| Active-matrix LCD techology |
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| Wednesday, 03 October 2007 | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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LCD monitors are rapidly becoming shipped with new computers by default, and are superior to the old CRT screens for the majority of applications. Although liquid crystal displays boast perfect image geometry and should be perfectly sharp, in reality they still have various shortcomings. Viewing-angle dependancies, poor blacks and motion-blur are still issues, particularly for photographic and LCD television applications. Certain flicker or shimmers can be caused by sub-optimal user-adjustment, or shortcuts taken at the factory. On this page I explain a little of LCD screen technology and give you some pointers to the factors which separate the best from the rest. A large number of test patterns are provided to aid setting and diagnosis of your display. Active-matrix LCD techologyA modern desktop liquid crystal display (LCD) computer-screen consists of an active-matrix panel and polarizers (which together act as an electrically-controlled pixel-level light filter) situated immediately in front of a backlight (large-area uniform light source - usually consisting of one to four cold-cathode florescent lamps (CCFL), a lightguide and diffuser). The panel is connected to a dozen or so row- and column-driver chips which address the display with the picture data, and are driven by the rest of the monitors electronics. I'm not going to explain the basics of an LCD panel in any more detail - this is already covered on hundreds of sites, for example: http://www.netbored.com/classroom/what_is_tft_lcd.htm.LCD vs CRTThe table below shows the main factors differentiating LCD and CRT displays from a users' perspective.
On the basis of image-quality alone, in my opinion LCD is the monitor of choice for "office" and technical/CAD applications (largely text-based, or detailed but colour-non-critical graphics), while CRT still has the upper hand for high-end photographic/art work and for television displays. LCD testsClock/phasing for Analog inputsLCD panels use discrete pixels and ideally should be connected to the image source via a digital interface such as DVI. In an analog video signal from a computer's "VGA connector" the luminance data for one pixel runs into the next in a time continuum. To properly recover the data back into the correct discrete pixels requires accurate synchronisation of a "sampling-clock" in the monitor to the "pixel clock" in the graphics card. Although possible, this can be technically tricky to get right (the VGA output was never designed for this!). Symptoms of incorrect clock/phase settings include blurry or shimmering text, and shimmering on fine crosshatch patterns. The "automatic" adjustment of many LCD monitors is not always successful, and manual tweaking may be needed. It is probably wise to allow the monitor (and PC) to warm up for 5- or 10minutes before running this test. For an illustrated explanation of what these controls do, and how to use them, see Clock/Pitch and Phase controls.InversionIn liquid crystal pixel cells, it is only the magnitude of the applied voltage which determines the light transmission (the transmission vs. voltage function is symmetrical about 0V). To prevent polarisation (and rapid permanent damage) of the liquid crystal material, the polarity of the cell voltage is reversed on alternate video frames. Unfortunately it is very difficult to get exactly the same voltage on the cell in both polarities, so the pixel-cell brightness will tend to flicker to some extent at half the frame-rate. If the polarity of the whole screen were inverted at once then the flicker would be highly objectionable. Instead, it is usual to have the polarity of nearby pixels in anti-phase, thus cancelling out the flicker over areas of any significant size. In this way the flicker can be made imperceptible for most "natural" images.
The following test patterns deliberately excite only one polarity-half of the inversion pattern for some common schemes, and one of them should cause your screen to flicker. This is not a fault with the screen, but enables you to find out which inversion scheme your screen uses. Note: if using an analog-input monitor, the following tests will not be meaningful unless the clock and phasing settings are correct (see previous test). Warning to anyone who suffers from epilepsy or other extreme flicker-sensitivity: one or more of the tests below is likely to make your LCD flicker at 30 to 40Hz.
On Internet Explorer, you can switch to full-screen (and back) using F11. You might also want to set the toolbar to auto-hide (right-button menu when in full-screen mode). Extreme tech: The inversion pattern for any given screen will inevitably flicker to some extent and is not a fault. If it really flickers a great deal then it may indicate that the common-electrode voltage has not been set up properly. In that case you might also perceive a "dot crawl" effect on plain colours of medium brightness. A grossly mis-set common-voltage will also make your screen more susceptible to temporary 'image sticking' problems.
Some diagonal cross-hatch patterns used for shading by CAD programs can interact with inversion and cause objectionable flicker on some LCD monitors - this should only be a problem if the manufacturer has left the common-electrode voltage grossly mis-set. Having seen the severity of the effect on some new monitors (and how easily it could have been tweaked out at the factory), I'm beginnning to form the opinion [January 2005] that in some cases this flicker could argueably be classed as a "fault" with the product. Laptop LCD screens tend to be optimised for lower power, with some relaxation of image-quality criterion. As well as (often) lower brightness and less saturated colours, laptop screens usually use a 'row inversion' (aka 'line inversion') scheme rather than the dot inversion now universal in desktop screens. If you look closely at a row-inversion LCD, particularly if it is showing a fairly plain, mid-brightness colour, you may see a slight horizontal line interference pattern on alternate lines, which may appear to drift up or down the screen. This is also not uncommon on colour mobile-phone displays, or personal DVD players. Cross-talkOwing to the way rows and columns in the display are addressed, and charge is pushed around, the data on one part of the display has the potential to influence what is displayed elsewhere. This is generally known as cross-talk, and in matrix displays typically occurs in the horizontal and vertical directions. Cross-talk used to be a serious problem in the old passive matrix (STN) displays, but is rarely discernable in modern active-matrix (TFT) displays.A fortunate side-effect of inversion (see above) is that, for most display material, what little cross-talk there is is largely cancelled out. For most practical purposes, the level of crosstalk in modern LCDs is negligible. Certain patterns, particularly those involving fine dots, can interact with the inversion and reveal visible cross-talk. If you try moving a small Window in front of the inversion pattern (above) which makes your screen flicker the most, you may well see cross-talk in the surrounding pattern. Different patterns are required to reveal cross-talk on different displays (depending on their inversion scheme). The following patterns may show cross-talk on your screen. These patterns are not comprehensive and should not be used blindy to rate one screen against another. The appearance of a visible cross-talk from any of these patterns does not indicate a "fault condition" with your display!
Colour, and viewing-angle dependenceColours displayed by LCD screens tends to vary with viewing angle. Please go to the following page to see this effect: Most low-price 15" and 17" LCD monitors use the Twisted Nematic ("TN") liquid crystal mode; with these displays the image looks much lighter if looked down on from above, and much darker if looked at from below.Newer liquid-crystal modes such as Vertical Alignment ("VA") or In-Plane Switching ("IPS") have less viewing- angle dependence, but may suffer slower response times and/or lower contrast. Greyscale setupAlthough not specific to LCD screens, I'm providing a greyscale test-page for completeness.Refresh rate, response time, flicker and motion-blurThere seems to be a lot of confusion and mis-information on these topics on the web; here's my clarification...Refresh rate is the rate at which the electronics in the monitor addresses (updates) the brightness of the pixels on the screen (typically 60 to 75Hz). For each pixel, an LCD monitor maintains a constant light output from one addressing cycle to the next (sometimes referred to as 'sample-and-hold'), so the display has no refresh-dependent flicker. Response time relates to the time taken for the light throughput of a pixel to fully react to a change in its electrically-programmed brightness. The viscosity of the liquid-crystal material means it takes a finite time to reorientate in response to a changed electric field. A second effect (which has a rather more complicated explanation) is that the capacitance of the LC material is affected by the molecule alignment, and so if a step change is brightness is programmed, as the LC realigns the cell voltage changes and the brightness to which it settles is not quite what was programmed. Unless 'overdrive' (which tries to pre-compensate for this effect) is employed, it may take several refreshes before the light output stablises to the correct value. Response rate for dark-to-light is normally different from light-to-dark, and is often slower still between mid-greys. VESA and others define standard ways of measuring response time, but a single figure rarely tells the whole story. The visual effect of motion blur is self-explanatory and it is fairly intuitive to realise that a slow pixel response-time will cause this problem. What is less obvious, but at least as important in causing motion-blur, is the 'sample-and-hold' effect: an image held on the screen for the duration of a frame-time blurs on the retina as the eye tracks the (average) motion from one frame to the next. By comparison, as the electron beam sweeps the surface of a cathode ray tube, it lights any given part of the screen only for a miniscule fraction of the frame time. It's a bit like comparing film or video footage shot with low- and high-shutter speeds. Motion-blur originating from sample-and-hold in the display can become less of an issue as the frame (refresh) rate is increased... provided that the source material (film, video, or game) contains that many unique frames. For LCD TV there is significant interest in the industry in strobing (flickering!) the backlight deliberately so as to reduce sample-and-hold motion-blur! Colour-temperatureMonitors, both flat-panel and CRT, often come with the ability to adjust the colour-temperature, perhaps to 3200K, 5000K, 6500K, or 9300K.The sRGB standard for PC graphics (along with western television standards) assumes a display colour temperature of 6500K. Under normal circumstances you will get most accurate colour-reproduction using the 6500K setting on your display. Setting too high a colour temperature (eg 9300K) will result in images which are rather blue ("cold", artistically-speaking), while too low a colour temperature (eg 3200K) will make the display look very yellowy. Common pixel-array sizes, or 'resolution'
Note that, in general, LCD panels in LC-TV products typically have much lower resolution (i.e. fewer, and bigger pixels) than similarly-sized LCD computer monitors.
Television-standard based pixel-array sizesIn 1981, the 'Rec.601' standard was finalised for the digitisation of television signals (for studio use). It is based upon a 13.5MHz-sampling rate of existing analog television signals. From this, follow a number of oft-quoted resolutions (note the number of active-lines is less than the system number of lines; "625-line" standards have 576 active lines, and "525-line" standards have 480 active lines):
Some mobile phone or PDA-type displays are specified from the CIF family. Whether CIF/QCIF formats have square pixels or not probably depends on the application in mind! For interest, modern cinema films are made either in 'widescreen' 1.85:1 or 'scope' 2.35:1 aspect ratios. Widescreen television standardised on 16:9 (1.78:1), so 'scope' films still have narrow black bars at the top and bottom when presented in a TV 16:9 frame. Widescreen-films (1.85:1) are usually shot with sufficient excess head- and footroom to fill a 16:9 TV frame when required. DVDs are mastered in either 4:3 or 16:9 frames, in both cases PAL-format DVDs have 576 active lines vertically (NTSC DVDs will have 480 active lines). The horizontal pixel-count for DVDs is universally 720 pixels (NTSC and PAL) for both 4:3 and 16:9 ratios; television and DVD pixels are not square!
HDTV and Digital Cinema pixel-array sizesModern HDTV standards are based upon square pixels, and 16:9 aspect ratio. We have "720" formats, based upon 720 active scan lines and 1280 pixels per line (1280×720) and "1080" formats based upon 1080 active scan lines (1920×1080). Interlaced and progressive versions exist. Both 50Hz and 60Hz refresh variants are standardised, as is a 24Hz progressive scan (directly compatible with film). There are many good engineering reasons to avoid interlaced systems, but they seem to live on anyway. It's looking as though most equipment, both professional and domestic, will handle most of the common variants natively.
External links for LCD technology
For a technical introduction to the various technologies and considerations which go to make LCD panels, I recommend the book "Active Matrix Liquid Crystal Displays: Fundamentals and Applications" by Willem den Boer (2005), available from [Amazon.com To see original article:http://www.techmind.org/ |
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