Variable Refresh Rates
G-sync and FreeSync

Simon Baker, 5 June 2015

 


Introduction

If you're a gamer, it's always been a challenge to balance the performance of your graphics card with your monitor. For many years people have had to live with issues like tearing, caused when a frame rate is out of sync with the refresh rate of the display. The only real option has been to use a feature called Vsync to bring both of them in sync with one another, but not without introducing some issues of its own at the same time. During 2014-15 we have seen a change in how refresh rates are handled between graphics card and monitor and the arrival (broadly speaking) of "variable / adaptive refresh rate" technologies. NVIDIA and AMD, the two major graphics card manufacturers, each have their own approach to making this work which we will look at in this article.

 
 

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Vsync

As an introduction, monitors typically operate at a fixed refresh rate, whether that is 60, 120 or 144Hz. When running graphically intense content like games, the frame rate will fluctuate somewhat and this poses a potential issue to the user. There are traditionally two main options available for how frames are passed from the graphics card to the monitor using a feature called Vsync, whether it is turned on or off.



 

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. This allows you to run at higher frame rates than the refresh rate of your monitor but can lead to a lot of problems. 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 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. Tearing can become particularly noticeable during faster horizontal motion (e.g. turning, panning, strafing), especially at lower refresh rates.
 


 

The solution to this tearing problem for many years has been 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 lag as there is an inherent delay before frames are sent to the monitor. On a 120Hz monitor the 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 game play 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. Stuttering can be a major problem when using the Vsync on option to reduce tearing.

During Vsync ON operation, there can also sometimes be a sudden slow down in frame rates when the GPU has to work harder. This creates situations where the frame rate suddenly halves, such as 60 frames per second slowing down to 30 frames per second. During Vsync ON, if your graphics card is not running flat-out, these frame rate transitions can be very jarring. These sudden changes to frame rates creates sudden changes in lag, and this can disrupt game play, especially in first-person shooters.

 


Variable Refresh Rate - Adaptive-Sync
 


 

To overcome these limitations with Vsync, both NVIDIA and AMD have introduced new technologies dubbed G-sync and FreeSync respectively. G-sync was launched mid 2014 with the first screen we tested being the Asus ROG Swift PG278Q. FreeSync was not launched until 19th March 2015, with the first screen we’ve tested being the BenQ XL2730Z. Both options currently require a DisplayPort 1.2 interface from the graphics card and monitor to operate. At the time of writing AMD are known to be experimenting with providing the same functionality over HDMI, but it is still early days and only in proof-of-concept stages.

The idea of both technologies is based on variable refresh rates. These technologies can be integrated into monitors allowing them to dynamically alter the monitor refresh rate depending on the graphics card output and frame rate. The frame rate of the monitor is still limited in much the same way it is without a variable refresh rate technology, but it adjusts dynamically to a refresh rate 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. You can get the benefit of higher frame rates from Vsync off but without the tearing, and without the lag and stuttering caused if you switch to Vsync On.

 


G-Sync vs. FreeSync

 

        

 

Both G-sync and FreeSync operate on this principle of dynamically controlling the refresh rate. There are a few differences between how the technology is implemented though. NVIDIA G-sync requires a proprietary G-sync module to be added to the monitor, which comes at quite a high cost premium. You will notice as a result that the retail price of compatible G-sync monitors is often £100 - 200 higher than similar competitors because of this module. There is another limitation with adding a G-sync module in that it is only designed to work with a single interface currently, and so supporting monitors only offer a single DisplayPort connection. That makes those monitors somewhat restrictive when it comes to attaching any other devices of computers. Updated G-sync modules will apparently allow additional connections to be offered, although we’ve yet to see any displays released at the time of writing. The screens are also provided without a scaler, and so hardware aspect ratio control options are not offered. That's not as important as on some screens given you are restricted to a single DisplayPort interface anyway, and the PC can handle the scaling for you. It does also mean that signal processing lag is incredibly low as a result, another positive for gaming.

G-sync modules also support a native
blur reduction mode dubbed ULMB (Ultra Low Motion Blur). This allows the user to opt for a strobe backlight system if they want, in order to reduce perceived motion blur in gaming. It cannot be used at the same time as G-sync since ULMB operates at a fixed refresh rate only, but it's a useful extra option for these gaming screens. Of course since G-sync/ULMB are an NVIDIA technology, it only works with specific G-sync compatible NVIDIA graphics cards. While you can still use a G-sync monitor from an AMD/Intel graphics card for other uses, you can't use the actual G-sync or ULMB functions.

 


 

On the other hand AMD FreeSync technology costs virtually nothing for a monitor manufacturer to adopt and so there is no price premium really for supporting monitors, hence the name. Most of them already had the relevant components in their supply chains, but need the right software to come along to expose latent capabilities. With the help of VESA, the DisplayPort Adaptive-Sync specification was born to do exactly that. DisplayPort Adaptive-Sync has no unique material or licensing costs, and AMD FreeSync technology builds on top of that industry standard to give gamers a benefit in all of their games. No licensing. No proprietary hardware. No incremental hardware costs. So as the name suggests, the key advantage of FreeSync really is in the cost!

Thankfully since you don't need a dedicated extra module added to the screen you can still offer multiple video inputs on the screen without problems. You can only use FreeSync over DisplayPort, but there's no issue with including HDMI, DVI, D-sub etc to offer the user multiple interface options. You're not limited to just a single DisplayPort like with G-sync is currently. Scalers can also be provided as normal for hardware aspect ratio control although there may be additional signal processing lag added depending on the electronics and scalers manufacturers use. They will need to focus closely on reducing lag as they do with current non-FreeSync monitors. There is no native blur reduction mode coupled with FreeSync support so it is down to the display manufacturer whether they add an extra blur reduction method themselves. FreeSync can only be used from compatible AMD graphics cards, and you cannot use FreeSync from an NVIDIA card. You can still use a FreeSync monitor from an NVIDIA card without problems, just not the actual FreeSync feature.

 

We don't want to get in to any kind of NVIDIA vs. AMD debate here. What this really boils down to is whether you're an NVIDIA or AMD graphics user. At the moment there isn't a single standard which works from all graphics cards, so you need to pick a monitor to match your graphics choice. AMD's option is cheaper and more versatile for manufacturers to adopt but we don't feel that it will mean we will have a much larger selection of FreeSync monitors to choose from. At the end of the day the monitor manufacturers need to cater for their audience, and with such a huge market share from NVIDIA they would be mad to ignore G-sync offerings. Maybe at some point there will be a common approach between NVIDIA and AMD but with both technologies being so new at the moment, we can't see that happening for a while. G-sync may be more expensive, and limited when it comes to connection options at the moment, but there is the added benefit of the native ULMB included don't forget.

 

For more information about G-sync see NVIDIA’s website, and for more information on FreeSync, see AMD's website.

 

G-Sync vs. FreeSync Reference Table

 

 

NVIDIA G-Sync

AMD FreeSync

Pros

  • Included ULMB Blur Reduction mode

  • Low processing lag since no scalers included

  • NVIDIA has larger graphics card market share than AMD

  • Very few bugs or issues with G-sync operation since introduction

  • Very low cost to implement

  • Multiple interface options can still be provided

  • Scalers can still be provided

  • Vsync on/off option for frequencies out of supported FreeSync range

  • Some very minor performance benefits over G-sync

Cons

  • Cost premium to add G-sync module

  • Limited connectivity interfaces at present (expected to change)

  • Lack of scaler support

 

  • No integrated blur reduction mode

  • Possible additional processing lag added by scalers

  • Some early teething problems with FreeSync affecting overdrive circuits on monitors.



G-sync Operation

 

There are plenty of reviews and tests of G-sync online which cover the operation of G-sync in more detail. Our friends over at http://www.blurbusters.com/ have done some G-sync testing in various games which is well worth a read. They've also carried out various lag tests which have confirmed that using G-sync doesn't seem to add any noticeable lag, compared with running with Vsync off.


Above: G-sync options in the NVIDIA control panel

It should be noted that the real benefits of G-sync really come into play when viewing lower frame rate content, around 45 - 60fps typically delivers the best results compared with Vsync on/off. At consistently higher frame rates as you get nearer to 144 fps the benefits of G-sync are not as great, but still apparent. There will be a gradual transition period for each user where the benefits of using G-sync decrease, and it may instead be better to use the ULMB feature included, which is not available when using G-sync. Higher end gaming machines might be able to push out higher frame rates more consistently and so you might find less benefit in using G-sync. The ULMB could then help in another very important area, helping to reduce the perceived motion blur caused by LCD displays. It's nice to have both G-sync and ULMB available to choose from certainly on these G-sync enabled displays. Very recently NVIDIA has added the option to choose how frequencies outside of the supported range are handled. Previously it would revert to Vsync on behaviour, but the user now has the choice for Vsync on or off.

 


FreeSync Operation

AMD FreeSync can support dynamic refresh rates between 9 and 240Hz but the actual supported ranges depend on the display, and this does vary. When you connect the display to a compatible graphics card, with the relevant driver package installed the display is detected as FreeSync compatible and gives the following pop up message:

 


 

Within the Catalyst Control Centre there is an added configuration option for FreeSync at the bottom as shown in the screenshot below. Once enabled, FreeSync ON is often also confirmed in the OSD menu of the display somewhere.
 


 

We don't want to go into too much depth about game play, frame rates and the performance of FreeSync here as we will end up moving away from characteristics of the monitor and into areas more associated with the operation of the graphics card and its output. FreeSync is a combined graphics card and monitor technology, but from a monitor point of view all it is doing is supporting this feature to allow the graphics card to operate in a new way. We'd encourage you to read some of the FreeSync reviews online as they go into a lot more detail about graphics card rendering, frame rates etc as well.

Within the AMD press material and presentation which we were invited to, they had carried out some tests of FreeSync and noted a minor improvement in frame rates when FreeSync was enabled. We're talking extremely minor here, up to about half a frame difference at best. Still, it's better than any drop in performance! On the other hand they found a minor drop in frame rate performance when testing NVIDIA G-sync, down by a couple of frames at most. NVIDIA had acknowledged this minor performance drop in the past although said they were working on it. For those interested, there's some more information here about the test environments used by AMD.

The results obtained by AMD were as follows:
 


Above: AMD tests of FreeSync and G-sync Frame rates

AMD concluded from their tests that enabling FreeSync maintains a more consistent performance vs. the competition:

Really any difference here is extremely minor and won't make any practical difference to the user. Anandtech actually carried out some further tests themselves and found no discernable difference between the two solutions. Perhaps there's some very minor difference somewhere, but not something to worry about at all. We only include it here as you're likely to hear about this in any AMD vs. NVIDIA conversations.


In addition to these tests, AMD also checked how each solution behaves if you operate outside of the supported range. i.e. what happens if you provide a frame rate above the maximum supported 144Hz, or below the bottom end of the range supported by the monitor. With AMD FreeSync you have the option as a user to either have V-sync on or off for operation above the maximum supported refresh rate. So if you have a powerful enough system you are able to output more frames if you want. Obviously you're back into the realms of possible tearing etc with V-sync off, or into the realms of some possible lag and stutter with V-sync on, but you have the choice as the user at least.

Their tests here confirm that operation, using V-sync off when out of range of FreeSync. Again test environment described here if you want more info.


It should be noted that the real benefits of variable refresh rate technologies really come into play when viewing lower frame rate content, around 40 - 75fps typically delivers the best results compared with Vsync on/off. At consistently higher frame rates as you get nearer to 144 fps the benefits of FreeSync (and G-sync) are not as great, but still apparent. There will be a gradual transition period for each user where the benefits of using FreeSync decrease, and it may instead be better to use a
Blur Reduction feature if it is provided. On FreeSync screens this is not an integrated feature however, so would need to be provided separately by the display manufacturer.


 

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