Writings on technology and society from Wellington, New Zealand

Thursday, April 17, 2008

Computer monitors

This week on Radio New Zealand National I talked about computer monitors. They may sound boring, but some of us spend a lot of time staring at them. I talked about where they have come from, where they are going and how they work.

Read on for my speaking notes, or listen to the podcast!

Q: OK…so computer monitors. These are the screens that computers all have, right?

A: Yes, flat panel screens are the most popular technology at the moment. But they are the latest in a long line. Many years ago, you used to program computers using paper tape or punch cards, and the answer came back the same way.

Q: Sounds a bit hard to read!

A: Yes, you had to really want to use the computer! Sometimes you could put the card deck or the paper tape that the computer had spat out into a printer and it would print out a great pile of paper you could read, or perhaps a lot of gas bills or bank statements or whatever the computer was doing for you. That was back in the days when computers saw their work as a series of jobs, each with its own input, probably on punch cards, then a processing step where the computer took the input and ran it through a program, and then an output step. Then the computer waited for its next job. And computers in those days were titanically expensive things, and it was important to keep them busy, so people would build up a queue of jobs for the computer to do. After a while the computer manufacturers got a little cleverer, and the computers could do several jobs at once, but they still saw what they did as all these separate jobs.

Q: How long ago was this?

A: Well, punch cards were still the primary way you talked to the first mainframe computer I got my hands on back in the late 70s. But even then, teletypes were coming along. They looked like a typewriter, but one that would sometimes type of its own accord. You’d use those to interact with the computer – you’d type a question or an instruction, and the computer would respond. And that was a complete shift in the way computers behaved. It wasn’t very efficient, because this expensive computer was sitting around waiting for a person to hit keys, so that was one of the reasons it was slow to take off, in the mainframe world, at least.

At the same time, there what were called mini-computers, some running Unix, which were both cheaper to build and better designed to be able to cope with getting other things done while they waited for people to tell them what to do next.

Q: And then we got computer screens, like we have today?

A: Yes – but the first computer screens were just glass teletypes. All they did was display text in a kind of typewriter font, usually in green, and it gently moved down the screen as you typed until you had a screen full, then it started at the top again. The computer programs hadn’t caught up with the fact that screen could do more than teletypes. But after a while, they did catch up and they starting putting characters all over the screen. Still in monochrome at first, and then colour started to get added – we are perhaps at the beginning of the 1980s now. And another thing that started to get added was the capability to do graphics – display simple pictures and charts. And this meant quite radical changes to the underlying computers, which until now had really just seen the world as a stream of characters in and a stream of characters out. PCs and early Macs started happening here, but initially the charge was led by other so-called microcomputers like the Atari, the Amiga, the Commodore PET and the BBC.

These machines were mostly used for gaming – although you could get useful work done a PET or a BBC. But they had better graphics capabilities built in, and software that was designed to use it. And these weren’t expensive machines – it didn’t matter if they sat around for a bit waiting for you to tell them what to do. So the graphics capabilities of the computers and of the screen was co-evolving, as an ecologist would put it.

Q: The graphics on those computers were really clunky!

A: By today’s standards, certainly. But they evolved rapidly to support a wide range of colours on screen and very high definition displays – by 1987 we had the so-called VGA standard on PCs which were 640 by 480 dots on the screen, each of which could be one of 256 different colours. They were totally dazzling. Every improvement since then has been one of degree, not kind. That’s when we started to hear the word “multimedia” to mean that the computer could show pictures and play sounds – it wasn’t until we got to the 486 chip in about 1990 that PC had enough grunt to play a stream of sound.

Q: Are computer screens just TVs, really?

A: They are and they aren’t – fundamentally both computer screens and TVs show patterns of coloured light, but you want different things from them. TVs have traditionally been quite low-definition fuzzy things. All they had to do was display a broadcast TV signal which was never that sharp in the first place, and you sat some distance away so the fuzz didn’t matter that much. But they had to be able to show rapidly moving pictures well. Computer screens were the opposite – you sit close to them and you want to see really sharp text and graphics – it’s much easier on the eyes that way, but the text and charts don’t tend to jump around the screen a lot so the display responding quickly wasn’t all that important.

Q: But now people watch movies on their computers.

A: Haven’t we come a long way! There’s a huge amount of processing power used to decode a movie on the fly and play it on a screen – it would have been unthinkable 20 years ago. Yes, there’s been a kind of convergence of the two things. Computers are used a lot to play DVDs, and TV has gone high definition which means that people are expecting a much sharper picture. But even today, a decent high definition TV doesn’t have the real crispness – measured in dots per inch – that a good computer screen does.

Q: Both TVs and computer displays have gone to flat panels.

A: Yes, let’s just talk about the underlying technology a bit. There’s four kinds – the old glass tube that all TVs had until recently, there’s plasma, there’s LCD, and on the horizon there’s LEDs.

Q: How do these things actually work?

A: They are all quite different. A glass TV tube – called a cathode ray tube or CRT – is a giant radio valve – you’re too young to remember radio valves, but all electronics had them in before about the 70s. So, a glass TV tube is a thick glass vessel with a vacuum inside – that’s why they implode so beautifully if you abuse them – and on the front surface, the screen part, it has a pattern of chemicals called phosphors painted on the inside. And these phosphors glow, they fluoresce, if you hit them with energy. So the trick is to hit them with the right amount of energy at the right time. And that’s done by spraying them with electrons – in the back of the tube, at the pointy end is an electrode called an electron gun – it’s just a lump of metal, really, which is charged up to a very high voltage, and because of that electrons stream from it through the vacuum in the tube to the screen and make the phosphors light up.

Q: How do you control that to get a detailed moving picture instead of just a glow?

A: There are magnetic coils wrapped around the neck of the tube and they steer the electron beam so that it scans its way across the screen from left to right and top to bottom. That’s why the TV tubes have to be made reasonably deep, especially for larger screen sizes – you need space to be able to steer the electron beams in. The brightness is controlled by controlling the current, the number of the electrons, that the electrode gives off.

Q: And how does it do colour?

A: Colour TVs came along a lot later than black and white, didn’t they? It’s harder – what you do is get phosphors in red, green and blue and lay them across the screen in a pattern of tiny dots. Then you arrange for the electron beam to have different strength according to which colour dot it’s hitting at the time, so that you can mix the red, green and blue to achieve the colour you want.

Q: So what about flat panels?

A: The most common kind of flat panels for computers, and for smaller TVs, is the LCD or liquid crystal display. That works by switching light. The trick they use is like Polaroid sunglasses – you know how if you get two pairs of Polaroid sunglasses and turn them at right angles, the whole thing goes dark?

Q: Yes – why is that?

A: Because light has a direction of polarization – think of light as a wave that’s coming towards you – is the wave going up and down or side to side or some other angle? Polaroids work by filtering out everything that’s going from side to side, which is mainly strong sunlight reflected off surfaces. In an LCD panel, there’s a strong light in the back, then a polarized layer, then thousands of tiny cells containing the so-called liquid crystal, which is something that changes its polarization direction when you put a voltage on it. That has the effect of shuttering the light behind the panel or letting it pass through. Now making these panels of a decent size has eluded manufacturers until recently – it involves a making wafer of immensely pure silicon and constructing all the millions of picture cells onto it so that each one works properly. That process is getting easier and cheaper, which is what has driven the price drop of laptops, and of LCD TVs.

Q: So what’s the difference between LCD and plasma?

A: Plasma is a different technology again. It works a little like a fluorescent light. The screen is divided into thousands of tiny cells, which each have a strong voltage placed across them front to back. In the front of each cell is a phosphor like you get in a TV tube. Each cell is filled with a gas – generally neon or xenon, which are chosen because they are totally unreactive and won’t try to chemically combine with the cell walls. The gas is heated to a plasma – that means it is got so hot that some of the electrons dissociate from the atoms, which become positively-charged ions. Because there’s a voltage across the cell, the ions race toward the front and smack into the phosphors so the phosphors give off light.

Plasma displays are a bit more power hungry than LCD, and heavier, which is why laptops are all LCDs, but plasma displays can produce more light and they are much faster at switching cells on and off so they tend to be better for action and sport. Plasma doesn’t usually get you quite as crisp a picture as LCD because its hard to make the cells as small as LCD cells.

Q: So which do you choose between plasma and LCD?

A: plasma and LCD are still a trade off between resolution – that’s how many dots there are per inch, LCD can do far more – and plasma for moving pictures – the cheapest plasma responds faster than the best LCD and you can see the difference when you have a rapidly-moving picture.

If you are after computer screen, or a small to medium TV, I’d definitely go to LCD. If you are after a big TV my best advice is look at several in a shop under challenging conditions, like fast moving pictures. For regular TVs above about 32 inches may you well be better off with plasma, unless you really want to go high definition – and there are some HD plasmas now. It’s tempting to see plasma as yesterday’s technology but I don’t think it’s quite done its dash yet. Big LCD panels are still more expensive and it’s hard for them to show motion without blurring.

Q: What’s tomorrow’s technology?

A: Ah yes, we’ll change again. The next big thing will probably be LEDs or light emitting diodes. These are the little lights – usually red or green – that you get as pilot lights on electronic equipment. They have grown up in size and modern traffic lights, for instance, are now mostly LEDs – next time you are waiting at a light, look closely and you might see that the light is an array of dots rather than just a glass lens over a bulb. Some cars are starting to use them as well. LEDs are very efficient light generators and they are long lasting. The giant displays you get at events are generally clusters of LEDs. We will see them turned into home display panels before much longer – there’s already a small LED DVD player from Sony but it’s only available in Japan.

Q: How do they work?

A: It’s a quantum effect – remember we talked about quantum theory before? A diode is a junction of two different types of material, one side with an excess of electrons so some of its atoms have an extra electron they don’t really want, and the other side of the junction has lack of electrons – it has atoms with holes where electrons should be. When you apply a voltage electrons flow across the boundary and they meet the atoms which are short of electrons, and – here’s the quantum bit – they all give off exactly the same amount of energy as they fall into the holes. That means that all the light from a LED is one wavelength, so it’s a pure colour. You get a mix of colours by combining LEDs of different colours and controlling how much current you give to each one. The problem has been making a large array of tiny LEDs , but progress is happening on that.


An article comparing LCD and Plasma, and another from Consumer.

Wikipedia on CRT, LCD, plasma and LED displays.

posted by colin at 11:53 am  

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