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LCD monitor masterclass

70 February 2009

Monitor masterclassEverything you ever wanted to know about LCD monitors – and a bit more – courtesy of our in-house fl at-screen afi cionado, Jeremy Laird

Everyone loves a fulsome fl at screen. And why not? You can now bag yourself 22 inches of half-decent LCD panelry for under £150. But how much do you really know about

the technology that goes into these widescreen wonders? The next time you pull the trigger on a monitor at your favourite online emporium, will you have a clue what you’re buying? Or will you be winging it based on little more than size, price and maybe the number of input options?

If you fall into the latter category, don’t turn on the hot tap and reach for the razor blades just yet. This month, we’re going to give you all the information you need to sort the pulchritudinous panels from the monged-out monitors. Monitor makers may have a nasty habit of fl uffi ng up

spec sheets with all manner of distracting disinformation. But we’ll help you keep them honest with the most powerful weapon of all – knowledge.

We’ll get you up to speed with with the three key LCD types – TN, IPS and VA – and help you understand where each respectively sucks or scores. We’ll also debunk all that disinformation and help you separate the specifi cations that count from those that serve merely to pad out press releases.

We’ll have a look at the technological developments you can expect from monitors over the next few years, too. Like just about every aspect of PC tech, LCD displays just keep getting better

and better. That trend is set to continue with the rise of new technologies such as LED backlights. However, there are also some less welcome developments to be aware of, including the more widespread use of cheap panel types along with a number dubious new image enhancement techniques. With all that in mind, may the fl at-panel pedantry begin.

THE SAME BUT DIFFERENTIf you come away with just one insight from this month’s LCD technology expose, for the love of liquid crystal make it the appreciation that not all LCD monitors are the same. In fact, there are three major types of LCD panel

“A dingy, washed-out image featuring mediocre contrast and very restricted viewing angles is the hallmark of TN”

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manufacture. Unsurprisingly, TN panels aren’t exactly the sexiest lookers. A dingy, washed-out image with mediocre contrast and very restricted viewing angles is the hallmark of a TN display. Much of this is because the default position for the crystals in a TN panel allows light to pass through. Only when a voltage is applied do the crystals ‘twist’ into a position that blocks light.

Fine control of the crystal states is another TN shortcoming, leading to poor colour accuracy. However, one of the upsides is rapid response. The crystals in TN panels change position more quickly than any other panel type, making them great for fast-paced frag fests and action movies. In terms of quoted specifi cations, TN panels therefore lead the pack for pixel response, achieving very low single digit milliseconds times.

Elsewhere, things aren’t so impressive. Static contrast ratios for TN screens are usually well below 1,000 :1 while viewing angles are typically below 170 degrees in both horizontal and vertical planes. However, all of these specifi cations are subject to abuse by manufacturers (see box on page 73).

IPS – IN-PLANE SWITCHINGAt the opposite end of the scale for everything from price and image accuracy to frequency of use is IPS or in-plane switching technology. Where other panel types have just one controller transistor per picture cell, IPS screens have a pair and therefore offer

February 2009 71

technology. Each has its own very distinct characteristics in metrics such as pixel response, contrast and viewing angles. No single panel type is superior across the board.

However, before we trot out the various pros and cons, it’s worth understanding the basic principles that underpin any LCD display, from a simple monochrome panel in an ancient pocket calculator to the latest high screens with millions of eye-popping colours. LCD, of course, stands for liquid crystal display. Each picture element, or pixel, in an LCD panel is made up of just that, a cell containing liquid crystals.

On a desktop LCD monitor, there are thousands – if not millions – of these pixels, arranged in a grid pattern. Behind this grid is a light source, literally the backlight. When an electric current is passed through the crystals courtesy of a pair of transparent electrodes, they realign themselves. Precise techniques vary between panel types, but the idea is that there are two basic positions or states the crystals can assume. In one, light is allowed to pass through, in the other it’s blocked out.

OK, it’s actually a little more complicated than that. The crystals don’t block light, they polarise it in a certain plane, and in combination with static polarisation fi lters, light either passes through the LCD panel or is blocked. What’s more, the crystals can assume intermediate positions, allowing some but not all light to pass through. But you get the idea.

Now, on a full colour LCD panel, every pixel is actually made up of three individual elements, each with a colour fi lter – red, green and blue. However, these elements are so tiny, the light emitted by them is amalgamated at normal viewing distances. Hence, by varying the intensity of light that passes through these elements, a full palate of hues is generated and Bob’s your mother’s technicolour brother.

Those principles all LCD panels share, but there are lots of detail differences. For instance: some panel types block light more effi ciently, allowing deeper blacks and greater contrast. In others, the liquid crystals change state more rapidly, delivering better pixel response and hence sharper moving images. Still others offer fi ner control of the positions that the crystals assume, giving superior colour accuracy. But which are best in what areas?

TN – TWISTED NEMATICTN or twisted nematic panels are easily the most common in today’s desktop monitors. For the most part, that’s because they are dirt cheap to

Widescreen 16:10 aspect monitors already dominate the desktop. But a new trend will see screens adopt the even wider 16:9 ratio more commonly used for HDTV sets. Already, most major monitor manufacturers have announced plans to rapidly shift the majority of their PC panels to the 16:9 ratio by the end of 2009.

The argument for doing so largely involves saving a few pennies on manufacturing costs. Simply put, you can cut slightly more 16:9 panels out of a given amount of LCD substrate. Of course, the 16:9 ratio also means you can watch 720p and 1080p content without those annoying black bars above and below the image.

However, that only applies to HDTV content. Almost all feature fi lms are even wider than 16:9, so those irritating black bars will remain. Still, there is some good news - the new 22-inch class of 16:9 monitors will pack 1,920 x 1,080 pixels. That’s full 1080p and a healthy boost over the 1,680 x 1,050 standard resolution of existing 22-inchers with 16:10 aspect ratios. It also means that you will soon be able to buy a full 1080p monitor for £150 or less. That is pretty damned awesome.

The other big trend in 2009 will be the gradual adoption of LED backlights. In theory, LED backlights deliver a cleaner, purer light enabling better colour fi delity. They also use less power and can be squeezed into smaller spaces. Oh, and they also last much longer than the CCFL tubes used by current monitors. So far, we’ve only seen one affordable monitor with an LED backlight, Viewsonic’s VLED221wm. It’s certainly more vibrant than other TN monitors – boding well for affordable monitors as LED production volumes ramp up and prices tumble over the next 12 months.

WIDER, BRIGHTER, BETTER

The more consistent liquid crystal control of IPS tech delivers better viewing angles

LCD monitor masterclass

PCF223.feat2 71 17/12/08 3:20:47 pm

Above Colours on IPS panels (left) are more stable at extreme angles than those on TN screens (right)

the best control and colour accuracy. But, this added complexity not only leads to increased cost. It also blocks more light and can lead to slightly less saturated colours.

Initially, this might sound like a somewhat disastrous disadvantage when compared to the competitor panel types, but it’s not: the fi ner crystal control of IPS technology delivers the widest and most consistent viewing angles. The reasons for this are a little complex, but involve the way crystals rotate about their axis rather than the way that they tilt up or twist at one end in a TN panel. Anyhow, the upshot is that crystals in an IPS panel present a more consistent face to the viewer at varying angles and therefore allow a more constant quantity of light to pass through each of the red, green and blue picture elements. That prevents the

colours from going wonky when viewed from an angle.

However, the defi nite area in which IPS panels are recognised to fall short is concerning pixel response. The greater range of movement makes the crystals in IPS panels a little more reluctant to shift. The longer that takes, the blurrier a panel looks when rendering moving images. That said, the latest IPS panels are capable of grey-to-grey response as low as 6ms, so you’d hardly call them slow. As for viewing angles and contrast, look for fi gures of at least 170 degrees and 1,000 :1, respectively.

VA – VERTICAL ALIGNMENTThe fi nal panel type is VA or vertical alignment. There are actually two types of VA technology: PVA (patterned vertical alignment) and MVA

(multidomain vertical alignment). PVA is much more common, but both share the same basic structure and give similar image quality.

In most regards, VA panels are a half-way house between IPS and TN technology. For price, pixel response and colour accuracy, they fall neatly between the two. However, VA panels have a few foibles that set them apart from other panels. For starters, the default position of the crystals blocks light from passing through. Consequently, VA panels have the deepest, inkiest blacks available and the highest contrast ratios.

They also boast the highest levels of colour saturation (though not colour accuracy) and viewing angles that are nearly on a par with the best IPS screens. All these factors together make VA the most vibrant, eye-catching panel technology of all. The richness and depth of a higher-end VA monitor gives TN and IPS technology a thoroughly good spanking. Static contrast ratios of 1,000:1 or more are common as are viewing angles in excess of 175 degrees.

But, there is one catch. VA panels are not the speediest. In an attempt to push response times below 10ms, many VA monitors use a technique known as pixel overdrive. While it’s a pretty effective measure for speeding up raw pixel response, it introduces a new problem: input lag (see left).

MAKE YOUR CHOICEThat’s how the different panel types stack up. Funnily enough though, many monitor makers are doing their best to avoid mentioning panel types at all, particularly when the panel in question

LCD monitor masterclass

72 February 2009

“With a few exceptions, IPS is restricted to the expensive LCD panels”

INPUT LAG Pixel response should be fragging fanatics’ number one concern when picking a panel, right? That used to be true, but now there’s something even more scary…

All LCD monitors suffer from some lag due to signal processing needed to drive a matrix of pixels. Symptoms are a tangible delay between the output signal from the video card and resulting image update on the panel; sluggish mouse response on the desktop or ‘laggy’ mouse inputs when gaming. Only aggressively overdriven VA panels suffer suffi cient lag to make it a real nuisance. Overdrive accelerates liquid crystals towards a new colour state by applying excessive voltage, then reducing it to the correct level before the

crystals can overshoot the target state, to give the desired colour faster. We’ve measured it – some suffer from as much as 50ms or more of additional lag compared with a typical TN monitor. So, individual pixel response below 10ms ain’t much use if the whole panel is dragging its heels.

Precisely why overdrive causes lag isn’t clear. Is the LCD industry colluding in a cover-up? Samsung, the biggest maker of PVA monitors, declined to comment to PCF.

More ways to process input at display level before showing means more lag

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Above Normal pixel response: a bit on the blurry side, but no hideous artefacts

importantly, armed with this information, manufacturers can no longer bamboozle you with superfi cially spectacular specifi cations. If you see a TN monitor with a sky high contrast ratio, for instance, you’ll know there’s some dodgy dynamic contrast technology at work. ¤

is TN. But there is tell-tale sign you can use for panel spotting: price. Pretty much all 22-inch widescreen monitors and certainly anything below £250 will be TN-based. Just about any 23 or 24-inch screen cheaper than £300 will almost defi nitely be powered by a TN panel. Likewise, there are a number of even larger 26 and 27-inch TN monitors priced around £250 to £350. PVA is the next most popular panel type. It’s most commonly seen in 24-inch monitors priced in the £350 to £500 range. A handful of MVA screens also fall into this range.

A number of manufacturers also knock out 26 and 27-inch PVA panels; again, these will be signifi cantly pricier than their TN-based equivalents. Finally, Samsung makes a monstrous 30-inch PVA panel. It’s arguably the most spectacular monitor currently on the market. As for IPS technology, it’s becoming increasingly rare. With a few exceptions, IPS is restricted to seriously expensive panels aimed at graphics professionals who are willing to fork out for the very best colour accuracy.

TRICKSY TECHAs important as panel type may seem, it’s not the only factor that defi nes image quality. Recently, several new electronic image enhancement technologies have emerged, not all of which have been unambiguously successful. The most infamous of these

is dynamic contrast. The theory behind it is simple enough. Motion video and 3D games contain scenes that vary in terms of light intensity from brightly lit daytime vistas to dingy dungeons and nightscapes. So why not vary the backlight and therefore enable deeper darker blacks to be rendered without compromising on vibrancy and brightness for brighter pictures?

That’s exactly what dynamic contrast attempts to do by analysing image data as it reaches the monitor and adjusting the backlight to suit. In theory, this all sounds rather impressive, but unfortunately in practice, the result always turns out to be pretty clunky. Typically, the backlight is noticeably slow to respond, and when it eventually does it tends to crush the fi ne details in darker colours. We’ve yet to see a dynamic contrast implementation that actually benefi ts image quality. Then there’s digital colour enhancement. Here, the monitor’s colour balance is digitally processed with a view to either compensating for the shortcomings of the panel itself or just jazzing up the visuals. As with dynamic contrast, we’ve yet to see a colour enhancement technology that really delivers.

FINAL SCREENINGBy now you’ll have a good idea why some monitors look so much better than others and how the different panel technologies compare. More

February 2009 73

With most PC components, you get pretty much what it says on the box. If AMD or Intel fl ogs you a 2.4GHz CPU, you can be pretty sure that’s how fast it runs. Likewise, if GPU makers told us porkies about their numbers of shader cores, there would be scandal of webtastic proportions.

Somehow, monitor makers have been getting away with frankly fanciful claims for years. How? Probably because making objective measurements of screen performance is far from trivial. To reveal a CPU’s frequency, you need only download a tiny freeware application. To measure the brightness of an LCD panel, you need extremely specialist hardware.

Another problem: there are often no hard and fast rules governing a given image quality metric. Take viewing angles. Here, a monitor maker effectively makes an arbitrary decision about just how much colour distortion is acceptable and then draws a line. Unsurprisingly, some choose to fl uff up the fi gures.

Pixel response is another tricky area. Broadly, there are two ways of measuring it. Black-to-white or on-off response is the time taken for the pixel to go from fully on to fully off. But what about grey-to-grey response? Here again, it’s up to the monitor maker to decide upon two largely arbitrary intermediate points from which to make a measurement. Often only a single response statistic is quoted and manufacturers don’t even have the good grace to identify which it is.

Contrast ratios are also a, well, grey area. Sources in the monitor industry tell PCF that most monitors are capable of nothing like the quoted contrast ratios. Likewise, colour depth fi gures can also be fudged. Though dithering is increasingly used to increase the colour depth of cheap 6-bit TN panels, it falls short of the fi delity of a true 8-bit panel… Not that you can tell from the specifi cations list.

DODGY DEALERS

“Is the LCD industry colluding in a cover-up? Samsung declined to comment to PCF…”

The horrors of overdrive: note the weird pink artefacts. Faster pixels sometimes

come at a cost of image fi delity

LCD monitor masterclass

PCF223.feat2 73 17/12/08 3:20:50 pm