how crt and lcd monitors work

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    How CRT and LCD monitors work

    We all spend an awful lot of time sat in front of computers. Whether we're gaming or working, we are at themercy 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 at60Hz can make us weep in pain and long for a nice LCD to while away our hours at. A good display makes all thedifference.

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

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

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

    The Basics

    So let's start with the easy stuff. The picture that appears on your monitor comes from the graphics card in yourcomputer, and the job of the graphics card is to render the picture suitable for the monitor. A wired output runsfrom 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 eachother. The standards are set out by VESA, which defines things like how monitors identify themselves to the

    computer.

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    CRTs

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

    CRTs receive their picture through an analogue cable, and that signal is decoded by the display controller, whichhandles 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 fireselectrons towards the front through a vacuum which exists in the tube of the monitor. The gun can also bereferred 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 thedisplay controller. As electrons pass the anode, they are shunted or pulled in one direction or the otherdepending on how magnetic the anode is at that time. This moves the electrons towards the correct part of thescreen.

    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 becomethe red, green and blue electron rays. However, higher quality monitors have an individual gun for each, whichcan increase picture quality.

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

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    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. Shadowmask is an older technology, and is l iterally a piece of metal with mil lions of holes in it which allow the differentcathode 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 themonitor.

    Aperture grille is a newer technology which defines the gaps through which electrons pass using a mesh of wiresrather than a sheet with holes in. Whereas a shadow mask is made of circular holes, the grill is made of verticalslots. Because it is by its nature thinner, it allows for brighter displays. However, the grill is fragile and prone tobeing knocked around. The grill is therefore strapped to the monitor using stiff horizontal wires - this is whatcauses 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 betterimage 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 thatmake 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?

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    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.

    LCD

    Flat panel monitors are a relatively recent product to enter the computer market. The clue to LCD technology is inthe name - crystals that are in liquid form. Because they are in a liquid form they are easily manipulable, and thisallows us to play with the way that light interacts with them. If you have a flat panel in front of you, try justpressing 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 videocontroller is decoded and understood by a display controller on the monitor itself. The controller has two things tocontrol - 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 dotpitch and image area to try and calculate - the native resolution of the monitor is simply the number of pixelscontained in the matrix. If i t's a 17" monitor, chances are there are 1280 pixels in the matrix horizontally, and1024 vertically.

    Perspective view

    Each pixel is made up of three sub-pixels, which have red, green and blue filters in front of them, just as eachpixel 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, alternatecurrent - 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 ispassed through, altering its polarisation and this then results in the correct colour coming out of the secondpolarising 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 singlecathode at the top, or one at the top and one at the bottom, or two at the top and two at the bottom for optimumbrightness 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.

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    Exploded view of LCD from side on

    Contrast ratio

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

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

    Put simply, the contrast ratio of a display is the ratio of the brightest possible white value compared to the darkestpossible black value. Most desktop TFT monitors have a contrast ratio of between 300:1 and 600:1 while atypical 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 lightwhen 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 whitedown. The greater the contrast ratio, the greater the difference between black and white you can maintain andthe better quality the display.

    For more details on contrast and brightness, check out our article on theBrightside 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 forone that's 8-bit. Professional quality panels will use 10-bit colour, and the newer ATI Radeon cards will support

    that output.

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    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 theelectron guns are told to fire by the video source. The refresh rate on a monitor is limited by how fast the gunscan 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 weknow why? The phosphors in a CRT illuminate when hit with electrons, but begin to fade as soon as the energyfrom the electron is used up. To keep the phosphor illuminated requires a constant stream of electrons. If they'renot coming in fast enough, the phospor will visbly fade then light up again - causing that horrendous 60Hz flickerwe 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 withenough 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 ispassed from the pixels for as long as the display controller tells it to. However, LCDs are nominally set up toreport 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 ratetime is the measurement of how many frames can be displayed per second. For an LCD, response time refers tohow quickly a liquid crystal can twist, then untwist to either pass or block the light of each pixel. The faster thecrystals 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 withoutghosting. Ghosting is the remnants of the old frame image 'below' the new frame image due to the fact that not allthe 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 andeven 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.

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    However, with suitably powerful graphics hardware, a constant minimum frame rate of 60FPS coupled with Vsync

    makes for optimum image quality and gameplay.Disassembling a LCD monitor

    To give you a further insight into the technology inside flat screens, we thought we'd take one apart and see whatwas inside. Many thanks to the guys atViewsonicfor giving us a dead unit from their stock to mutilate.

    Here's the stock unit: a standard 17" monitor, 1280x1024 resolution. The back is held on with a couple of screwsin the stand, a couple at the bottom of the monitor and then plastic bezel clips between the front and back plastichalves.

    With the back off, you can see the display controller. The circuitry here has cables going off it to the power unit, tothe cathodes that light the display and to the actual electrodes controlling the liquid crystals.

    Here you can see the power circuitry on the left, and the circuit that handles the front panel buttons on the right.

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    With a couple more screws removed on the side, this back panel circuitry is removed and we are left with the

    actual flat panel. Are we going to stop here? Of course not.

    Next up: unscrew the metal backing and prise it off - not an easy task, these things are put together pretty solidly.

    Here you can see we're getting to the guts of the panel. The big white board is the major part of the diffusingmechanism, with multiple layers of thing plastic in front and behind it to provide extra relectivity and diffusion.

    Here are the actual cathodes that are attached to the display on the top and the bottom. This particular modelhas dual-dual cathodes for maximum brightness.

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    With the diffusing material and backlight pulled away, we're left with the panel. Attached to the bottom of thepanel is a secondary circuit board that controls the electrodes used to manipulate the liquid crystals.

    Here you can see the polarising filters that are on each side of the display. They are incredibly sticky and rather

    hard to peel off! We managed to prise back a couple of corners. Of course, they look pretty similar to the eye butallow light 'waving' in different directions to pass through.

    If you look closely at the panel here, you can just about make out the individual pixels that make up the display

    matrix.

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    Future technology in LCDs

    What does the future hold for display technology?

    Arguably, there is not much of a future at all for CRT technology. As LCDs get better and better, with fasterresponse times, better contrast and higher resolutions, the uses for the analogue output it provides will becomefewer, given the convenience of the form factor of LCD displays.

    Philips and other companies have come up with technology to create flatter CRTs, which occupy less than halfthe space of a traditional CRT of any given specification, but even these look bulky compared to today's svelteLCD designs.

    Connections: The first area that LCDs will evolve in is through different connector types. VGA and DVI are,effectively, connections that are becoming obsolete. The new standards coming in are HDMI and UDI, which wehave written about previously. These standards should allow for better quality connections with more featureset,such as the ability to carry audio and video along a single cable.

    LED backlighting: We've seen that LCDs are currently backlit by a number of cold cathodes above and belowthe screen. New technology replaces these heat-producing, power-intensive cathodes with cooler, more efficientLEDs. Laptops using LED backlighting are now starting to come into the mainstream, with the Sony TX2 sportinga 7 hour battery life thanks to the efficiency of its backlight. LCD monitors are now also beginning to approachreference CRT quality thanks to the technology - check out this review of theNEC Spectraviewfor more details.

    The Sony TX2 laptop with LED backlightBetter panel types: Currently, the vast majority of monitors on the market are sold with Twisted Nematic (TN)LCD panels. This type of LCD panel usually has a great response time, but the colour range and viewing anglecan often be limited.

    Multi-domain Vertical Alignment (MVA) panels have fast response time, wider viewing angles and a much bettercontrast, resulting in a better picture overall. However, it is a more expensive technology. As costs fall, however,

    MVA panels could deliver better image quality to TFT monitors.

    Leaping ahead

    If we were to leap into the future, we would see some fairly hefty transitions in display technology.

    SEDs: Surface-conduction Electron-emitter displays are being touted as a replacement for LCD panels. SEDsuse a phosphor coating, like CRTs, that is charged with electrodes - only rather than from behind through avacuum, like a CRT, the electrodes are within the matrix of pixels, like with a LCD. They are said to producebetter images than LCD with a cost that will eventually be lower. Prototypes have already been shown off bycompanies like Toshiba and displays are expected to be available on the market in 2007. However, some remainsceptical about the technology and manufacturing problems and costs that are yet to be overcome.

    Curved displays: At the Consumer Electronics Show in Las Vegas this year, Bill Gates showed off a number ofconcept displays that he believed would be on the market within the next 10 years. Perhaps the most awesome

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    of these was a field-of-view curved display. Around the size of two 24" panels 'sewn' together, the curved displaysat on the desktop and allowed for things on the monitor to be seen in the peripheral vision outside of our 90degree standard view. Imagine gaming on one of those! Since the display is curved, we have to image it was anOLED display, which can be produced in that way.

    A picture of the desk-sized display Bill showed at CES (courtesy www.tsr.ch)

    Table displays: Bill also showed off a concept display built into a coffee table, where the surface of the table

    becomes the screen when a device, like a PDA, is placed on top of it. This could bring a 'big-screen' interface to

    PDAs and Gates suggested that they'd be used at airports and other places where keyboard-mouse-monitor

    access was limited.

    OLED: Short for Organic Light Emitting Diode, OLED could well replace LED as the display of choice over

    coming years. OLED has similarities to SED technology, in that the l ighting material is an organic substrate.

    When current is applied, it glows. However, rather than the material being on the panel, it is within the LED itself,

    meaning that LEDs can be coloured as required to emit the correct light - with no RGB sub-pixels needed.

    Because the electricity can operate directly on the substrate, you don't need to have a backlight - this saves on

    power and space.

    OLED is already starting to make its way into the market, such as with thisBenQ phone.

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