Authors: Mungler and Agi
Editor: Agi
Introduction
The Cathode Ray Tube (CRT) has been the standard for PCs from the start - from the ancient, tiny green screens to today’s high powered 20" + SVGA behemoths. However, while the image quality of CRT's has continued to improve, there are other areas that are still a concern: mainly power consumption, physical size and radiation emissions. Thanks to standards like TCO'95, the issues of power consumption and emissions are being addressed. These standards are evolving as quickly as the technology. The issue of size, though, has not been as easy to overcome. In fact, as monitor prices continue to fall, users are purchasing larger and larger monitors. The downside of this new purchasing power is that these big monitors consume an immense amount of the space on your desk.
There is a solution that addresses all three of the concerns listed above: Thin Film Transistor Liquid Crystal Display (TFT LCD). These types of computer monitors are compact and yet meet the strictest of standards imposed on today's VDU’s (Visual Display Units). The only problem is, with these new technologies comes a whole new set of issues and somewhat confusing specifications. This article will attempt to explain the meaning of various technical specifications, their importance when identifying a good display model and how these things affect the gaming experience.
So what is a TFT LCD
The Austrian botanist Fredreich Rheinizer discovered liquid crystal back in 1888. "Liquid crystal" is neither a liquid nor a solid crystal; instead it remains in a state much like oil. In the mid-1960s, scientists demonstrated that liquid crystals could change the properties of light passing through it when you apply an electric field. Prototype displays were made soon after, but were too unstable for any kind of mass production. However, a British researcher proposed using a stable, liquid crystal type material (biphenyl). LCDs then went on to be featured in things like calculator screens and other gadgets displays.
How do they work
Instead of making an image by firing a scanning beam of electrons at tiny phosphor dots on the inside of the screen (like CRTs), LCDs use a strong backlight as their light source, and then control how much of this light is allowed to reach the dots (pixels) by selectively blocking the path of that light. If you then place coloured glass over each pixel you can create colour images on the display.
Thin Film Transistors (TFT LCD) are an extension of the older Liquid Crystal Display technology. TFT LCD has a sandwich-like structure with liquid crystal filled between two glass plates. TFT Glass has as many TFTs as the number of pixels displayed, but these transistors are lighter than in previous LCD models. Liquid crystals then move according to the difference in voltage between the Colour Filter Glass and the TFT Glass.
Liquid crystals remain transparent unless a voltage is placed across them. While the voltage is applied, the portion of the display under its influence remains opaque. The speed at which the liquid crystal can be switched on and off is relatively slow (this will be discussed later). But in 1992, Thorn EMI announced a new type of LCD called FLCD (ferroelectric LCDs). This technology switches more quickly and also has the ability for each pixel to remain in whatever state it’s in without the need for the switching voltage to be constantly applied. This greatly increases battery life for portable equipment such as laptops and PDA’s. While FLCDs hold much promise, not much of that technology has been seen on the open market as of yet. The sensitivity of the FLCD screens to shock and vibration, which dislodges the crystals, is believed to have been solved and the contrast ratio is also improving as the elimitation of defects increase.
The future for ferroelectric LCDs is still uncertain, but if a higher contrast and wider viewing angle is achieved, they will compete with current LCDs. What is the response time for the FLCD? The FLCD has the advantage that the on and off switching times are the same, unlike other displays and the response time is a staggering 70µsec!!! at normal projector working temperatures. But I digress :)
The Major Issues with TFT LCD
For most users, TFT LCD monitors more than meet the requirements of everyday computing; coping adequately with the Windows environment, photos, video, documents, etc. For gamers, however, it's not quite that straightforward. While CRTs have issues with size, emissions and power consumption, TFT LCD’s have a different set of issues - issues that directly affect aspects of PC gaming. These concerns include native resolution, non-native resolution, response time, dot pitch and a few more.
Native Resolution
The Cathode Ray Tube of regular CRT monitors extends from the glass you see in the front of the screen, into a cone shape to the back of the unit and makes up the bulk of the monitor (in size as well as weight). At the very back of the tube is an electron gun. This gun fires a stream of electrons towards the screen. Magnets steer the electrons from the guns. These magnets are what you adjust with your control panel buttons on the monitor, allowing you to fill the viewable area of the screen. On the other side of the glass is a masking layer of three colored phosphers, the stream of electrons hits this layer and shines either red, green or blue. These primary colours combine to make your image.
TFT LCD monitors work on a completely different principle allowing manufacturers to significantly reduce the screen in both size and weight, while keeping the same viewable area. These screens have a fixed amount of pixels built into the display and are therefore designed to operate at a specific resolution; this is termed the native resolution of the display. Native Resolution is not usually a problem with everyday tasks...but can cripple the gaming experience. Why? If the video card does not have enough horsepower to run a game at the native resolution, the game will turn into a dull slideshow. One solution to get the game to run at an acceptable frame rate (at the native resolution) is to reduce various graphical details options. Considering today’s photorealistic games, however, the resultant image quality is rarely satisfying to the people that want to play these games. So one thing to consider when choosing a TFT LCD is whether your system can handle the images at its native resolution.
Non-native Resolution
If the computer does not have enough horsepower to run at the native resolution, another option available is to change the resolution of the display to something less than the native resolution. This may seem to be a simple answer but this usually uncovers another weakness of TFT LCD’s...most do not display acceptable images at anything other than their native resolution. Two things can happen when a screen is not in its native resolution; either the screen size may be reduced and black bars are added or the image is stretched to fill the screen. If you have a screen with a native resolution of 1024x768 and you play a game at 800x600 then there may be nice black bars round the edge of your screen of 306432 pixels that are simply not being used. Much like watching a letterbox format movie (really really widescreen) on a standard size TV. Alternatively, the screen may use pixel doubling or ‘zooming’. This is where the non-native screen resolution is mapped onto the native resolution. You will not have any black bars round the edge of the screen, but each pixel being output by the video card may be mapped onto several pixels on the actual screen. This gives rise to fuzzy edges on objects, even when the picture is completely static. This situation should be avoided as much as possible. Make sure you look for an TFT LCD monitor that is the right size for your uses and for your computer.
Response Time
Since their debut, a major black mark against TFT LCD’s in the gaming world is their relatively slow response times, hinted at earlier. The screens simply do not refresh at the same rate as the CRT cousins and this leads to ghosting and jagged pixel effects on you display. Ghosting is when the previous image displayed on the screen can still be seen for moments after the image has changed. For instance, in any first person shooter style game, if you look around quickly, a hazy after image of walls etc will trail the actual image on the screen. The image below shows that effect.
(Far Cry image taken with digital camera)
In the photo below, the enlarged version of the picture above, you can just about see the effects of ghosting on the edge of the wall bleeding into the light area to it's right. However, it's difficult to completely convey this effect in static images.
(enlarged view showing ghosting)
CRT’s have a refresh rate measured in Hz. The higher this number, the faster the screen refreshes the display. A CRT monitor redraws the whole display every pass, picking up even the smallest changes at a much faster rate than the human eye. Having a refresh rate that is too low, the CRT monitor will appear to flicker. In the States, the sine wave used by electric utilities is 60Hz. That's why CRT monitors will flicker when run at 60Hz. (for more on the effects of Refresh Rate and VSYNC, click here and here) TFT LCD’s are slightly different and are not hindered by the flicker that plagues CRT monitors. Consequently, TFT LCD’s typically operate at 60Hz and when the image on the screen changes, only the affected pixels on the display are altered. The rest remain unchanged. But changing pixels takes time to alter from one state to another, this is commonly called rising and falling. The time the pixels take to rise or fall is then referred to as the response time. The smaller the response time, the faster the pixels can change and this will result in less of the ghosting effect. This is, in my opinion, the greatest issue with TFT LCD’s and gaming. This response time is given in milliseconds on the specifications for the display. However, gray-to-gray response time is much more important for determining if an LCD is the best for gaming, rather than rising and falling response time. Unfortunately, grey-to-grey response time is rarely given on sales specifications.
Viewable Area
One continuous criticism of CRT monitors was the screen size. Invariably CRT monitors would be sold according to their tube size. So the “18” monitor” refers to the size of the cathode ray tube in the monitor. The viewable area of the screen may be significantly different from 19”. The viewable area may range from 15” to 17.1” depending on the manufacturer of the CRT in question. When referring to LCD’s and TFT 19” is 19”. The viewable area of an 17” CRT is equivalent to a 15” LCD or TFT screen. So if you are perfectly happy with the size of your screen and just wish for it to take up less space. You should go for the next size down (i.e. if you have a 19" CRT desk hog, look at buying the next size down, a 17" or 15" LCD/TFT)
There are a few more minor issues that may or may not affect your buying decision.
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