As any serious gamer knows that the responsiveness of a PC monitor can be a potential ‘winner’ or ‘deal breaker’ when it comes to the purchasing decision. Unfortunately for them the marketing machine is whirring away and has, for the past few years, been well ahead of the game on this front. Sometime around 2005 monitors came to market with ‘revolutionary’ claims of response times as low as 2ms for grey to grey transitions (see the monitor technology article for further information). Whilst this is a good marketing tool and a convenient and eye-opening claim to make these numbers are often criticised for representing the ‘best case’ response times and not representing ‘real world’ scenarios where the response times can reach far greater values. There is no industry-wide standard for measuring a monitor’s responsiveness and, alongside quoted contrast ratios; they should be taken with a large pinch of salt. The consumers meanwhile are largely unaware of this criticism, have become obsessed with low response time figures and are fixated on this one single figure. In most cases, through no real fault of their own, they have no idea whatsoever how it was derived or what it means in practice.
An important thing to factor in, other than the ‘spread’ of response times, is how the particularly rapid transitions are actually achieved – more often than not it will be with the aid of Response Time Compensation (RTC) technologies or overdrive (see this article for more information). The picture above illustrates some of the potential problems that can ensue by lowering the grey-to-grey response times (which are typically above about 12ms ‘unaided’) using overdrive. The picture was taken using the ‘PixPerAn’ (Pixel Performance Analyser) test at 60Hz on a BenQ XL2410T with AMA enabled (see the XL2410T review for further information). This is a TN (Twisted Nematic) panel monitor that is known to use particularly heavy overdrive to boost pixel transition performance. You can clearly see a black ‘overdrive trail’ and possibly even a tertiary white halo; typical RTC artifacts in place of conventional trails. This same type of pixel overdrive (RTC technology) has been implemented into IPS monitors with a good degree of success. Despite the figures ‘on paper’ appearing to show IPS monitors in a bad and unresponsive light, the majority of transitions fall within a comfortable range for good performance at 60Hz. This means that, in practice, IPS monitors can appear every bit as responsive as those ‘super responsive’ TN-panel monitors at 60Hz – and more depends on how effective the RTC algorithms are at ‘hiding’ conventional trails without producing overly conspicuous artifacts. One particularly good example of a highly responsive IPS monitor is the Dell U2410. We reviewed the U2410 in some depth and were pleasantly surprised by how responsive it was. The low input lag (affecting how responsive a monitor ‘feels’ as the result of user interaction) coupled with well-implemented overdrive meant that responsiveness in the real world was actually superior to some 2ms-rated TN panels. But what if we try to raise the bar a little higher by pushing up other factors affecting perceived responsiveness?
Well there is a third and rather important aspect of the computer monitors responsiveness that is receiving recent renewed attention – the refresh rate. Until the recent spate of 120Hz ‘3D’ monitors this was often brushed under the rug as some sort of CRTesque relic. After all; the backlight of an LCD monitor isn’t linked in any way to the refresh rate of the panel so it doesn’t ‘flicker’ even at ‘60Hz’. This refresh rate does, however, have important consequences for the apparent smoothness of motion on the screen. In the simplest terms; a 60Hz monitor is limited to a transition time between frames of 16.67ms (1/60= 0.0167s= 16.67ms). At 120Hz the transition time between frames is lowered to 8.33ms (1/120= 0.0083s= 8.33ms) which can increase the apparent fluidity of the image if certain other conditions are met. Don’t get these frame transition times confused with pixel response times as they are not directly interdependent – refresh rates dictate the rate at which frames are sent to the monitor whilst response times describe the time the monitor’s pixels spend transitioning to a new frame. For optimal visual performance at a 120Hz refresh rate the monitor would have to complete most pixel transitions within 8.33ms to prevent ‘frame overlap’ – with even lower values being preferable. In reality that is a big ask for any LCD monitor and, as with the 60Hz monitors, it relies on the use of RTC technology or ‘overdrive’. The PixPerAn image below should be familiar to those who have read our XL2410T review as it was taken with the XL2410T set to 120Hz and AMA overdrive enabled. As you can see the ‘overdrive trailing’ is significantly reduced and the overall visible trailing is minimal. It is still there and is more significant in this particular test than on certain other 120Hz monitors such as the Samsung 2233RZ [source]. In practice each frame is displayed for half the length of time that it would be on a 60Hz monitor and with the pixel transitions themselves being nice and fast the small amount of overdrive trailing is itself difficult to detect – coupled with low input lag the apparent responsiveness is very good.
Unfortunately for the hypothetical 120Hz IPS monitor the current technology is unable to achieve consistent pixel transitions below this 8.33ms threshold – so they don’t even reach the bare minimum for 120Hz to work effectively. The overdrive used to even go some way to achieving this goal would have to be so aggressive that the resulting artifacts would be hugely distracting on a PC monitor. This kind of tehnology may lend itself well to high-quality 3D TVs. This is largely because such artifacts are not so problematic at a distance and neither is the remaining trailing. The current bandwidth of HDMI is, however, too restrictive for this. Given the projected push and current considerable investment made by the major IPS panel producer (LG Display) towards OLED monitors it may be that we never see IPS technology suitable for highly responsive 120Hz monitors. If we do see 120Hz IPS (and possibly VA) panels then they will probably aimed primarily at ‘enabling’ stereoscopic 3D viewing – without ‘secondary benefits’ such as a significant reduction in visible trailing and superior 2D responsiveness. Looking on the bright side – OLED monitors have projected pixel transition times of around 0.01-0.2ms without any sort of overdrive whatsoever. This leaves a lot of breathing space for very comfortable and artifact-free 120Hz performance but will also allow refresh rates to be pushed even further. Soon it will be up to the rest of the computer to catch up with the monitor and not the other way around.