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NVIDIA G-SYNC Display Technology Explored
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Date: Dec 12, 2013
Section:Graphics/Sound
Author: Marco Chiappetta
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What Is NVIDIA G-SYNC

Back in September at a press event held in Montreal, NVIDIA CEO Jen-Hsun Huang announced what he called “one of the most important works NVIDIA has done for computer graphics.” The technology was called G-SYNC. G-SYNC is an end-to-end graphics and display architecture that starts with a Kepler-based GPU and ends with a G-SYNC module within a monitor. The G-SYNC module is piece of hardware that replaces the scaler inside a display/gaming monitor that essentially does away with the fixed vertical refresh rates of current displays.

To put it simply, what G-SYNC does is keep a display and the output from a Kepler-based GPU in sync, regardless of frame rates or whether or not V-Sync is enabled. Instead of the monitor controlling the timing, and refreshing at say every 60Hz, with G-SYNC the timing control is transferred to the GPU. The GPU scans a frame out and the monitor then draws it. The monitor doesn't update until a frame is done drawing. And as soon as a frame is done drawing, the monitor will update as soon as it can with the next frame, in lockstep with the GPU.


NVIDIA G-SYNC Display Technology Demo

During the original announcement, we recorded some video of G-SYNC in action, as you can see above. We must point out that this live demo of the technology doesn’t really do it justice because the camera can’t pick up all of the issues being displayed on-screen, and the camera's recording frame rate is out of sync with the monitors too. To see it live is to truly appreciate G-SYNC, but the tech is explained in plain English in the video, which some of you may find helpful. If you were there live, what you would have seen was that the animation on the G-SYNC enabled screen was as smooth as silk and devoid of any synchronization-related anomalies or stutters, whereas the traditional screen showed significant stutter and tearing. The difference live was like night and day.


Asus, Ben-Q, Philips and Viewsonic have signed up to offer G-SYNC enabled monitors.

Just recently, we were given the opportunity to experience G-SYNC on our own and experiment with the technology in-house. In this piece, we're going to talk about some of our experiences with G-SYNC, but before we do, we want to dive a little deeper into the technology and explain what really makes G-SYNC tick. To do so, we must first talk about what G-SYNC is designed to fix.

As it stands today, gamers can typically choose to play their games with V-Sync (vertical sync) enabled or disabled. V-Sync is an ancient technology that essentially allowed the output from a video source to synchronize properly with a display at a given frequency--the most common of which is 60Hz. That may sound well and good, but if the graphics output is coming at a rate above or below the vertical refresh rate of the screen, a number of issues are introduced. Disabling V-Sync may seem like the simple answer, but that causes a whole new set of problems.


V-Sync Enabled - Stutters and Lag

The diagram above illustrates what happens between a GPU and a display when V-Sync is enabled. In the image, the panel is refreshing every 16ms (roughly 60Hz), but the GPU is rendering frames at different intervals. Frame 1 renders faster than 16ms, so a bit of lag is introduced before the screen updates. Frame 2 takes longer than 16ms, so that frame is shown on-screen twice, which causes stutter during the animation and input lag. And so on. V-Sync would be an ideal solution if the frames were rendered and output at 60Hz as well, but that's not how today's games work. It's common for today's games to exhibit significant variations in frame time and it's rare that the GPU and display are actually in sync.


V-Sync Disabled - Tearing

Disabling V-Sync does away with the input lag, but introduces tearing on-screen. The diagram above illustrates what happens between the GPU and display when V-Sync is disabled. The GPU essentially pumps out frames as fast as it can, regardless of whether or not the display can keep up. What results is that unfinished parts of adjacent frames are displayed on-screen, and since the positioning of the scene's components are usually in different positions, tear lines are introduced.


On-Screen Tearing with V-Sync Disabled

As you can see, this scene exhibits tears at the very top and just above the gun, because three adjacent frames were output to the display faster than the display could draw them. This is what happens in virtually every game when playing with V-Sync disabled. If frame rates are high enough, the tearing may be tough to see, but rest assured, it's there.


Tomb Raider Frame Rates Over Time: Red Line-V-Sync On, Green Line-V-Sync Off


Crysis 3 Frame Rates Over Time: Red Line-V-Sync On, Green Line-V-Sync Off

The two graphs above show how frame rates are affected when enabling / disabling V-Sync in a couple of games, with a GeForce GTX 760, namely Tomb Raider and Crysis 3. With V-Sync enabled (red line), and the games configured for high image quality settings to target the 40-60 FPS range, it is not uncommon to see frame rates locked at 30 FPS for a time (half the monitor's refresh rate), which means many frames are duplicated, which introduces lag. Or, as is the case with Tomb Raider, you see wild, constant fluctuations between 30 and 60 FPS. With V-Sync disabled (green line), each frame it output at a completely different rate, which is never actually in sync with the display.

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How Does G-SYNC Work

Now that we’ve explained what happens when V-Sync is enabled or disabled, understanding what NVIDIA’s G-SYNC technology does should be simple. The image below illustrates how frames are output to a G-SYNC equipped screen, when G-SYNC is enabled in NVIDIA’s drivers.


G-SYNC Enabled

As you can see, when a frame is complete, it is scanned out to the screen. Frame 1 finishes is X amount of time, and it’s sent to the display. Frame 2 takes a bit longer, but when it is done, it is sent out to the screen, and so on. G-SYNC removes the fixed refresh rate limitation of today’s screens and transfers the timing to the GPU. The screen’s refresh rate is dynamically adjusted to stay in-sync with the GPU, regardless of the frame rate, though G-SYNC is limited to refresh rates between 30 and 144Hz. In doing so, G-SYNC eliminates the screen tearing associated with disabling V-Sync, and eliminates the lag and stutter that come when V-Sync is enabled.


 
NVIDIA G-SYNC Module

NVIDIA achieved this feat by developing the G-SYNC module, which will be featured in a number of new monitors starting next year. The G-SYNC module replaces the scaler and controller boards in current displays and allows for the dynamic refresh rates mentioned earlier.

The module is comprised of an FPGA—programmed by NVIDIA—a bit of DRAM, and a DisplayPort input. At this time, G-SYNC requires a Kepler-based GPU, with a DP output, and obviously a G-SYNC enabled display. To fully appreciate the technology, a high-DPI gaming mouse is also recommended.

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Our Experience With G-SYNC

The experience, when we experimented with NVIDIA’s G-SYNC technology, felt very polished. Although the display we used was a pre-production sample, that’s not quite ready for prime-time just yet, once we had everything connected up properly, G-SYNC “just worked”.


NVIDIA's Public Drivers Already Support G-SYNC

NVIDIA’s public drivers already have G-SYNC support built in. Once we plugged in the display, we received a notification that a G-SYNC capable display was detected, we clicked it, and simply enabled G-SYNC. For best results, we also had to manually set the max refresh rate to 144Hz, but that was it.

We wish there was an easy way to convey how G-SYNC affects on-screen animation, but there isn’t. We don’t have a means to capture DisplayPort feeds and shooting video of the screen and hosting it on-line doesn’t capture the full effect either. In lieu of an easy visual method to show how effective G-SYNC is, we’ll just have to write about it. Luckily, G-SYNC can be summed up in one word—impressive.


ASUS Was The First To Announce A G-SYNC Enabled Display

If you’re the type that gets irked by input lag and can easily pick-up the synchronization related artifacts of your current display setup, G-SYNC will be a jaw-dropper. Tearing is simply gone, on-screen animations look smooth and fluid, and input lag is virtually non-existent. An added benefit of using a screen with a high refresh rate is that desktop animations and mouse movements are also more precise and fluid.

In addition to G-SYNC being rather impressive, we’re also pretty psyched around here that there’s finally some innovation happening in the monitor space. We’ve been stuck with current display technologies for too long. With 4K displays here now and G-SYNC coming soon, the desktop display market is about to heat up, and we're excited for it. Did we mention that G-SYNC is also compatible with 4K resolutions? It is, and we hope some manufacturers will make a 30-inch+ 4K display with this new NVIDIA technology on board. If they do, they can take our money right now…



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