The Adverse Effect on Overdrive of Running a TFT at 75Hz
Simon Baker, 24 October 2006


A recent round up at X Bit labs here reviewed a lot of the new monitors using overdrive technology (Response Time Compensation - RTC). This included several popular models from Samsung and Viewsonic and is well worth a read.

One of the most interesting points to me was the findings from the Samsung 970P where Oleg Artamanov (the reviewer) tested the response time at not only 60Hz (the normal setting) but at 75Hz refresh rate. TFT documentation always lists the "optimum" and "recommended" refresh rate at 60Hz, but it is interesting to see the effect that 75Hz had on the panel's performance.

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The Samsung 970P response time under normal conditions:

A typical response time graph for an overdriven PVA / MVA panel, in this case, the 970P (at 60hz). Note the response time from code 0 to code 75 is very hard to measure because the transitions are very dark. Tom's Hardware Guide don't really bother recording these in their tests. Tests are taken using a photosensor (photodiodе + low-noise operational amplifier) attached directly to the screen and taking an oscillogram while switching LCD pixels under the photosensor. This shows the response time of the pixels across the entire grey range from dark transitions on the left (code 0), to the lighter transitions on the right, reaching black > white at code 255.

At 60Hz, the graph behaves as expected and in a similar fashion to AU Optronics' 8ms P-MVA panel found here Response time is mostly around the 15ms - 20ms mark, as with Viewsonic's VP191B / VP930. Responsiveness of the panel has been improved greatly compared with old style PVA panels thanks to a heavy dose of overdrive from Samsung. Oleg says in his review:


The monitor is still rather slow on the darkest tones, but its speed improves quickly towards the lighter colours and doesn’t change much on them. Compared with the 193P+, the SyncMaster 970P is obviously faster.

...And at 75Hz

Using the screen with a refresh rate of 75Hz has an adverse effect on the actual performance of the panel, and the responsiveness of the pixels! This graph shows the 970P at 75Hz and you can see that the responsiveness has dropped quite considerably. This graph is much more like a traditional MVA / PVA panel would show. For comparison, the Sony HS95P's MVA panel is without overdrive and shows a similar form (here.) Now I know this is a different panel, but I just wanted to show the similarity between a non RTC monitor using MVA / PVA technology and one which does use RTC in the shape of the graph. Weirdly, at 75Hz the 970P responds like a normal PVA panel without overdrive, not as slow, but certainly not as fast as with the fully functioning overdrive. Oleg says in his review:


The monitor has become considerably slower and the decrease of the response time from the darks to the lights is now less abrupt. The shape of the graph resembles a graph of an ordinary PVA matrix without response time compensation (well, an ordinary PVA is still much slower than that). Most of RTC errors have disappeared, too, because the overdrive impulse is obviously too weak, so where could the errors come from?

So running at 75Hz causes the RTC control to function incorrectly, reducing responsiveness, but having the positive effect that RTC errors / overshoot are reduced. Benoit Dupont over at Tom's Hardware also noticed that the artefacts and overshoot on some RTC enabled monitors was reduced by moving away from the 60Hz recommended refresh rate:


On displays with which the overdrive is poorly controlled, some people can see visual artefacts in FPS games during lateral movements. A halo of colour appears temporarily around the moving object. This phenomenon is due to the overdrive technology used on this type of monitor. In the worst cases, the colour displayed is not the right one for 3 whole frames, which can be visible in the form of unwanted colours. (If you have this problem, one trick that works fairly well is to increase your display's refresh rate to 70 Hz instead of 60 Hz.)


Vincent Alzieu at BeHardware has also tested some models at 60Hz and 75Hz and noticed no improvements in running at a higher refresh rate in practice. He also comments that "2 said that they supported 75 Hz, but when we measured the time between images we realised that they were in fact at 60 Hz" and also that "finally, the last two really ran at 75 Hz…partially. In fact the monitors really displayed four images and then skipped the fifth. The sixth one was displayed normally. When we looked at the results, we realised that this skipped 5th image was to resynchronise the monitor at 60Hz. In fact, it really displayed 4 images in 67 ms whether it was at 60 or 75 Hz." More details from BeHardware here.

So what does this mean?

I spoke with Oleg as this interested me a lot. He said:


The Correct theory is that you need two different sets of overdrive impulse parameters (two different LUTs) to work with two different refresh rates. Maybe Samsung monitors use only one LUT calculated for the 60 Hz refresh rate or may be they use two LUTs, but the 75 Hz one was calculated inaccurately, there it no easy way to check this. My personal opinion is that they use only one "60 Hz" LUT, maybe with some rough on-the-fly corrections when monitor works at 75 Hz.

To translate a little, the LUT (Look Up Table) in this context is an algorithm used by the panel to decide how much voltage it needs to apply to the liquid crystals in order to rotate them the desired amount. The screen receives a request from the graphics card to change the image, and so then has to decide how much voltage is applied to the pixels to change to this image. At 60Hz, the LUT is capable of sending a correct amount of voltage to the liquid crystals, and this is based on the application of overdrive, giving the panel an over impulse to speed things up. This is the principle behind overdrive, and is explained a bit more here if you want to read about it. So at 60Hz the LUT sends a good amount of overdrive driven voltage to the crystals and we get a nice responsive panel.

However, at 75Hz Oleg is suggesting that the LUT does not really operate correctly. The interface chip on the panel recognises the refresh rate is set outside of the optimum refresh rate and in trying to operate the pixels (this is in very basic terms) it cannot interpret the voltage properly. Instead, overdrive is adversely affected and the panel does not respond as quickly as it should.

Oleg has also told me that this was the first of the RTC monitors he tested at 75Hz and so cannot comment on the relevance to other models. It is quite likely that this is a common trait, particularly between other Samsung models I would imagine. He has told me he will certainly test this in any future RTC enabled monitor reviews.

Can anyone test this themselves?

Anyone with an overdriven TFT, can you test this yourselves? We obviously can't use the equipment they have at X-Bit or THG, but if you can compare some fast gaming then that would be excellent. Maybe try something like Pixperan (like BeHardware use) to test comparisons. Does running at 75Hz noticeably decrease performance in real use to you on your screens?




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