NVIDIA G-Sync Reviewby Anand Lal Shimpi on December 12, 2013 9:00 AM EST
It started at CES, nearly 12 months ago. NVIDIA announced GeForce Experience, a software solution to the problem of choosing optimal graphics settings for your PC in the games you play. With console games, the developer has already selected what it believes is the right balance of visual quality and frame rate. On the PC, these decisions are left up to the end user. We’ve seen some games try and solve the problem by limiting the number of available graphical options, but other than that it’s a problem that didn’t see much widespread attention. After all, PC gamers are used to fiddling around with settings - it’s just an expected part of the experience. In an attempt to broaden the PC gaming user base (likely somewhat motivated by a lack of next-gen console wins), NVIDIA came up with GeForce Experience. NVIDIA already tests a huge number of games across a broad range of NVIDIA hardware, so it has a good idea of what the best settings may be for each game/PC combination.
Also at CES 2013 NVIDIA announced Project Shield, later renamed to just Shield. The somewhat odd but surprisingly decent portable Android gaming system served another function: it could be used to play PC games on your TV, streaming directly from your PC.
Finally, NVIDIA has been quietly (and lately not-so-quietly) engaged with Valve in its SteamOS and Steam Machine efforts (admittedly, so is AMD).
From where I stand, it sure does look like NVIDIA is trying to bring aspects of console gaming to PCs. You could go one step further and say that NVIDIA appears to be highly motivated to improve gaming in more ways than pushing for higher quality graphics and higher frame rates.
All of this makes sense after all. With ATI and AMD fully integrated, and Intel finally taking graphics (somewhat) seriously, NVIDIA needs to do a lot more to remain relevant (and dominant) in the industry going forward. Simply putting out good GPUs will only take the company so far.
NVIDIA’s latest attempt is G-Sync, a hardware solution for displays that enables a semi-variable refresh rate driven by a supported NVIDIA graphics card. The premise is pretty simple to understand. Displays and GPUs update content asynchronously by nature. A display panel updates itself at a fixed interval (its refresh rate), usually 60 times per second (60Hz) for the majority of panels. Gaming specific displays might support even higher refresh rates of 120Hz or 144Hz. GPUs on the other hand render frames as quickly as possible, presenting them to the display whenever they’re done.
When you have a frame that arrives in the middle of a refresh, the display ends up drawing parts of multiple frames on the screen at the same time. Drawing parts of multiple frames at the same time can result in visual artifacts, or tears, separating the individual frames. You’ll notice tearing as horizontal lines/artifacts that seem to scroll across the screen. It can be incredibly distracting.
You can avoid tearing by keeping the GPU and display in sync. Enabling vsync does just this. The GPU will only ship frames off to the display in sync with the panel’s refresh rate. Tearing goes away, but you get a new artifact: stuttering.
Because the content of each frame of a game can vary wildly, the GPU’s frame rate can be similarly variable. Once again we find ourselves in a situation where the GPU wants to present a frame out of sync with the display. With vsync enabled, the GPU will wait to deliver the frame until the next refresh period, resulting in a repeated frame in the interim. This repeated frame manifests itself as stuttering. As long as you have a frame rate that isn’t perfectly aligned with your refresh rate, you’ve got the potential for visible stuttering.
G-Sync purports to offer the best of both worlds. Simply put, G-Sync attempts to make the display wait to refresh itself until the GPU is ready with a new frame. No tearing, no stuttering - just buttery smoothness. And of course, only available on NVIDIA GPUs with a G-Sync display. As always, the devil is in the details.
How it Works
G-Sync is a hardware solution, and in this case the hardware resides inside a G-Sync enabled display. NVIDIA swaps out the display’s scaler for a G-Sync board, leaving the panel and timing controller (TCON) untouched. Despite its physical location in the display chain, the current G-Sync board doesn’t actually feature a hardware scaler. For its intended purpose, the lack of any scaling hardware isn’t a big deal since you’ll have a more than capable GPU driving the panel and handling all scaling duties.
G-Sync works by manipulating the display’s VBLANK (vertical blanking interval). VBLANK is the period of time between the display rasterizing the last line of the current frame and drawing the first line of the next frame. It’s called an interval because during this period of time no screen updates happen, the display remains static displaying the current frame before drawing the next one. VBLANK is a remnant of the CRT days where it was necessary to give the CRTs time to begin scanning at the top of the display once again. The interval remains today in LCD flat panels, although it’s technically unnecessary. The G-Sync module inside the display modifies VBLANK to cause the display to hold the present frame until the GPU is ready to deliver a new one.
With a G-Sync enabled display, when the monitor is done drawing the current frame it waits until the GPU has another one ready for display before starting the next draw process. The delay is controlled purely by playing with the VBLANK interval.
You can only do so much with VBLANK manipulation though. In present implementations the longest NVIDIA can hold a single frame is 33.3ms (30Hz). If the next frame isn’t ready by then, the G-Sync module will tell the display to redraw the last frame. The upper bound is limited by the panel/TCON at this point, with the only G-Sync monitor available today going as high as 6.94ms (144Hz). NVIDIA made it a point to mention that the 144Hz limitation isn’t a G-Sync limit, but a panel limit.
The G-Sync board itself features an FPGA and 768MB of DDR3 memory. NVIDIA claims the on-board DRAM isn’t much greater than what you’d typically find on a scaler inside a display. The added DRAM is partially necessary to allow for more bandwidth to memory (additional physical DRAM devices). NVIDIA uses the memory for a number of things, one of which is to store the previous frame so that it can be compared to the incoming frame for overdrive calculations.
The first G-Sync module only supports output over DisplayPort 1.2, though there is nothing technically stopping NVIDIA from adding support for HDMI/DVI in future versions. Similarly, the current G-Sync board doesn’t support audio but NVIDIA claims it could be added in future versions (NVIDIA’s thinking here is that most gamers will want something other than speakers integrated into their displays). The final limitation of the first G-Sync implementation is that it can only connect to displays over LVDS. NVIDIA plans on enabling V-by-One support in the next version of the G-Sync module, although there’s nothing stopping it from enabling eDP support as well.
Enabling G-Sync does have a small but measurable performance impact on frame rate. After the GPU renders a frame with G-Sync enabled, it will start polling the display to see if it’s in a VBLANK period or not to ensure that the GPU won’t scan in the middle of a scan out. The polling takes about 1ms, which translates to a 3 - 5% performance impact compared to v-sync on. NVIDIA is working on eliminating the polling entirely, but for now that’s how it’s done.
NVIDIA retrofitted an ASUS VG248QE display with its first generation G-Sync board to demo the technology. The V248QE is a 144Hz 24” 1080p TN display, a good fit for gamers but not exactly the best looking display in the world. Given its current price point ($250 - $280) and focus on a very high refresh rate, there are bound to be tradeoffs (the lack of an IPS panel being the big one here). Despite NVIDIA’s first choice being a TN display, G-Sync will work just fine with an IPS panel and I’m expecting to see new G-Sync displays announced in the not too distant future. There’s also nothing stopping a display manufacturer from building a 4K G-Sync display. DisplayPort 1.2 is fully supported, so 4K/60Hz is the max you’ll see at this point. That being said, I think it’s far more likely that we’ll see a 2560 x 1440 IPS display with G-Sync rather than a 4K model in the near term.
Naturally I disassembled the VG248QE to get a look at the extent of the modifications to get G-Sync working on the display. Thankfully taking apart the display is rather simple. After unscrewing the VESA mount, I just had to pry the bezel away from the back of the display. With the monitor on its back, I used a flathead screw driver to begin separating the plastic using the two cutouts at the bottom edge of the display. I then went along the edge of the panel, separating the bezel from the back of the monitor until I unhooked all of the latches. It was really pretty easy to take apart.
Once inside, it’s just a matter of removing some cables and unscrewing a few screws. I’m not sure what the VG248QE looks like normally, but inside the G-Sync modified version the metal cage that’s home to the main PCB is simply taped to the back of the display panel. You can also see that NVIDIA left the speakers intact, there’s just no place for them to connect to.
It looks like NVIDIA may have built a custom PCB for the VG248QE and then mounted the G-Sync module to it.
The G-Sync module itself looks similar to what NVIDIA included in its press materials. The 3 x 2Gb DDR3 devices are clearly visible, while the FPGA is hidden behind a heatsink. Removing the heatsink reveals what appears to be an Altera Arria V GX FPGA.
The FPGA includes an integrated LVDS interface, which makes it perfect for its role here.
Post Your CommentPlease log in or sign up to comment.
View All Comments
JimmiG - Thursday, December 12, 2013 - linkWhat about Triple Buffering? Seems we already have a solution to this problem in software..
tipoo - Thursday, December 12, 2013 - linkStutter/lag.
NicePants42 - Thursday, December 12, 2013 - linkI came here to post this, and am quite surprised that there was no mention of triple buffering in the article. It seems very disingenuous on the part of both nVidia (assuming there were no slides mentioning triple buffering) and Anandtech to omit this issue.
In fact, it was Anandtech who did an excellent (IMHO) job of informing me about the advantages of triple buffering back in 2009, in this article: http://www.anandtech.com/show/2794/2
I'm glad nVidia is bringing more hardware solutions to improve gaming, but not addressing triple buffering here makes me think that nVidia's marketing department wasn't impressed with the comparison.
What gives, Anand?
Zink - Thursday, December 12, 2013 - linkTriple buffering is a type of v-sync. On the first page of this review it explains the issue. The buffers hold on to what the GPU rendered for anywhere from 0 ms to 17 ms so there is no way for the rate of motion of objects in the frames from the GPU to actually match up with when the frames are displayed.
PEJUman - Thursday, December 12, 2013 - linkMy thoughts exactly, triple buffering is a software solution, while this is a hardware based solution. If I understand correctly, the input lag for both should be the very similar. The distinction comes from the memory requirements; on already taxed hardware (think 4K), G-sync would work better.
ultimately, Unless Nvidia can get the G-sync compatible LCDs under ~$30 premium, it (G-sync) will only make sense for ultra high 4K monitors; If you have middle of the road stuff, your money is better spent into better GPU imho.
Traciatim - Thursday, December 12, 2013 - linkTriple Buffering causes lag, since you never get to see anything that happens in the game until the third frame is scanned on the screen. That's a pretty huge deal when you are playing games where reflexes affect the outcomes. If you get a chance to react up to 33ms faster than the next player, all else being equal you win.
JimmiG - Friday, December 13, 2013 - linkOf course triple buffering has drawbacks (input lag) just like G-Sync, however it's a perfectly viable solution to the same problem that this hardware+software combination tries to solve. I doubt competitive twitch FPS players are going to jump on G-Sync anyway as it may causes input lag too - they will keep playing with double-buffering + no VSync.
With triple-buffering, as long as your actual frame rate is over 60 FPS, there should always be a frame ready in one of the back buffers when the screen redraws. I've always found that in the few games that support it, the frame rate is very even and smooth.
PEJUman - Friday, December 13, 2013 - linkso does G-sync, isn't telling the screen to wait until the next frame is ready = lag? The only difference is G-sync waits for the next frame, buffering grabs the latest finished frame. Both creates extra lag.
psuedonymous - Friday, December 13, 2013 - linkTriple-buffering solves the issue when you have performance to spare (i.e. spend most of your time rendering at above the display refresh rate) and a very high tolerance for update delay (lag). When performance constrained, triple-buffering offers little to no benefit over double-buffering (as you're never filling that other buffer before display update), and you still get that frame-by-frame variance between render time and display time when performance varies.
oranos - Thursday, December 12, 2013 - linkjust because it's niche doesn't mean it's not worth an article.