames per second, the microstutters are however very minor (for 300fps, edge vibrating amplitude only 1/300ths the current motion speed -- e.g. 3 pixel back-forth microstutter during 1000 pixels/second motion) and only becomes noticeable when other weak links are eliminated (fix motion clarity by LightBoost, fix mouse microstutter by good 1000Hz mouse & configurations)

[THX1138]

Thanks very much guys!