Index


RISC World

LCD Monitors

Robert Richards throws is old CRT monitor in the bin....

Well it had to happen. Having got well and truly fed up with a rather worn out and uncomfortable AKF60 I decided it was time to treat myself to a new monitor. I always worked in 800x600, as 1024x768 was just too flickery for comfort at a 60Hz frame rate. I also found the AKF60 gave rather too bright a picture with poor contrast.

For a RISC PC owner upgrading their monitor, there are only two sensible choices:

  • A ViewFinder card and large, high-quality CRT
  • An LCD flat panel.

I chose the LCD route because of three considerations. It would be a fair bit cheaper than buying the ViewFinder card and a large CRT to do it justice, I don't have the physical space to sensibly use such a large monitor, and I don't have need for extremely high resolutions. If money is no object, you may consider a very large, high resolution LCD monitor coupled with a ViewFinder card. But we're talking megabucks here.

So how does an LCD panel compare to an old CRT? And are there any specific advantages to using LCDs on RISC OS computers? Let's have a look at how the technologies differ.

Technology

A CRT works using the same principles as a television. Three beams of electrons are fired from devices called electron guns. These work by boiling electrons off the surface of a hot filament, and then accelerating them towards a highly positively charged metal plate. A small hole in the plate allows a proportion of the electrons to whizz past, escape it's electric field and hit the front of the tube. The whole tube is evacuated - electrons can't more large distances through air. Monochrome screens have one electron gun, and the electrons strike a phosphor coating on the front of the tube. Without turning this into a chemistry lesson, the energy released by the impact is absorbed by the phosphor and re-emitted a short time later as visible light. Electrons will only travel in a straight line if left to their own devices - so the device as described above will only create a small bright circle of light on the screen. A picture is formed by using electromagnets to swing the beam horizontally and vertically. Electrically charged particles can be manipulated using either electric or magnetic fields. To see this simply for yourself, hold a fairly strong magnet near to a black and white television and see the image distort. (Don't try this on a colour TV - as we'll see later colour sets are easily damaged by this).

This is synchronised with circuitry that adjusts the power of the beam to create light and dark patches. The beam is scanned horizontally, and when a line has been completed, it is scanned across the line below and so on. If this is done fast enough, the latent glow of the phosphor dots and effects due to the way our eyes work will create the illusion of an image. Sounds fairly simple, but it's more complicated in reality. As the beam scans towards the centre of a line, it travels less distance, so the vertical deflection will change. One way of getting around this to use a "goldfish bowl" but this can't practically compensate for the effect. The electromagnets therefore have to be continually adjusted to take account of this effect. This is why most monitors have controls for the shape of the image - trapezoid, pincushion and rotation are all provided in order to calibrate the electronics.

Colour screens are more complex yet. There are three guns, and three different types of phosphor - red, blue and green. Look very closely at a television screen (switch it off first!), and you would just be able to see that each pixel is made up of a triangle of dots - one of each colour.

Each gun is responsible for lighting only one colour of phosphor. To ensure that no stray electrons cause mushy colour, a very fine metal mesh is placed just behind the phosphor to stop them. It is this metal mesh that gets damaged when strong magnets are placed near - if it gets bent out of shape it's bust with no chance of repair. Well I did warn you not to try it on your brand new widescreen home cinema!

Now, CRT monitors work in exactly the same fashion as TV sets, but at a much higher resolution, so the electronics must be designed to a much higher standard. The bigger the monitor, the higher the resolution, the better the electronics need to be - hence the sharp cost increase for larger, higher resolution screens. Most monitors are controlled by analogue circuits, but digital controls are becoming the norm for new units. The critical factor with CRT monitors is the frame rate they are capable of at any given resolution. Many cheaper models will offer high resolutions at the price of a slow frame rate. Slow frame rates are what give rise to the flicker that many people find so distracting - even to the point of causing headaches and eye strain.

The other point to bear in mind with CRT monitors is that X-rays are emitted. All those high-energy electrons blasting into metal decay in a burst of energy that is emitted mainly as X-rays. This is the principle used by hospital X-ray machines, though they are many times more efficient at producing them! The good news is that the dosage reduces with the square of the distance. If you sit four feet from the screen, you receive a quarter of the dosage you would were you to sit two feet away, which is a quarter the dosage received from one foot away and so on. Some people may say it's scaremongering, but if you sit for long hours in front of a CRT, there is medical evidence which suggests you increase the risk of developing cataracts. Pregnant women in the US were for a time given the right by law to be given a lead-based apron to wear by their employer - I don't know if this is still the case.

In case your worrying about all that time spent in front of the television - it's not an issue because people tend to sit a lot further away from the TV set.

To sum up, CRT's are large and bulky, can cause eye strain and have been implicated with various other health risks. In addition, many find the "fish bowl" gives a distorted picture. Flat screen CRT technologies solve this problem, but are expensive and because of the way the technology works, there are thin black lines across the image. Some find this annoying, others don't even notice.

Let's have a look at the alternative - LCD technology.

LCDs work in a completely different manner to CRTs. Firstly, the light source is not a transient glow from a dot of phosphor, but (usually) a continuously lit backlight. I say usually as there are other ways of generating the light but these are not widely seen at present, and in any case the effect is the same. So what does this mean? Well, because the light source is of a consistent intensity, there is no flicker whatsoever. Ever. So how do we form a picture? Well, the screen itself is formed of an array of thin-film transistors (TFT). Instead of amplifiers and electromagnets being driven by the computer video signal, the signal is fed into a digital system, which switches the state of the appropriate transistors to generate the image.

Advantages of LCD monitors.

We've already touched on the major advantage of an LCD monitor from a useability point of view - a guaranteed elimination of flicker and a corresponding reduction in the likelihood of eyestrain being a problem. However, LCD monitors have many other advantages. From the point of view of image quality, most LCD monitors boast much higher contrast ratios than their CRT counterparts. Look at your CRT screen when it's switched off - it's grey. Black parts of an image only appear black because of the bright pixels around them, in reality they can be no darker than grey. The higher you have the CRT brightness turned up, the lighter that grey becomes, which results in a further loss of contrast. A CRT screen is much closer to a true black, and even turning the brightness up to 100% will results in only a little lightening of the dark areas of a picture. The more expensive and higher-quality models will boast a better contrast ratio than the cheaper models, but even the budget model I purchased is incomparably better than the AKF60 it replaced in this respect.

Power consumption is an issue that is becoming increasingly important in the world. An LCD screen will typically consume between 25 and 30 Watts in operation, and less than 5 Watts when on standby. This is a mere fraction of the power requirements of a CRT monitor. To put this into context, it's currently costing me four times as much to light the room that I'm working in than it is to run the monitor I'm using. And I've only got a single 100W bulb burning! Let's try and bring this into context - are there any specific advantages offered by LCD monitors that are particularly relevant to RISC OS users?

Well yes there is one! Remember that the LCD monitor image is built up using entirely digital means - there's no need to keep refreshing the image as with phosphor dots. This means that the refresh, or frame rate can be set to a much lower value than with CRT monitors. For those using ARM7500 based machines, this brings the obvious benefit of reducing the proportion of CPU power dedicated to generating the display. A slower frame rate means more of the CPU time can be spent on number crunching. In short - the machine won't slow down so markedly when running in high resolution modes. RISC PC users may also notice an improvement. If you take a 600 model and run it alongside a later 700 model with the same monitor, configuration and so on, you will notice that the 600 displays much crisper images in high resolution and high colour modes. Anti-aliased fonts in particular will look much more pleasing to the eye - if you're using a 700 machine, no the fonts are not supposed to look "fuzzy" like that. Yes, anti-aliasing works by "fuzzing" the edges of the fonts, but it's supposed to make the font look more like printed text rather than as though it's been smudged.

So why do we see this curious effect? The answer lies on the motherboard. After the RISC PC was released, the EU rules on electromagnetic emissions were tightened, and the RISC PC failed the new tests. Acorns answer was to choke the output from the VIDC chip and dampen the high frequency signals. It's these same high frequency signals that give definition to high resolution images. In effect, the bandwidth of the video output was reduced and image quality suffered. Now, if you use an LCD monitor, you can use a lower frame rate, and hence a lower pixel rate, and minimise the effect of the choke by using less bandwidth in the first place. Net result is a sharper image.

I should probably point out that because ViewFinder bypasses VIDC, it doesn't suffer in any way from the VIDC choke. And finally, an LCD panel is of course significantly smaller and lighter than a traditional CRT monitor. Mine is around 3" thick. Also remember that a 17" CRT may only have 15" screen viewable, whereas a 15" LCD has 15" screen viewable.

OK. So those are the advantages - better contrast, lower power consumption, smaller, lighter, no flicker or eye strain, a faster ARM7500 machine and a clearer RISC PC image. But everything in this world gets thumbs down for some aspect.

Disadvantages.

Resolution. Currently a big problem for LCD monitors. Anything over moderately high resolutions and you'll need very deep pockets. 1600x1200 panels are about the absolute maximum at the moment. Expect to pay around 900 for this though. Of course, if money is no object you could buy a ViewFinder and an LCD monitor. My budget model can do 16 million colours with a maximum resolution of 1024x768. But the 2MB VRAM limit is reached with only 32 thousand colours. So there you go - a bottom of the range panel, and a standard RISC PC still can't drive it to it's full potential! LCD monitors are designed to run at a specific resolution. Because each pixel consists of a distinct "square", running the screen at a lower resolution means that the pixels will become blocky (more so than a CRT screen) and possibly rectangular. However, games are the only time you'd really want to lower the resolution, and the effect is most noticeable with text. So I at least can live with it. I have to say that the RISC OS desktop and decoration may initially look ugly compared to a CRT image. In particular, window title bars stood out a mile when I first used the LCD - the yellow CRT, but looks like a mess of oddly coloured pixels on an LCD. A fiddle with the colour balance and a period of "getting used to it" has reduced the distraction. It still doesn't look right. Ironically, this is because of the much greater definition an LCD panel offers compared to a CRT.

Summing up

If you've got a fuzzy old AKF60 and are wondering whether to go for an LCD or new CRT I'd recommend giving the LCD serious consideration. Although it is significantly more expensive, the benefit in terms of viewing comfort more than compensates. Don't be put off by the bottom of the range models (so long as it's capable of at least 1024x768 and 15" viewable) there are actually only a few manufacturers of LCD panels - the brand name companies just stick them in a casing and apply a few logos. Don't be put off by horror stories of "dead" pixels. This certainly used to be an issue (TFT manufacture is a highly sensitive process, with many thousands of transistors a few might be defective leading to a permanently bright or dark pixel - a "dead" pixel). More recently, the processes have been refined and you're unlikely to see any problems like this. I haven't noticed anything amiss with my budget model - badged as "Video7" from an unheard of German company.

If you do go for an LCD, make sure you buy from a RISC OS dealer (unless using it with ViewFinder - this should be capable of driving any of the higher resolution models). Because of the digital circuits that control LCD panels, the timing and sync of the analogue signals from the video port are critical. A properly constructed MDF is required to assure a decent picture. Mine gives a picture with the standard AKF60 and default MDFs, but there are bright lines over the screen. It's good enough to configure the machine in an emergency, but no good for proper use. I bought mine from ExpLAN, who provide full instructions, an MDF with several modes defined from 400x352 to 1024x786 for the 15" budget model and a three year manufacturers guarantee. Looking in Computer Shopper, the price I paid was similar to, and in some cases less than some PC-centric dealers were selling similar or identical kit for. RISC OS dealers are often accused of overcharging (occassionally rightly so), but this is one area where there are a number of good deals. Shop around - I've seen reasonably priced LCDs from CTA, CJE, APDL and Liquid Silicon to name a few. There are also some terrible deals around. Either some people have forgotten to update their price list or there's something special about those monitors they aren't telling us!

And don't worry about getting your old games to work with a relatively fussy LCD. With this particular model, when it comes across a screen mode it hasn't seen before, I press a button and it automatically configures and calibrates itself to the new mode. The calibration is then saved - the process only ever needs to be done once. The monitor will subsequently select the appropriate calibration from it's internal memory when changing modes. If you need high resolutions, the cost ramps up significantly and you'll also need a ViewFinder to make the investment worthwhile.

If you can afford the extra cost, an LCD monitor is definitely the way to go. Don't hold off thinking the price will drop further, it won't because the current manufacturing processes have been made about as efficient as possible. Last year LCD's dropped to around a third of their original price, and prices may well rise this year. Simply put, the LCD monitor has been probably the most useful and appreciated computer purchase I've made for quite some time. I'm quite sure that anyone who makes the decision will not regret it.

Do beware - once you've used and LCD you'll never want to look at a CRT monitor again. Best start being nice to the boss so you get one at work too. Just tell him it'll leave more space on the desk for a bigger "In" pile!

Robert Richards

 Index