Notebook screens explained

Q I am shopping around for a laptop -- how times have changed, as I have not yet needed to leave the house! I am becoming more and more confused with the different types of displays on offer from the different manufacturers: Passive DSTN, Passive XGA, TFT Active Colour Screen, XGA TFT Colour Screen etc. Are these all different or are they just trying to confuse me by changing the titles? Please help by describing the types of displays and explaining which is the best value for money.

- Shane Chiddy

A Computer technology changes so quickly that it's difficult to keep up to date with all the new terms and acronyms that appear almost daily. You could read piles of computer books and magazines and still get blind-sided by a term that you don't recognise or remember. My favourite computer acronym is PCMCIA -- People Can't Memorise Computer Industry Acronyms, although the Personal Computer Memory Card International Association may not approve of that interpretation!

Manufacturers of notebook computers generally use flat LCD (liquid crystal display) screens to create displays that are light and non-bulky. An LCD screen consists of a fluorescent panel that emits light, a layer of liquid crystal material that is sandwiched between two polarising plates, several colour filters and a front panel. Most notebook screens are either active-matrix or passive-matrix displays. Active-matrix displays commonly use TFT (thin film transistor) technology, which allocates one to four thin film transistors to each pixel on the screen. This means that the electrical current that illuminates the pixel is smaller and can be turned on and off more quickly, providing good on-screen contrast. The display refreshes quickly, producing a responsive image that is capable of keeping up with the cursor. Active-matrix displays operate at high resolutions to generate sharp images in brighter colours, and provide a wider viewing angle than passive-matrix displays. TFT displays are the best notebook screens currently available, but they are often expensive because of their complexity.

A passive-matrix display consists of a grid of vertical and horizontal wires with an LCD picture element, or pixel, at each intersection of wires. The columns and rows of pixels are each controlled by a single transistor, causing the display to be drawn line by line. During a screen refresh, any given row will only be activated for a short time, resulting in poor contrast. Passive-matrix displays suffer from slow response times, so the refreshing of the screen is quite visible, and if you move the cursor, it may disappear until the display catches up. Most passive-matrix displays don't generate sharp, bright images. They can also be difficult to view at an angle, and are usually limited to 256 colours. Their advantages are that they are cheaper than active-matrix displays, and use less power, which means that the notebook's battery lasts longer.

Most passive-matrix displays use variations of a technology called Super-Twist Nematic, which gets its name from the way light is manipulated as it travels from the fluorescent panel at the back of the display through the liquid crystal, polarising panels and colour filters to the front display panel. Some common types of passive-matrix displays are:

DSTN (Double-layer Super-Twist Nematic). This is also known as a dual-scan screen. In the process of drawing the screen, the display is halved, and both halves are refreshed simultaneously. This is much faster than redrawing the entire display at once. Although dual-scan displays aren't as crisp or bright as active-matrix screens, they use less power.

FRSTN (Fast Refresh Super-Twist Nematic). This is an attempt to improve the performance of passive displays while keeping the costs down. The display is better than DSTN, but it is still not a great choice for running presentations and high-performance graphics.

CSTN (Colour Super-Twist Nematic). Displays of this kind are an affordable, viable alternative to active-matrix screens. They are relatively fast, can be viewed from a wide angle, and provide high-quality colour.

XGA (Extended Graphics Array). A high-resolution graphics format that was introduced by IBM in 1990. XGA video cards can support 256-colour graphics at a resolution of 1024 x 768, or a resolution of 640 x 480 in "high colour" (16 bits per pixel or 65536 simultaneous colours). XGA-2 is an extension of XGA which supports 1024 x 768 in high colour, and has higher refresh rates than XGA.

Other acronyms you may see are HPA and HPD. HPA (high performance addressing) displays are relatively cheap, and are sometimes used in entry level notebooks. HPA displays are not as fast or as sharp as active TFT displays. The HPD (hybrid passive display) uses a different formulation of liquid crystal that improves the contrast and performance of the display. HPD is a new technology, and may prove to be an acceptable, inexpensive alternative to TFT.

If you can afford it, buy a notebook with a high-resolution active-matrix display. Active-matrix displays provide better quality images than passive ones, and as you'll spend quite a lot of time looking at the screen, your eyes will thank you at the end of the day.

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Belinda Taylor

PC World
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