Overclocked Refresh Rates
Simon Baker, 8 April 2016

 


Introduction

With a sudden influx of new monitors with "overclocked" refresh rates we thought it might be useful to provide a bit more information about the situation. We will look at the reasons a user might want to overclock their monitor in the first place, what has been available in the past and what we have seen so far from the new range of overclocked monitors. We will update this article later on as well when we've had chance to test some more of the new overclocked monitors in more detail.
 


     

 

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Why Overclock a Refresh Rate?

The refresh rate of a monitor has an impact on the gaming experience in a couple of areas. For those using Vsync to reduce tearing in games, it allows for a higher maximum frame rate on the screen. A 60Hz Vsync screen could support up to 60fps maximum, but a screen with 100Hz for example could support 100fps with Vsync on. The higher the refresh rate the better then in this regard, and while some panels will natively support 120Hz and above, users are always looking to boost this and squeeze more out of their screens. Much like overclocking other components, getting more for your money is always a good thing.

Secondly, there is a correlation between refresh rate and perceived motion blur. While pixel response times do have an influence on blurring, refresh rate also comes in to play. You will immediately notice the more fluid feel on a high refresh rate screen and the smoother gaming experience. To demonstrate this as well, take a look at the pursuit camera pictures from our Acer Predator XB270HU review shown below:


Above: Acer XB270HU comparison of overdrive settings, to show perceived motion blur differences between different refresh rates

Pursuit cameras are used to capture motion blur as a user might experience it on a display. They are simply cameras which follow the on-screen motion and are extremely accurate at measuring motion blur, ghosting and overdrive artefacts of moving images. Since they simulate the eye tracking motion of moving eyes, they can be useful in giving an idea of how a moving image appears to the end user. It is the blurring caused by eye tracking on continuously-displayed refreshes (sample-and-hold) that we are keen to analyse with this new approach. This is not pixel persistence caused by response times; but a different cause of display motion blur which cannot be captured using static camera tests. Low response times do have a positive impact on motion blur, and higher refresh rates also help reduce blurring to a degree. You can see from the above pictures that there is an improvement in perceived motion blur as you increase refresh rate. This motion clarity improvement goes hand in hand with the more fluid feel of the higher refresh rate screen.

So not only do you increase your maximum frame rate by increasing refresh rate, you also help reduce blurring and overall improve the fluidity of movement. While there are more and more native high refresh rate panels available in the market now (120 and 144Hz being common), there are also many that are still stuck at the 60Hz level. This is particularly the case with non-TN Film technologies like IPS and VA. To overcome this limitation on many 60Hz panels the subject of overclocking a monitor's refresh rate is therefore an attractive one. It might allow manufacturers and users to squeeze a bit more out of their screen and offer improved gaming performance, or something that otherwise wouldn't be available from the given panels in production.
 


Considerations for Increased Refresh Rates

It's all very well pushing a refresh rate up, but there are some considerations which should be made. Firstly, you need to remember that the native refresh rate of the panel is fixed, and pushing it up beyond this could lead to complications. In some cases it may void your warranty perhaps, although whether a manufacturer would be able to tell you'd tried it is questionable. More often than not if you try to increase the refresh rate and it's not supported you will just get an "out of range" type message from the monitor and have to revert back to your previous refresh rate setting. In other cases the screen may show visible flickering or artefacts if you have tried to push the refresh rate too high or beyond suitable limits. The ability to overclock is commonly linked to the controller board used in the monitor and some are capable of supporting an overclock while others simply won't allow anything.

In some cases a screen will support a higher refresh rate happily in Windows and all will look well in casual use. However, the real test is when you actually game on the screen or view multimedia content. In many cases if you run at a refresh rate above the supported maximum the screen will just drop some frames and therefore make the whole thing pointless. The best way to check is to use something like this BlurBusters.com frame skipping test. You need to take a photo of the screen which captures the moving square at a fairly slow shutter speed. You are supposed to see a continuous line without any breaks in it, but if frames are being dropped the line will appear broken. Frame dropping is one of the main problems with overclocked refresh rates.

Another factor you need to consider is the response times of the screen. Each refresh rate requires the screen to be updated at a fixed interval, for instance a 60Hz screen is updated every 16.6ms (1000 ms / 60). A 100Hz screen would be refreshed every 10ms. To keep up with this refresh interval, the response times of the panel need to be reliably producing figures (grey to grey) under that interval. So a 100Hz refresh rate screen needs to have response times which are reliably under 10ms G2G in practice, not just on paper. If they are not, you have a panel which cannot change the pixels fast enough and it leads to obvious blurring and smearing problems. It again makes the overclocked refresh rate somewhat pointless. This is one of the reasons why it has taken so long to see high refresh rate IPS-type panels, as manufacturers have found it hard to drive response times low enough (without introducing loads of overshoot) to support the higher refresh rate. AU Optronics managed it nicely with their recent 27" 2560 x 1440 AHVA panel (as featured in the Acer XB270Hu, Asus MG279Q etc) but LG.Display have yet to release a high refresh rate IPS panel of their own.

We have provided a table below showing how often a screen is refreshed at different refresh rates, and the response times which need to be reliably met to make them practical:

Refresh Rate (Hz)

Refresh Interval (ms)

60

16.66

75

13.33

100

10.00

120

8.33

144

6.94

165

6.06

200

5.00


Historical Overclocked Monitors

In the past we've seen overclocked refresh rates from a range of Korean monitor models primarily, as the focus has been on pushing non TN Film technologies higher than the native 60Hz that was available. These are displays specifically marketed as being overclockable by the manufacturers and have been around for a few years now. Some can reach up to around 100Hz and even 120Hz but results really do vary and can be hit and miss. The maximum supported refresh rate varies depending on the users system, interface being used, the actual monitor model and their specific scaler, controller and internal electronics. Some Korean screens are sold with guaranteed overclocking expectations though. Overall there's a bit of a risk with these screens are they are generally not guaranteed and have often limited support and warranty. Not to mention the fact they are normally cost-focused and so don't tend to be of the highest quality or design.

TN Film panels have been available for many years with native high refresh rates, although the enthusiast community has also looked at how far those can be pushed. The overclocking itself requires the user to create a custom resolution/refresh rate and generally you need to test various settings to see what can and can't be supported. BlurBusters talks a bit more about the process for overclocking and some of the steps the user needs to make.

With the Korean overclockable screens while there is certainly an improvement in fluidity and frame rates, response times have been an issue in many cases. Certainly all the Korean monitors we've ever tested have not been that fast in real-life response times, despite some adventurous manufacturer specs. Claimed figures like 5ms G2G are rarely achieved and in reality the response times can often be far too slow to practically keep up with a boosted refresh rate (or sometimes even the native 60Hz!). As a result of the slow response times blurring and smearing is a big issue on many Korean screens and they can't always keep up with the frame rates and refresh rate they can reach. We've also seen very limited success in actually overclocking some of the Korean models we've tested despite their marketing and specs, with many of them working fine in windows at the overclocked setting but then dropping frames when tested. Admittedly we've not tested that many Korean models like this and we're sure some can handle the increased refresh rate without problems like that. Response times could still be a limiting factor though in many cases from what we've seen.

Some enthusiast users have also started to modify their screens with different controller boards in an effort to overclock the screens as high as possible. Often this is only for lower resolutions, weird scaling settings and really only for testing purposes and a bit of fun and curiosity. We know some users have managed to overclock a 27" TN Film panel with a native 144Hz refresh rate up to as high as 240Hz for instance. Whether the panel response times are suitable for that kind of refresh rate is questionable, and whether it really brings about any significant improvements in real use is also debatable we would think. Let alone the demands of course on your system trying to run at such a high refresh rate for any real-life uses. Still, it's an interesting experiment and shows what is possible in theory where the technology is pushed.

 


The New Wave of Overclocked Screens

With the arrival of new gaming technologies like NVIDIA G-sync, AMD FreeSync and various Blur Reduction Backlight systems manufacturers have been looking at other ways to boost the gaming performance and specs of their screens. We have now started to see some screens appear with advertised overclocked refresh rates. Again these are focused currently on boosting the refresh rates of non TN Film panels, particularly where there are limits with available panels with only a 60Hz native support. These screens also have specific overclocking settings in the menu and "officially" support this as a feature. You need to do little more than enable the overclock in the OSD menu and set your graphics card to the new refresh rate.

Acer for instance have already released a few gaming monitors in their Predator range with enhanced refresh rates. The Acer Predator XR341CK (with AMD FreeSync support) was the first we tested in July 2015, boosting the native 60Hz panel refresh rate slightly up to 75Hz. While it might not sound much, it's still a 25% improvement and does provide some definite improvements in practice as well as we saw from our review. This wasn't a full overclock as such, as all they were doing was allowing reliable support of the panels maximum supported refresh rate. Often that is not supported but it is at least within the panel spec guidelines.

Later we saw the Acer Predator X34, based on the exact same 60Hz panel as the XR341CK but this time using an NVIDIA G-sync module. Acer had managed to offer an overclocked refresh rate here of up to 100Hz so this is obviously a more impressive overclock! It seems to be the presence of the G-sync module, or perhaps the absence of an additional scaler, that is allowing this high overclock and other screens since have all followed this pattern. Again, with the additional overclocked refresh rate we saw improvements in perceived motion clarity (as a result of refresh rate), increased frame rate support and also a pleasing improvement in measured response times as well. As we explained before, the response time consideration is important for overclocked refresh rates, and so far the G-sync modules have done a nice job of improving response times as refresh rate increases as well.

We have also seen some other overclocked screens released so far, including models we have tested like the IPS Asus ROG Swift PG279Q (boosted from 144Hz native to 165Hz overclocked), the VA Acer Predator Z35 (boosted from 144Hz native to 200Hz) and the IPS Asus ROG Swift PG348Q (from 60 to 100Hz). These screens all have NVIDIA G-sync in common, so that appears to be the easiest way for manufacturers to overclock refresh rates at the moment.

The results of these overclocked refresh rates does vary somewhat, and may lead to some issues on some screens and systems. It's probably worth noting that these overclocked refresh rates are generally not guaranteed by the manufacturers and are an "up to" spec. Some manufacturers stipulate you need certain (high end) graphics cards for optimal performance, some simply state that the maximum overclock may vary from system to system. We have had mixed results during our testing. Some models like the Acer Predator X34 seemed to work fine at its maximum 100Hz without much fuss at all, offering stable images with no artefacts or flickering from both our NVIDIA and AMD test systems. Neither system dropped frames and the 100Hz seemed easy to achieve and stable. We know some users have had issues reaching the maximum 100Hz in some cases, but the next step down of 95Hz tends to be fine if that is the case. On the other hand, the Asus ROG Swift PG348Q is based on the exact same panel as the Acer X34 but was far more fussy. We had issues with flickering from an AMD system at anything above 60Hz, and frames were dropped from a lower end NVIDIA card at all refresh rates. From a higher end card (that Asus recommend) the screen behaved normally and worked fine at the maximum overclock. This just goes to show that results can really vary and you need to be mindful that this overclock is a "best endeavours" kind of thing.

The Acer Predator Z35 with its maximum 200Hz overclocked refresh rate is another interesting model to look at. With the overdrive setting switched to off you can see a classic case of where the response times are not sufficiently fast enough to keep up with the refresh rate demands (as we talked about earlier). Anything above 60Hz with overdrive off introduces obvious and distracting blurring and smearing as the VA panel's response times are just too slow to cope. This is the kind of thing you will often see from Korean models that have been overclocked, but that still suffer from slow response times. The Z35 is a VA-type panel and we found that although you can comfortably overclock the refresh rate up to 200Hz without much issue, the pixel response times weren't sufficient to cope. Too much overshoot was introduced above about 120Hz and so this made the overclock somewhat pointless. Be careful of bold claims as refresh rates continue to be pushed, as panel technology and subsequent pixel response times will certainly have an impact on whether the refresh rate is viable or useable.

While we have had fairly decent experience with most overclocked refresh rates when it comes to issues like frame skipping, we have noticed an odd bug affecting the overdrive control on a couple of screens when maxed out. The Acer Predator Z35 at maximum 200Hz and the Asus ROG Swift PG348Q at the top 95/100Hz refresh rates both showed a similar issue in our testing. In simple motion tests like PixPerAn we observed an odd "jump" in the overdrive control which caused a dark overshoot artefact to periodically appear in front of the moving image. This would then jump back to normal behaviour before cycling again. It seems that on some screens if you push them to the limit it might impact the overdrive control somewhat.

Overall the boosted refresh rates hold some promise for the future, certainly for screens which are currently limited to lower 60Hz refresh rates. Manufacturers are limited by available panels, especially in current popular formats like ultra-wide screens and so overclocking the refresh rate is a good compromise and seems to offer nice additional benefits and performance improvements. Perhaps once DisplayPort 1.3 is available we will begin to see higher refresh rates natively from some panel manufacturers as well and so overclocking will be unnecessary. When you start talking about overclocking high 144Hz refresh rates we do feel you start to enter an area of diminishing returns to be honest. You have to consider whether it's going to be possible or practical for you to power a screen at the higher refresh rates reliably anyway for a start. Then we feel the improvements in actual perceived motion blur above 144Hz are far less obvious than were you moving from 60 to 100Hz for instance. Getting response times low enough without a lot of overshoot is also a big challenge and so there reaches a point where the additional refresh rate is rather pointless other than sounding good on a spec sheet. We've not so far seen any real benefit at all with anything overclocked above native 144Hz panels in fact, but there's definitely improvements with the 60Hz overclocked panels when done properly.

 


Calculating Bandwidth Requirements

Even if we ignore the other factors such as the panel capabilities, response times etc there is a limit to how far a refresh rate can be pushed which is dictated by the bandwidth of the video interface being used. The bandwidth needed is related to several factors including the resolution, refresh rate and colour depth. The video interface being used needs to be able to support the bandwidth requirements of the screen to function. At the moment, the maximum bandwidth capacity for a single cable is DisplayPort 1.2 which has a 21.6 Gbps overall limit (17.28 Gbps effective) as per the DisplayPort specs.

To calculate the bandwidth requirements for any given display you need to consider this rather complicated equation:

        ( (horizontal res + H blanking) x (vertical resolution + V blanking) )   x  colour bit depth   x    refresh rate   x  TMDS 8-bit>10-bit conversion = total bandwidth requirement

For example if we look at the Asus ROG Swift PG279Q with 165Hz overclocked refresh rate the equation would look like this:

        (2720 x 1460)   x 24  x 165   x 1.25   = 19.66Gbps, so within spec of the 21.6Gbps

Asus have altered the V blanking width to accommodate the higher refresh rate. Here, the v blanking is 20, while H blanking is 160. There reaches a point though where overclocking a refresh rate is not possible as you then exceed the bandwidth specs of the interface. When DisplayPort 1.3 starts to be used for graphics cards and monitors there will be a big increase in supported bandwidth. We expect we will start to see more higher refresh panels by then as well with native 100Hz+ and without the need to overclock as regularly.

 


Conclusion

In summary we feel there have been some decent improvements in refresh rates in recent times, thanks to the new-found ability to offer overclocking features. Manufacturers continue try to maximise the capabilities of fairly low refresh rate panels, working with the tools they have available to them at the present time. We've seen the most benefit so far when boosting low 60Hz panels up to around 100Hz. This has allowed display manufacturers to greatly improve the gaming experience on high end IPS panels in new and exciting formats, without needing to wait for the panel manufacturers to catch up with the market demands. Thankfully other areas of performance, most notably the response times have also been improved and so these overclocks are more practical and beneficial than older custom methods on Korean import screens (on the most part). We don't feel that overclocking native 144Hz panels has really brought about much positive change so far, although from the work done by enthusiasts it is clear that higher refresh rates will be possible from a technical point of view. Maybe we will see some screens with a boosted refresh rate that really makes a big difference in the future. Once DP 1.3 is widely adopted by the graphics card and monitor industry we may well start to see more higher refresh panels produced anyway, with native support for 120/144Hz and so the need and desire to overclock may diminish. We expect manufacturers will still try to push things from a marketing and specification point of view so don't expect it to die out completely as a feature. We hope to see more positive overclocking experiences in the future and will continue to test interesting monitors as they appear.

 

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