Published: October 15, 2008 11:00 PM
By Vincent Alzieu
 
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Now that more and more 120 Hz LCD TVs and computer monitors are on their way, they pose more questions than they answer. In advance of some in-depth reviews of the latest 120 Hz screens, this guide is a brief look at the technology and what it promises for the future.

We start by looking at what 120 Hz screens are and how they work, before looking at our reactions to the very first samples that we've taken a look at.

Then we look at two specific ways to use display equipment--watching movies and playing videogames--and rate their performance for these two applications.

Contents

Introduction
First Impressions
Ghosting
120 Hz Gameplay
120 Hz Movies

» Introduction

Working on the same principle as your TV or even analog moviereels, your computer monitor redraws the screen it's showing several times a second to create the impression of a moving image. 

Measured in Hertz, a monitor's refresh rate is the number of times per second it can show a new image. 

In the days of cathode ray tube monitors and TVs, it was an important characteristic, because if the refresh rate was too low, users would perceive  a visble flicker as screen was redrawn.  The general rule that a higher refresh rate is better still holds true today.

Flicker, however, is a less obvious problem on flatscreen LCDs, which are more prone to ghosting, another visual artefact that leads to 'phantom' images trailing behind the currently displayed frame, which is one of the benefits that an increased refresh rate hopes to improve.

What good 120 Hz?
The next step is increasing refresh rates is the move to 120 Hz, but a lot of people are unsure about whether this will bring any genuine improvements.

Taking a quick peek at our mailbag, here's what most of you want to know about them:

  • what's the point of refreshing the image on the screen 120 times a second if the computer can only send 60 still frames in that time?
  • what's it like for games?  and movies?
  • do they work in 3D?
  • is it just a piece of marketing to con us into buying new screens and expensive graphics cards?
There's certainly a lot to wonder about …
» First Impressions, Two Modes

So, what are 120 Hz actually like?  The first thing you need to know is that manufacturers foresee two different modes for 120 Hz screens, and we've tested both of them.  The higher refresh rate can be put to good use in one of two ways:
  • to show 120 images per second--whether by reading 120 images from your computer, or by computing extra frames to insert ('interpolation')--this is 'pure 120 Hz'
  • to alternate continuously between two 60 Hz streams. In this second mode, a special pair of glasses, which is synchronized with the graphics card to only show you one of the 60 Hz streams at a time gives objects the appearance of being 3D.
First Look at 3D
At the IFA 2008 trade fair in Berlin, we tried out the Samsung SyncMaster 2233Hz to play a pretty convincing test game which made excellent use of the 3D effects. 

Samsung's racing demo gave the most convincing 3D experience that we've had to date, using all the different technologies that have been attempted.

It was a lot better than NVIDIA's earlier attempt based on CRT screens: there was no jumpiness meaning the image was nice and sharp.

There were plenty of non-tech journalists at the IFA, and everybody we spoke to who'd given it a go was taken in.

You can also see our video of the Samsung SyncMaster 2333Hz in action too.


First Look at pure 120 Hz
We've also had a chance to try screens running at 'pure' 120 Hz to show 2D images, rather than alternating between two 60 Hz streams.

This time, it was the turn of another Samsung, the 2253BW (left), which we've just tested and found to be an excellent screen, to go head-to-head against the 2243BW (right), a 120 Hz model.

Despite the former's excellent quality, the latter beats it hands down thanks to the improved refresh rate.

» Ghosting Explained

One of the reasons the 2253BW did so well as a pure 120 Hz monitor was because it reduced ghosting to a minimum.  But just what is this phenomenon, and why is getting rid of it so important?

These two test images show the undesirable effect of ghosting, a problem which affects LCD moniotrs leading to the presence of a fainter, offset replica of the actual image. 

It occurs when images are moving rapidly across the screen and one pixel is not fully deactivated after the image has moved on to activate others.


The first line, which was photographed with a very fast camera, shows what's actually happening, while the second shows how the images appear to our eyes.   Note that while this second line is by definition a simulation, it's not random: we've carefully calculated the appearance of the design based on the speed at which it was moving and the length of time between each frame.

As you can see, with a 120 Hz refresh rate, ghosting is not entirely eliminated, and there is still a slight blur.  

However, if you compare it with the text at 60 Hz, which is rendered almost illegible due to ghosting, the text at 120 Hz is a definite improvement and easily readable.  To the eye, the difference is very noticeable: in one you can read the text, in the other, you can't

Of course, this test image on a simple white background is an ideal scenario, and 120 Hz screens won't always perform as well as this in reality.

In practice, it's possible that certain combinations of colors result in a second phantom image because of the incredibly fast refresh rate.  This would make the blurriness worse than suggested by the simulations above.

» 120 Hz Gameplay

Some avid gamers are wont to complain that there's no such thing as 120 Hz games.  Well, au contraire, we say.

Even if some 3D games like Far Cry only rarely get up to 120 Hz, plenty of others benefit from the extra sharpness that a higher refresh rate offers.

Plenty of strategy games and simulations--The Sims, for instance, or the whole Civilization series--just look better on a 120 Hz.

Movements in particular are handled with more finesse, whether it's dragging soldiers across the battlefield or scrolling across a map.  

Everything is neater and sharper with less ghosting trailing behind.

Personally, as a big Civilization fan, I now find myself complaining about how blurry it is to drag units around the screen on a 60 Hz monitor, even though I never even thought about worrying about ghosting in this game before.

Even for occasional players, sharper images with less ghosting are a clear advantage.

Interpolation
First-person shooter games are a lot trickier to handle, though.

Indeed, all sorts of action games--whether they involve running, driving or shooting--find it hard to reach the dizzy heights of 120 Hz for extended periods of time.

The number of hardcore gamers who seek to push their harware to achieve results like this is limited, but they are out there, and a lot of them read DigitalVersus (as well as working here too!) so we're going to look at what 120 Hz brings to this.

So, what to do if your graphics card can't manage 120 Hz all the time?  Well, the first thing to remember is that when it does output a 120 Hz signal, the results will be good with sharp, clear images and reduced ghosting.

But when it doesn't, there are a few tricks that you can try.  The first is quite simply repeating every 60 Hz frame so it's showed twice, effectively creating a 120 Hz signal.

This doesn't require an enormous amount of work on the part of the graphics card, but it does allow for a decent reduction in ghosting on screen.

However, if the graphics card's driver is configured properly, it should be able to tell whether the signal it's receiving is a 'true' 120 Hz, or whether it's been interpolated for better results.

In the second case, the ideal solution is to go one further by not just repeating the interpolated images, but by redrawing them so they are the average of the frames that immediately follow and precede them.

This process of 'creating' new intermediary frames is more easily described wih a diagram:



120 Hz Interpolation: The Drawbacks
This system isn't just a hypothetical solution though: it's already used by the best modern TVs.

Whenever we've tried it on a television for either gaming or movies, we've found the improvements in fluidity striking and a definite improvement.

Indeed, manufacturers like Sony are going one further and using advanced interpolation to create refresh rates of up to 200 Hz on their Bravia 40Z4500 that we reviewed recently.

However, a computer graphics card is not necessarily able to match the performance of dedicated interpolation circuits integrated into these very high-end TVs.

By definition, the frames to be interpolated have to be calculated, and it's not yet clear whether this will lead to a time lag.

The image shown is already effectively behind the real time source signal because the graphics card has to buffer several frames of data to compare the preceding and following frames to allow for the calculation of the interpolated frame.

If this calculation is instantaneous--which is far from certain--then the signal which is eventually sent on to the graphics card shouldn't be delayed.

But any delay in calculating the interpolated frames--at the rate of up to sixty per second, remember--will slow down the display performance of the whole system.  
» 120 Hz Movies

When watching a movie, switching up to 120 Hz is a real personal choice.

Given that movies are regularly shot at 24 fps, getting up to 120 Hz requires one part of the system to take care of interpolating the images.

It can either be the graphics card, or the screen itself, but somewhere, some fairly complicated processing has to take place to allow 120 images per second to be fed to the screen, improving the fluidity of movement.

This is not a new idea: modern TV sets have been working on the idea for over a year, with a varying degree of success.


Over time, the process has been improved.

The very first models we tested (Samsungs … again) had less powerful processors than today's models, and objects had incredibly clear edges, which made movies look far too much like TV reports.

Now though, TVs running at 100 Hz are available from Philips, Samsung and, above all, Sony, that really deliver the goods when the higher refresh rate is activated.

As I said in the Introduction though, these high refresh rates are  question of taste, and if you don't like them, you can deactivate them.

Those who prefer sharper movements, with panning shots that don't inspire nausea can flip to 100/120 Hz on their TV, but it's not compulsory.

The same effects will probably be found with monitors.

A Samsung test card shows some of the gains hoped for from the transition from 100 (left) to 120 Hz (right)



 
Beware of dodgy demos
Of course, the images shown here have been cherrypicked by Samsung to show off their 120 Hz technology in its best light, and so it's worth taking them with a pinch of salt.

If you're out there looking for 120 Hz hardware, you should be aware of the kind of demos that tend to produce the most striking results.

In most cases, the easiest interpolations to calculate involve moving objects that follow a straight line without changing shape or speed, although, as you can imagine, this is rare in films.

It is easy to calculate the interpolations with some fairly easy geometry, though, which is why of course, manufacturers like Samsung use them for their trade show demos.
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