Nvidia G-SYNC – variable refresh rate technology
Traditionally monitors operate at a fixed refresh rate – commonly 60Hz, 120Hz or 144Hz. When running graphically intensive content, such as games, frame rate can be expected to fluctuate in response to varying levels of ‘demand’ from the game or application. This introduces something of a dilemma for gamers in particular. They are forced to choose one of two main options that determine how the GPU handles its ‘passing of frames’ to the monitor– ‘VSync off’ or ‘VSync on’. There is an excellent detailed technical explanation of these two modes and what they involve, using a 60Hz monitor in their example, on this AnandTech article.
At the most basic level ‘VSync off’ allows the GPU to send frames to the monitor as soon as they have been processed, irrespective of whether the monitor has finished its refresh and is ready to move onto the next frame. When the frame rate of the game and refresh rate of the monitor are different, things become unsynchronised. This lack of synchronisation coupled with the nature of monitor refreshes (typically from top to bottom) causes the monitor to display a different frame towards the top of the screen vs. the bottom. This results in a distinctive ‘tearing’ on the monitor that really bothers some users. Even on a 120Hz or 144Hz monitor, where some users incorrectly claim that there is no tearing, the tearing is still there. It is generally less noticeable but it is definitely still there. Even if users don’t notice distinct tearing, the effect of that tearing (texture displacement and ‘juddering’) is often more obvious. Because only some of the screen is displaying the most recent information this can be thought of as a source of ‘visual latency’, too.
The solution to this tearing is the ‘VSync on’ option which essentially forces the GPU to hold a frame until the monitor is ready to display it, as it has finished displaying the previous frame. It also locks the frame rate to a maximum equal to the monitor’s refresh rate. Whilst this eliminates tearing, it also increases input lag as there is an inherent delay before frames are sent to the monitor. On a 120Hz monitor the input lag penalty is half that of a 60Hz monitor and on a 144Hz monitor is even lower. It is still there, though, and some users feel it disconnects them from gameplay somewhat. When the frame rate drops below the refresh rate of the monitor this disconnected feeling increases to a level that will bother a large number of users. Some frames will be processed by the GPU more slowly than the monitor is able to display them. In other words the monitor is ready to move onto a new frame before the GPU is ready to send it. So instead of displaying a new frame the monitor displays the previous frame again, resulting in stutter. This stutter is most pronounced when ‘Triple Buffering’ is disabled, as the frame rate will suddenly drop to half of the initial value rather than simply reducing by a few FPS. It is still very noticeable to some people even with ‘Triple Buffering’ enabled, however.
Nvidia G-SYNC – a variable frame rate solution
Nvidia have come up with a solution to this issue; G-SYNC. By integrating some clever electronics (below) into specific monitors it is possible to get the monitor to adopt a variable refresh rate, adjusting in real-time to accommodate the frame rate of a game. The frame rate of the monitor is still limited in much the same way it is without G-SYNC, but it adjusts dynamically to a refresh rate as low as 30Hz (current models) to match the frame rate of the game. By doing this the monitor refresh rate is perfectly synchronised with the GPU. You don’t get the screen tearing or ‘visual latency’ of having VSync disabled nor do you get the stuttering or input lag associates with using VSync.
G-SYNC is currently designed to work via DisplayPort only as it relies on its data packet capabilities. The module will lock off access to any other port (HDMI, DVI etc.) and audio via DP capabilities, although newer iterations will offer seperate scalers for HDMI use. To use G-SYNC you need a compatible monitor and GPU. On the GPU side that’s any Nvidia GPU that’s a GeForce GTX 650 Ti Boost or ‘higher’ – this is a proprietary Nvidia technology and is not compatible with AMD GPUs. On the monitor side there was a DIY modification kit for use with the ASUS VG248QE which was available for around $175 (USD) as of January 2014. Installation is apparently a simple 20-30 minute job using a Phillips screwdriver. This kit is not currently available locally outside of the US, for example in the UK. Modified VG248QEs with the technology built into them will are available from select retailers at an RRP of $399. The most pleasing news is that some of the major monitor manufacturers are extremely excited about this technology. AOC, Acer, ASUS, BenQ, Philips and ViewSonic all plan to release G-SYNC ready PC monitors or already have. Some of the models will have 2560 x 1440 resolutions and ‘4K’ UHD resolution (3840 x 2160). Some IPS-type panels are also in the pipeline, so there is nothing limiting G-SYNC to TN panels specifically.
How does G-SYNC act at the refresh rate ceiling of the monitor?
One of the attractions of this variable refresh technology is the low latency compared to using VSync. By using what Nvidia call a ‘lookaside buffer’, it adds very little latency compared to ‘VSync off’ (perhaps 1-2ms). There has been a bit of confusion about what happens when the monitor reaches the upper limit of its operation – 144fps at 144Hz, for example. If you observe an in-game frame rate counter or utility like FRAPS you will see that the frame rate only ever seems to reach 141-143fps rather than 144fps. However; Nvidia’s Tom Peterson has stated in a number of videos (here, for example) that G-SYNC monitors behave like VSync on when it reaches this ceiling. The game queues up frames, which induces similar latency behaviour to VSync on. Either way, interaction with the game world felt very smooth to us on the G-SYNC models we’ve tested even at this ceiling – just be aware of the possibility of a touch of extra latency here if you’re one of those people who swear by VSync off for minimal latency.
How does G-SYNC act below the refresh rate floor of the monitor?
Below the floor of operation (i.e. lowest refresh rate supported by a monitor with the technology, currently 30Hz) the monitor will stick to multiples of the refresh rate. This essentially eliminates stuttering and tearing with similar effectiveness to an exact refresh rate = frame rate situation. So if the game ran at 20fps, the monitor would set itself to 40Hz to avoid stuttering or tearing rather than staying at 30Hz or its maximum static refresh rate. As we explore in reviews where this technology is used, though, low frame rates are low frame rates regardless of the technology. And the perceived blur is considerably higher and overall connected feel considerably worse at low frame rates compared to significantly higher ones.
What about motion blur?
Whilst G-SYNC can provide a smoother experience by eliminating tearing, reducing stuttering and decreasing latency it does not affect motion blur caused by either the movement of our own eyes or the pixel responses of the monitor. Many gamers are quite excited about an alternative technology that does fill this gap, LightBoost, and in particular the motion blur reduction its forced activation can bring to 2D rather than 3D viewing. When asked if the technology can be used alongside LightBoost and particularly the forced 2D activation Nvidia’s Andrew Burnes stated;
“We have a superior, low-persistence mode that should outperform that unofficial implementation, and importantly, it will be available on every G-SYNC monitor. Details will be available at a later date.”.
This feature is called ULMB (Ultra Low Motion Blur) and it can be activated on most of these new ‘G-d up’ monitors. Importantly, though, it can’t currently be activated at the same time as G-SYNC. It’s worth remembering that the main operating mode lends itself to variable frame rates, whereas ULMB lends itself to a fixed frame rate at the top end of the monitor’s supported refresh rates. Anybody interested in such technology should also have a look at our detailed article that covers the factors affecting monitor responsiveness in which G-SYNC and strobe backlights are both covered.
A new version of G-SYNC which combines the variable refresh rate element with support for HDR (High Dynamic Range) technology has also been developed. The aptly named G-SYNC HDR offers supports for HDR10 via DisplayPort 1.4, an HDR standard that will make its way onto various games and movies. This calls for far greater contrast and a broader colour gamut than typical LCDs produce, in addition to enhanced bit depth (10-bits per subpixel). To achieve superior contrast, monitors supporting G-SYNC HDR use full array (local dimming) backlights with 384 dimming zones currently employed. The backlight is controlled for each of these zones individually, meaning that light and dark elements of the image can receive an appropriate brightness level simultaneously. This contrasts (pun intended) with current LCD backlight technology where the backlight is controlled as an individual BLU (Backlight Unit) and intricate mixes of bright and dark shades can’t be displayed optimally. This will also decrease pheonomenon such as ‘IPS glow’ or backlight bleed, as such things are reduced at the lower luminances that will be used by the backlight zones covering dark content. Furthermore, the zones can be boosted to much higher brightnesses than traditional backlights (1000 cd/m²+). Increased colour gamut, meanwhile, can be achieved using enhanced backlight technologies such as Quantum Dots. In the near-term the target is DCI-P3 (~25% greater than sRGB) and in the longer term Rec. 2020 (~72% greater than sRGB).
Confirmed G-SYNC HDR monitors
This is a quick list of currently confirmed monitors which have or will have the HDR version of the technology integrated into them:
Acer XB272-HDR (27″ 3840×2160, 144Hz ‘IPS’)
ASUS PG27UQ (27″ 3840×2160, 144Hz ‘IPS’)
Confirmed G-SYNC monitors
This is a quick list of currently confirmed monitors which have or will have the standard version of the technology integrated into them:
Acer X34 (34″ 3440×1440, 100Hz IPS)
Acer X34P (34″ 3440×1440, 100Hz IPS)
Acer XB240HA (24″ FHD, 144Hz TN)
Acer XB241H (24″ FHD, 144Hz TN)
Acer XB251HQT (24.5″ FHD, 240Hz TN)
Acer XB252Q (24.5″ FHD, 240Hz TN)
Acer XB270HA (27″ FHD, 144Hz TN)
Acer XB270HU (27″ WQHD, 144Hz ‘IPS’)
Acer XB271HK (27″ UHD, 60Hz ‘IPS’)
Acer XB271HU (27″ WQHD, 144Hz ‘IPS’)
Acer XB271HUT (27″ WQHD, 144Hz ‘IPS’)
Acer XB272 (27″ FHD, 240Hz TN)
Acer XB280HK (28″ UHD, 60Hz TN)
Acer XB281HK (28″ UHD, 60Hz TN)
Acer XB321HK (32″ UHD, 60Hz ‘IPS’)
Acer Z271 (27″ FHD, 144Hz VA)
Acer Z271T (27″ FHD, 144Hz VA)
Acer Z301CT (30″ 2560×1080, 200Hz VA)
Acer Z35 (35″ 2560×1080, 200Hz VA)
AOC g2460Pg (24″ FHD, 144Hz TN)
AOC AG241QG (23.8″ WQHD, 165Hz TN)
AOC AG271QG (27″ WQHD, 165Hz ‘IPS’)
AOC AG352UCG (35″ 3440×1440, 100Hz VA)
ASUS PG248Q (24″ FHD, 180Hz TN)
ASUS PG258Q (24.5″ FHD, 240Hz TN)
ASUS PG278Q (27″ WQHD, 144Hz TN)
ASUS PG279Q (27″ WQHD, 165Hz ‘IPS’)
ASUS PG27AQ (27″ UHD, 60Hz ‘IPS’)
ASUS PG348Q (34″ 3440×1440, 100Hz IPS)
ASUS VG248QE G-SYNC (24″ FHD, 144Hz)
BenQ XL2420G (24″ FHD, 144Hz TN)
Dell S2417DG (23.8″ WQHD, 165Hz TN)
Dell S2716DG (27″ WQHD, 144Hz TN)
HP Omen X 35 (35″ 3440×1440, 100Hz VA)
Lenovo Y27g (27″ FHD, 144Hz VA)
Philips 272G5DYEB (27″ FHD, 144Hz TN)
ViewSonic XG2703-GS (27″ WQHD, 165Hz ‘IPS’)