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TekSpek Monitors
LCD Monitors

LCD Monitors
Date issued: 21/10/2008

Now shipping with all the but the cheapest complete PCs are LCD monitors. Advances in display manufacturing and associated cost reductions with economies of scale have brought LCD monitors into the mainstream, shipping with budget systems that start at just £400. LCD monitors come in all shapes and sizes, have differing resolutions and inputs. The purpose of this TekSpek is to provide a basic understanding of how LCDs work, delineate their desirable characteristics, and to offer basic buying advice.

The technology

Underlying technology
LCDs work in a relatively easy method to understand. Firstly, behind each LCD screen, where screens are defined by the number of pixels on a given panel, a light source is shone through from the behind to panel, through two panels of glass sandwiching the LCD, to your eyes. Each pixel on the LCD display has an array of liquid crystals (from whence the name arrives) whose molecules can be charged with a variable amount of voltage from electrodes over each subpixel (where each pixel is divided into red, blue and green filters), resulting in varying levels of light, from the source, passing through. Oversimplifying it somewhat, it's the combination of subpixel voltage-switching and light source that make up the images you see.

LCD technology encompasses digital watches right up to ultra-high resolution displays. For the latter, each pixel can display one colour to another, activated by the variable voltage-switching from the pixels' transistors, detailed above. Faster switching between opposing colours (usually black-to-white), quoted in milliseconds, is usually a reasonable indicator of the panel's effectiveness in displaying fast-moving images. The lower the ms time, the less time needed to redraw an image and, consequently, the lower the chance of ghosting or smearing when displaying fast-moving images. Manufacturers often have different methods of determining pixel response time, so it requires a little time investment on your part, to correctly determine whether the quoted figures between competing manufacturers is a consistent measure.

Types of LCDs
LCDs can be defined by the technology used to control the amount of refracted light (and by inference, colour) passing through from the source to your eyes. Broadly speaking, three technologies exist, and each has its own benefits and disadvantages. Twisted Nematic (TN) LCDs literally twist the liquid crystals in the display. The greater the twisting, controlled by the pixels' transistors, the less light passes through the LCD. Colours can then displayed in relation to just how much light is being passed through each pixel. This approach offers fast colour switching (response time) but usually at the cost of poor colour rendition for dark colours, especially blacks.

In-plane switching (IPS) uses two transistors per pixel, where the voltage is applied at each end of the horizontally-placed liquid crystal. The benefit is a better viewing angle, resulting from the alignment of the liquid crystal molecules. Vertical Alignment (VA) take the viewing angle benefits of IPS displays but only use a single transistor to do so. With no voltage applied by the pixel's transistor to the naturally vertically-aligned liquid crystal, deep blacks are displayed, from the complete blocking off of light. Voltage application turns the liquid crystal from vertical to horizontal, matching the underlying layer and producing a white image. Colour consistency, therefore, is one of the hallmarks of VA displays.

The use of millions of transistors to create an electric field around liquid crystals can lead to the unwanted side-effect of dead/stuck-on (sub)pixels, arising from transistors not working correctly and thus not applying the correct voltage (or at all) to the LCs. LCD manufacturers generally claim that a non-perfect screen, that is, one with a small number of erroneously behaving pixels is a by-product of keeping LCD costs down. Recently, though, advances in manufacturing have allowed certain manufacturers to offer a zero bright-dot policy, where the display will be replaced if it ships with dead pixels.

Size and brightness/contrast considerations
LCDs are usually offered in a number of sizes, measured from corner to corner, with the display's native resolution (number of vertical and horizontal pixels) and pixel pitch largely determining the optimum size. Pixel pitch, the space taken up by a single pixel, defined in miilimetres, usually ranges from 0.2mm-0.3mm. A lower pixel pitch adds in more pixels for a given size of display, thereby offering a sharper image. We've listed a number of LCD sizes and the native resolutions you're likely to find with each.

LCD size 15" 17" 17.1" (WD) 18.1" 19"
Usual resolution 1024x768 1280x1024 1280x768 1280x1024 1280x1024
Resolution acronym XGA SXGA WXGA SXGA SXGA
Pixel pitch (mm) 0.297 0.254 0.291 0.28 0.294
Current street price £90+ £120+ £150+ £150+ £150+

LCD size 20" 20" (WD) 24" (WD) 30" (WD)
Usual resolution 1600x1200 1680x1050 1920x1200 2560x1600
Resolution acronym UXGA WSXGA WUXGA WQXGA
Pixel pitch (mm) 0.255 0.258 0.27 0.25
Current street price £275+ £299+ £680+ £1400+

WD=Widescreen The above table highlights a couple of interesting factors that need to be taken into account when considering an LCD monitor purchase. The jump from a 19" to 20" LCD is accompanied by a near-doubling of street price. Typically, 19" panels ship with an SXGA resolution and 20" screens with UXGA. The UXGA panels' pixel count is almost 50% higher than 19" models, leading to a lower pixel pitch and far greater pixel density. Consumers looking to invest in an LCD monitor for CAD modelling and video-editing purposes, for example, will be best served with panels with the greatest pixel density, so whilst 30-inch wide-aspect panels have the greatest screen estate from the selection above, the ultra-high WQXGA resolution ensures that they carry the lowest pixel pitch.

LCDs can also be defined in terms of brightness and contrast ratios. You'll often see, for example, a particular panel quoted as having 500cd/m² (nits) brightness and a contrast of 1000:1. The brightness figure refers to the candela per square metre, with a higher figure offering greater brightness levels than a lower one. The contrast ratio is defined as the ratio difference in light intensity between the whitest white and deepest black. A higher nits figure and contrast ratio generally gives rise to a brighter screen with better colour representation.

Connectivity
Low-end LCD monitors tend to ship with analogue inputs, fed into the screen via an HD15 or DVI-I cable. Graphics cards natively output digital video signals, so having to convert between digital-to-analog at the source and back from analog-to-digital at the LCD's end adds in the possibility of an inferior image than from a straight digital-to-digital (DVI-D) link, where digital inputs exist on higher-specified models. The degree of I.Q. inferiority between analogue and digital inputs is dependant upon the quality of the panel's convertor, and it's always preferable to use DVI-D for the cleanest, sharpest image quality. LCDs may well have both analogue and digital inputs, and switching between the two is usually just a matter of pressing a mode-select button.

LCD monitors may also carry a number of other inputs, including S-Video and composite. Again, it's worth checking the exact specifications of each panel, to correctly determine the inputs on offer.

The market, trend, and players

The current LCD monitor market is dominated by a handful of players who manufacture the displays themselves. Samsung, LG Philips, and AU are the most cited manufacturers, and virtually every retail LCD, be it Dell, Acer or HP, contains a panel from one of the trio. Advances in LCD design have lead to a lowering of cost, increases in panel performance, and lower instances of multi-defective pixels on a single display.

Year on year LCD panel manufacturers raise the bar with respect to response time, brightness and contrast ratio. A £300 20-inch UXGA panel today, then, is undoubtedly better than a similar £1,000 panel released 3 years ago. Each passing month brings the retail price of a particular type/size of display down a notch or two, and it is possible to buy a perfectly decent 19-inch DVI-capable LCD monitor for <£200. This is precisely why bulky, unwieldy CRTs are now a dying breed, and the low pixel response time, high brightness and contrast ratio figures of current LCD monitors make them a reasonable choice for fast-paced gaming, too.

A recent trend has been to introduce PC-orientated LCD monitors with wide-aspect displays, usually with a 16:10 viewing ratio, that work well as multimedia monitors for playing back content. We expect this trend to continue unabated throughout 2006. We also expect LCD manufacturers to further reduce black-to-white pixel response time with each new iteration of panel, making ghosting/smearing in fast-moving games a thing of the past.

Consumers looking for a LCD monitor to fit a particular budget are urged to check the panel's response time, evaluate its brightness and contrast ratio in relation to its competition, and to make a note of its inputs. The maturity of the underlying technology and stiffness of competition is such that it is difficult to buy a bad LCD now. It always pays to do your research, though!