Tìm hiểu về LCD Monitor

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How CRT and LCD monitors work
Author: Wil Harris
Published: 20th March 2006

We all spend an awful lot of time sat in front of computers. Whether we're gaming or working, we are at the mercy of what many would argue is the most important element of any system - the monitor.

A well-defined monitor can make using a system a pleasure. Likewise, being forced to squint at a 15" CRT at 60Hz can make us weep in pain and long for a nice LCD to while away our hours at. A good display makes all the difference.

Monitors are widely used and rarely understood. Sure, you know that the difference between LCD and CRT is that one is flat and one is massive and heavy. But do you really understand the technology that goes into these things?

In this article, we're going to investigate how CRTs and LCDs work, and also examine some of the issues pertaining to monitors, such as Refresh Rate and Vsync as well as looking into our crystal ball to see the future of displays.

For a primer on resolutions, you might like to check out our previous article here.



The Basics
So let's start with the easy stuff. The picture that appears on your monitor comes from the graphics card in your computer, and the job of the graphics card is to render the picture suitable for the monitor. A wired output runs from the graphics card to the monitor.

But you knew that already.

Both the graphics card and monitor adhere to the same set of specifications, so that they can happily talk to each other. The standards are set out by VESA, which defines things like how monitors identify themselves to the computer.

CRTs
CRT stands for Cathode Ray Tube, and is descriptive of the technology inside that chunky monitor you might have on your desk.

CRTs receive their picture through an analogue cable, and that signal is decoded by the display controller, which handles the internal components of the monitor - think of it as the mini-CPU for the monitor.

CRTs have a distinctive funnel shape. At the very back of a monitor is an electron gun. The electron gun fires electrons towards the front through a vacuum which exists in the tube of the monitor. The gun can also be referred to as a cathode - hence the electrons fired foward are called Cathode Rays.

These rays correspond to to the red, green and blue channels of the display and video card.

At the neck of the funnel-shaped monitor is an anode, which is magnetised according to instructions from the display controller. As electrons pass the anode, they are shunted or pulled in one direction or the other depending on how magnetic the anode is at that time. This moves the electrons towards the correct part of the screen.



The electrons pass through a mesh, and this mesh defines the individual pixels and resolution on the screen. Electrons that pass through the mesh then hit the phosphor coating which is on the inside of the glass screen. When the particles hit the phosphor, they immediately light up - causing the light to shine through the front of the monitor, thus making up the picture on the screen. There are three differently coloured phosphours for each pixel (known as phosphor triads), and depending on which phosphor the electron hits, that's which colour the pixel will light up.

Differences in components
Different monitors differ in quality, and this is often dependent on the technology and components used internally.

Some CRT monitors use a single electron gun at the rear of the monitor to produce the electrons that will become the red, green and blue electron rays. However, higher quality monitors have an individual gun for each, which can increase picture quality.

The metal used for the mesh at the front of the monitor will also affect quality. Electrons also produce ions because of imperfections in the vacuum, and these electrons are destructive to image quality if they hit the phosphor. Consequently, meshes are made of relatively thick metal to prevent phosphor damage. However, in better quality monitors, a thinner, yet tougher metal alloy is used for the mesh. Because it's thinner, it means more light can get through, making for increased brightness and higher contrast.

Aperture Grille v Shadow Mask
Each CRT has a sheet of metal at the front of the monitor which (partly) defines the pixels on the screen. Shadow mask is an older technology, and is literally a piece of metal with millions of holes in it which allow the different cathode rays through to hit the phosphour. Because a shadow mask covers the whole back of the screen, protecting the phosphor from stray ions, it also limits the strength of the rays, reducing the brightness of the monitor.

Aperture grille is a newer technology which defines the gaps through which electrons pass using a mesh of wires rather than a sheet with holes in. Whereas a shadow mask is made of circular holes, the grill is made of vertical slots. Because it is by its nature thinner, it allows for brighter displays. However, the grill is fragile and prone to being knocked around. The grill is therefore strapped to the monitor using stiff horizontal wires - this is what causes the distinctive pair of lines across high-end aperture grille monitors.

Invar mask is a variant of shadow mask, and uses a thinner, stronger metal to form the mask, allowing for better image quality whilst remaining cheaper than aperture grill to produce.

Sony's Trinitron brand and Mitsubishi's Diamondtron brand are both variants of Aperture grill.


Dot pitch and resolution
Each pixel on the CRT screen is defined by lighting up combinations of the red, blue and green phosphors that make up the pixel. With a varying strength of electron gun operating on each phosphor, different colours are produced - with red, blue and green all fired on maximum strength, that means bright white is produced.

Dot pitch is measured on most monitors as the distance, diagonally, between two phosphors of the same colour. However, some manufacturers quote dot pitch on monitors as the horizontal distance between phosphors, which can make them appear better specified, on paper, than perhaps they are.



Dot pitch combined with viewable image area defines the maximum resolution of the screen. For example, if you have a 21" monitor with a viewable area of 401mm x 298mm, and a dot pitch of 0.26mm, you will have a CRT capable of displaying a maximum resolution of 1758 horizontally. How so?

Well, if we take 1 as the diagonal dot pitch, Pythagorus dictates that the horizontal dot pitch (ie the gap between pixels as rendered horizontally by the graphics card) will be 0.87. 0.26 diagonal dot pitch multiplied by 0.87 makes for a diagonal equivalent pitch of 0.228 horizontally. 401mm horizontal viewable screen area multiplied by 0.228 is 1758, hence a maximum of 1758 pixels are usable on the screen.

Got that?

In general, the lower the dot pitch, the better the sharpness of the image.

LCD
Flat panel monitors are a relatively recent product to enter the computer market. The clue to LCD technology is in the name - crystals that are in liquid form. Because they are in a liquid form they are easily manipulable, and this allows us to play with the way that light interacts with them. If you have a flat panel in front of you, try just pressing gently on the surface - you can see the crystals move around and alter the picture.

LCD panels are fairly simple to understand. The signal comes in and, as with a CRT, the signal from the video controller is decoded and understood by a display controller on the monitor itself. The controller has two things to control - the electrics of the pixels and the light source.

The actual image on a TFT is made up of a matrix of pixels. Unlike with CRTs, there's no complex equation of dot pitch and image area to try and calculate - the native resolution of the monitor is simply the number of pixels contained in the matrix. If it's a 17" monitor, chances are there are 1280 pixels in the matrix horizontally, and 1024 vertically.



Each pixel is made up of three sub-pixels, which have red, green and blue filters in front of them, just as each pixel on a CRT has RGB phosphors. The subpixels are made up of a group of liquid crystal molecules. These molecules are suspended between transparent electrodes and are mashed between two polarising filters.

The two filters are exact opposites of each other. As the light from the light source behind the first filter comes in, the filter effectively whites it out - which means that if it was to pass through the liquid crystals with no interaction, the filter on the other side would polarise it back to black, leaving no colour being emitted. In fact, alternate current - leaving the crystals 'dead in the water' - is how black is created on a panel.

However, if the electrodes apply current to the liquid crystals they twist and change the way that the light is passed through, altering its polarisation and this then results in the correct colour coming out of the second polarising filter and being displayed to the user.

The backlight itself is a cold cathode. Depending on how expensive the display is, there will be either a single cathode at the top, or one at the top and one at the bottom, or two at the top and two at the bottom for optimum brightness and clarity. These cathodes are diffused through a layer of plastic and then through multiple layers of diffusing material of the kind you might find on a flashgun diffuser for photography.



Contrast ratio
One of the major factors affecting a TFT is the amount of contrast it has. Traditionally, the contrast is lower than on CRT monitors, allowing for less differentiation between blacks and whites - and meaning that blacks are often ill-defined.

This is the reason that many gamers still prefer CRTs for games like Doom 3 and FEAR, which have an awful lot of black in them.

Put simply, the contrast ratio of a display is the ratio of the brightest possible white value compared to the darkest possible black value. Most desktop TFT monitors have a contrast ratio of between 300:1 and 600:1 while a typical LCD TV will raise that to between 800:1 and 1200:1.

Because the light of the backlight is so bright, the second polarising filter is not able to keep out all of the light when the display calls for black, and this means that blacks can sometimes appear a little more grey. Alternatively, if you turn the backlight brightness down to get pure blacks, this drags the brightness of bright white down. The greater the contrast ratio, the greater the difference between black and white you can maintain and the better quality the display.

For more details on contrast and brightness, check out our article on the Brightside display.

Bit depth
One of the most overlooked features of LCD panels is the colour depth of the panel. To achieve the ultra-low response times that companies often want to sell, colour depth is sometimes compromised by optimising panels for speed rather than quality.

Good quality panels use 8 bits of colour per RGB channel, resulting in 16.7m colours displayable. However, on many modern TFT panels, only 6 bits per channel are used, resulting in just over 16m colours with the rest being dithered, or 'faked', by algorithms.

If you're doing image editing, you will notice the dithering and if you want a high quality panel, you should look for one that's 8-bit. Professional quality panels will use 10-bit colour, and the newer ATI Radeon cards will support that output.

Don't confuse 8-bit colour with 32-bit colour on the desktop, and think that LCD panels aren't utilising the full potential of your awesome rig. 32-bit colour is actually 8 bits of alpha channel and then 8-bit RGB - the same as in your monitor. Good LCD screens can handle everything that the video card can put out.

Refresh rates
60Hz, 100Hz, Vsync... how do all these terms inter-relate?

On a CRT, the refresh rate is how many times, per second, the display is drawn - i.e. how many times the electron guns are told to fire by the video source. The refresh rate on a monitor is limited by how fast the guns can fire - more expensive guns can obviously fire faster. The refresh rate is also limited by the resolution, because the higher the number of vertical lines to refresh, the longer it takes the guns to refresh them all.

We all know from experience that a higher refresh rate makes for a better image that has less flicker, but do we know why? The phosphors in a CRT illuminate when hit with electrons, but begin to fade as soon as the energy from the electron is used up. To keep the phosphor illuminated requires a constant stream of electrons. If they're not coming in fast enough, the phospor will visbly fade then light up again - causing that horrendous 60Hz flicker we all know and love, which is especially visible on high resolution screens with lots of vertical lines to scan. 60Hz is more tolerable on a lower resolution screen where there are less lines to scan.

On a CRT monitor with a resolution of 1600x1200 or above, 100Hz is ideal to keep all the lines supplied with enough electrons to stop the phosphors fading and flicker occurring.



Refresh and Response on LCDs
All of this doesn't really apply to LCDs. The pixels and subpixels in LCD panels don't fade as phosphor does, since the light from the backlight is constant and the current from the electrodes is constant, meaning that light is passed from the pixels for as long as the display controller tells it to. However, LCDs are nominally set up to report a 60Hz refresh back to the video controller, which often requires at least some value to work to.

What does matter on a LCD, however, is the response time. This is not the same as a refresh rate. Refresh rate time is the measurement of how many frames can be displayed per second. For an LCD, response time refers to how quickly a liquid crystal can twist, then untwist to either pass or block the light of each pixel. The faster the crystals can react, the faster the motion that can be displayed on screen.

This is why a low response time is essential for applications like movies and games to be watchable without ghosting. Ghosting is the remnants of the old frame image 'below' the new frame image due to the fact that not all the crystals have updated with the new frame in time to display it.

Any response time below 16ms is fast enough for the eye to perceive full motion, and today's displays of 8, 4 and even 2 milliseconds will all provide a great viewing experience.

Vsync
This is an option used in games to present optimum image quality. When Vsync is enabled, the video controller sends the output to the monitor in line with the refresh rate of the monitor - so 60 frames a second are sent to the monitor if the monitor has a 60Hz refresh rate. Where 80 frames are sent to a 60Hz monitor, the monitor will spend some of its time trying to draw a new frame when the old frame hasn't finished being displayed across all of the monitor. This results in the image 'tearing' that you see occur.

Obviously, Vsync limits frame rate which most people would see as detrimental to a gaming experience. However, with suitably powerful graphics hardware, a constant minimum frame rate of 60FPS coupled with Vsync makes for optimum image quality and gameplay.

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