Expert: Invisibleman - 1/19/2004

Question
  I would like to buy an LCD monitor but I'm worried about response time. I've heard that, when gaming, LCD monitors create ghosting and blurring because of low response time. I was wondering if this is something that can be avoided by getting a good LCD monitor with a low response time, and if so, what is a good response time would be good enough to avoid ghosting and give a clear picture. And also, what other features are good to look for in a LCD monitor.

Answer
A Liquid Crystal Display (LCD) is an optical device that is commonly used to display simple ASCII characters, and images on digital items such as watches, calculators, and portable game consoles. LCD is also the technology used for displays in notebooks and other small computers. Like light-emitting diode and gas-plasma technologies, LCD allows display devices to be much thinner than those employing cathode ray tube (CRT) technology. LCD devices consume much less power than LED and gas-display displays because they work on the principle of blocking rather than emitting light.

How are LCDs made?

LCDs are created from two glass plates separated from each other at a distance of a few microns. Plates are filled with liquid crystal, then sealed together. The top plate is colored with an RGB pattern to create a color filter. Then polarizers are glued to both plates. This combination is sometimes called a 'glass' or 'cell'. The LCD cell is assembled into a 'module' by adding the backlight, driver electronics and frame.

LCD TFT components include:

  1. Polarizing filter: Regulates in and out
  2. Glass substrate: With electrodes
  3. Transparent electrodes: These electrodes with high transparency ratios are used to operate the LCD materials.
  4. Alignment layer: Films used to align liquid crystal elements in a fixed direction
  5. Liquid crystal
  6. Spacer: Maintains a consistent gap between two glass plates
  7. Color filter: Displays all color with RGB filter
  8. Backlighting: Sends light to the screen

In the active matrix LCD, a switching transistor (TFT) and a diode are attached to each color element to switch each element on and off. X and Y electrodes are attached to the same circuit board as the TFT. A switching signal is impressed on X electrodes; video signals go to the Y electrode.

What is polarization?

Light is an electromagnetic wave. Electric and magnetic fields oscillate in a direction perpendicular to the propagation of the light beam. The direction of these fields is called 'polarization direction'. Normal or non-polarized light has fields in multiple directions; polarized light has fields in only one direction.

What is a polarizer?

A polarizer is a sheet of special plastic that transmits light with a given polarization direction and absorbs all light with different polarization directions.

What differentiates passive matrix LCDs from active matrix LCDs?

A LCD is made with either a passive matrix or an active matrix display grid. An active matrix has a transistor located at each pixel intersection, requiring less current to control the luminance of a pixel. For this reason, the current in an active matrix display can be switched on and off more frequently, improving the screen refresh time.

For example, with an active matrix LCD, your mouse will appear to move more smoothly across the screen.

On the other hand, a passive matrix LCD has a grid of conductors with pixels located at each intersection in the grid.

How does a TFT LCD panel work?

On the TFT LCD panel, a data source drive is attached to each column while a gate drive is attached to each row. Each cell's TFT drain is connected to the electrode. The molecular arrangement of liquid crystal elements differs depending on whether or not it is impressed with voltage. The voltage varies the direction of polarized light and the amount of light by passing it through different arrays of liquid crystal elements. When two polarized filters are arranged vertically on a polarized light pole, the light that passes through the upper polarized panel is turned 90 degrees along with spiral structure of the liquid crystal molecules and passes through the polarized filter at the bottom. When impressed with voltage, liquid crystal molecules are arranged vertically from the original spiral structure and the direction of the light is not turned 90 degrees. In this case, light that comes through the top polarized panel may not go through the polarized panel at the bottom.

What are the advantages of TFT LCD compared to CRT?

In a CRT monitor, a gun shoots electrons and general light by colliding polarized electrons on fluorescent glass. Therefore, CRT monitor basically operate with analog RGB signals. A TFT LCD monitor displays an input image by operating a liquid crystal panel. The TFT's structure is fundamentally different than that of a CRT: Each cell has an active matrix structure and independent active elements. A TFT LCD is comprised of two glass panels. The space between them is filled with liquid crystal. When each cell is connected with electrodes and impressed with voltage, the molecular structure of liquid crystal is altered. This controls the amount of inlet lighting used to display images. A TFT LCD has several advantages over a CRT: It can be very thin and does not flicker because it does not use the scanning method.

Why is a vertical frequency of 60 Hz optimal for an LCD monitor?

Unlike a CRT monitor, the TFT LCD panel has fixed resolution. For example, an XGA monitor has 1024 x 3(R,G,B) x 768 pixels and higher resolution may not be available without additional software processing. The panel is designed to optimize display for a 65 MHz dot clock, one of the standards for XGA displays. Since the vertical/horizontal frequency for this dot clock is 60/48 Hz, the optimum frequency for this monitor is 60 Hz.

What kind of wide angle technology is available. How does it work?

The TFT LCD panel controls/displays input of a backlight using the dual refraction of liquid crystal. Employing the property that makes the projection of input light refract toward the major axis of the liquid element, it controls the direction of input light and displays it. Since the refraction ratio of input light on liquid crystal varies with the inlet angle of the light, the viewing angle of a TFT is much narrower than that of a CRT. Usually, viewing angle refers to the point where the contrast ratio is 10. Many methods to widen the viewing angle are currently being developed. The most common approach is to use wide viewing angle film, which increases the viewing angle by varying refraction ratio. Also IPS (In Plane Switching) or MVA (multi Vertical Aligned) is used to widen the viewing angle. The new range of LCD monitors employs advanced IPS Technology.

Why is there no flicker on an LCD monitor?

Technically speaking, LCDs do flicker; however, the cause of the phenomena is different than that of a CRT monitor -- and has no impact on ease of viewing. In an LCD monitor, 'flicker' refers to usually undetectable luminance caused by the difference between positive and negative voltage. On the other hand, CRT flicker - the kind that can irritate the human eye -- occurs when the on/off action on the fluorescent object becomes visible. Since the reaction speed of liquid crystal in an LCD panel is much slower, this troublesome form of flicker is not present in LCD display.

Why is an LCD monitor virtually free of Electro Magnetic Interface?

Unlike a CRT, the LCD monitor does not have key points that generate Electro Magnetic Interfaces, especially magnetic fields. Also, since LCD display utilizes relatively little power, its power supply is extremely quiet.

At what percentage is the brightness level preset?

At the factory, Contrast Level is preset at 100% and Brightness Level is set to the maximum level.

Liquid Chrystal Displays

Liquid chrystal display is the type of display used in laptop computers, in desk computers LCDs have not yet become as common. For their advatages LCDs will possibly replace the traditional CRT monitors, but the development has not been as fast as first was believed. It has been estimated that LCDs have 50% of the market before the year 2004. The advatages of LCDs compared to CRTs are

   * LCDs consume less power
   * do not produce electromagnetic radiation as CRTs do
   * flicker less than CRTs
   * are light and small in size

The drawbacks are

   * poorer viewing angle
   * higher price
   * problems with contrast ratio and response time
   * less accurate colour performance

Other differences:

  1. resolutions:
     CRTs are able to display more resolutions than LCDs which have only one resolution at full screen size. Lower resolutions are possible by using only a part of the sreeen. If a panel with 1024 x 768 resolution needs to display a resolution of 640 x 480 it uses only 66% of the screen.
  2. diagonal measurement:
     LCD's viewable area is the same as its diagonal measurement. CRTs lose about an inch behind the faceplate or bezel.
  3. convergence:
     In LCDs each cell is switched on or off individually so there is no convergence problems as in CRTs in which use electron guns and need faultless convergence if a sharp picture is wanted.
  4. signals:
     CRTs require analogue and LCDs digital signals. Current graphic cards have analogue outputs, hence analogue signals must be converted into digital: 1) graphics signal generated digitally in PC, 2) converted to analogue signal by the graphics card, and 3) fed into LCD where converted back into digital.

How LCD works
Liquid chrystal displays are based on technique in which current is lead through liquid chrystals. Liquid chrystals are substances that are almost transparent and have properties of both solid and liquid matter. There are two kinds of LCDs: DSTN (dual-scan twisted nematic) and TFT (thin film transistor), which are also known as passive and active matrix displays. LCD is made of several layers that are arranged according to the following order:

E) polarising filter
D) sheet of glass
C) electrode
B) alignment layer
A) liquid chrystals
B) alignment layer
C) electrode
D) sheet of glass
E) polarising filter

The same in picture:

To enter the page where the picture was taken: http://plc.cwru.edu/tutorial/enhanced/files/lcd/tn/tn.htm

Liquid chrystals are placed between two sheets of glass which are grooved to control the alignment of the molecules, i.e. the way they arrange themselves. The molecules follow the alignment of the grooves: if the grooves are parallel to each other so are the liquid chrystal molecules. If the grooves on one sheet of glass are aligned north to south and the grooves on the other east to west, the molecules twist in between the sheets of glass. Light goes through the molecules and follows their alignment twisting 90 degrees as well. A voltage makes the molecules arrange themselves vertically which causes the light go through them untwisted.

On the two sheets of glass there are polarising filters which prevent all other light waves expect the ones that come parallel to the filters. The filters are placed so that one is at 90 degrees to the other. This would block the light because to let the light go through the filters should be parallel. But as the liquid chrystals are twisted the light passes through the second filter by following the twist of the molecules. Because liquid chrystal molecules rearrange themselves vertically when a voltage is applied it is possible to block the light when wanted and prevent the light emerging from the other end. Thus there are two states: 1) no voltage, which means light passing through and 2) applied voltage, which prevents the light coming out of the other end.

The orientation of the alignment layers varies between 90 and 270 degrees. By the degree of the twist nematic LCDs can be divided into two groups: LCDs with 90 degrees twist are called twisted nematic (TN) and with 270 degrees super twisted nematic (STN) displays. The diffrence between nematic displays and TFT screens is that TFTs have an extra matrix of transistors connected to the LCD panel. In TFTs the response time (i.e. how fast the pixels respond to the command received from the panel controller) is much faster than in nematic displays.

New LCD types are developed all the time. One of them is HPD (hybrid passive display), which uses lower viscosity liquid chrystal that switches between states more quickly, and hence has faster response time. HPD has a response time of 150ms, whereas DSTN has a response time of 300ms and TFT 25ms.

Before LCDs overtake the market from CRTs a few problems have to be solved. The most important is probably the problem of converting analogue signals into digital. The process would be simpler if the panel was driven directly with a digital signal instead of the digital-analogue-digital conversion.

SOURCES:
Flat Panel Display Technology Overview: http://www.pctechguide.com/07panels.htm
Overview of LCD types:
http://plc.cwru.edu/ tutorial/enhanced/files/lcdindx.htm

Types of Monitors

LCDs

In the opposing corner are flat panel displays or LCDs (liquid crystal displays) commonly used in laptops and fast becoming popular as desktop monitors. Their major selling points are a slim profile and light weight. A CRT can be deeper than it is wide, whereas a LCD with a base is only about a handspan deep. No heavy lifting required with a LCD; they weigh less than half the average CRT. LCDs require half the power of CRTs and emit much less electromagnetic radiation which can interfere with other electronic devices.

In the screen of a LCD monitor, each pixel is produced by a tiny cell which contains a thin layer of liquid crystals. These rod-shaped molecules bend light in response to an electric current. It's the same display technology that resides in your digital watch but more sophisticated.

LCDs tend to be clearer than CRTs which can suffer from convergence or focus difficulties. Their improved clarity means that even small LCDs can display higher resolutions than the corresponding sized CRT. They also make small text easier to read. Unlike CRTs, LCD monitors have only one optimal resolution. At lower resolutions, the screen is redrawn as a smaller area or all the pixels in the image are blown-up to fill the screen. The latter solution can make images look jagged and blocky so be sure the resolution of the LCD is the resolution you want to use.

LCD Features

Viewing Angle

One of the main disadvantages of LCDs when compared to CRTs is their limited viewing angle. When viewing a LCD straight on it looks fine. But the screen will appear washed-out if you move your head over to the side and look at it from an extreme angle. Low-end LCDs can have viewing angles of only 100 degrees which won't give everyone crowded round your desk a clear view. For a standard 15 inch LCD try to get a 140 degree viewing angle. Up that by 20-40 degrees when shopping for an 18 inch LCD.

Brightness

The brightness of LCD monitors is another important factor. LCD monitors have several backlights that provide illumination. Brightness is measured in units called nits. The majority of LCDs produce 150-200 nits which is fine for most users. The backlights in a LCD are good for 10 to 50 thousand hours of operation.

Since they're fairly fragile and more likely to break, backlights usually come with only a one year warranty. This warranty is separate from the one for the screen so you might want to extend the backlight warranty to match the duration of your screen warranty.

Positioning

LCDs can provide a range of options for positioning a display. The common way to view a screen is landscape mode (longer than wide). Some LCDs let you pivot the screen 90 degrees so you can view it in portrait mode (taller than wide) which is great if you're growing tired of scrolling so often. You should also check out whether the screen can both tilt and swivel. Easy adjustment is important if you'll be doing presentations. You can even mount some LCDs on the wall like a picture.

Analogue vs Digital

One reason why LCD is the wave of the future is that LCDs are inherently digital. CRTs are analogue which means the digital information from your computer must be translated by the video card into analogue information for your monitor. Some LCDs come with a video card that has digital output which eliminates the extra conversion step. In theory, this all-digital solution should produce more accurate colour and faster frame rates. But abandoning your current digital-to-analogue video card might mean losing its 3D acceleration features.

Most LCDs are compatible with regular digital-to-analogue video cards, but the additional conversion of the analogue output of the video card back to digital information for the LCD can degrade image quality. Fortunately, the industry is working to create video cards that will be able to produce both analogue and digital output plus 3D acceleration.

CRTs

The most prevalent type of monitor today is the cathode ray tube (CRT). Despite its rather sci-fi sounding name, a CRT is the same as the picture tube inside your TV. They work by firing beams of electrons at phosphor dots on the inside of a glass tube. The phosphors in a CRT are chemicals that emit red, green or blue light when hit by electrons. These monitors are capable of multiple resolutions, give the best look to full-motion video and provide better control over color calibration for graphic artists.

On the down side, they hog a lot of room and weigh more than several sacks of potatoes. You can get more compact CRTs called short-depth or short-neck monitors which are a couple of inches shallower than regular CRTs. Unless space is a primary consideration, most people buy a CRT display because they offer good performance at an affordable price.

CRT Features

Refresh Rate

If you've decided to get a new CRT then it should have a sufficiently high refresh rate. This refers to how often the screen is redrawn per second. With low refresh rates you can get screen flicker and eye strain. Aim for a rate of 75 Hz for a monitor up to 17 inches in size and 85 Hz for any larger monitor. LCDs are basically flicker free so refresh rates aren't important.

Dot Pitch

Another consideration for CRTs is dot pitch. This is the distance in millimeters between phosphors of the same color. The smaller the dot pitch, the sharper the image. Opt for a dot pitch of 0.26 mm or smaller. You can measure dot pitch both horizontally and vertically, but monitor specs usually quote horizontal dot pitch. Occasionally, the dot pitch is measured diagonally. By multiplying diagonal dot pitch by 0.866, you can calculate horizontal dot pitch.

General Features

Size

Regardless of what type of monitor you're considering, size is a priority. A bigger monitor offers easier to read text, better graphics and the ability to have more and larger windows open on your desktop. All monitor sizes are measured diagonally across the screen, but that number isn't the true viewable area. The plastic frame around the monitor hides a certain amount of the screen so there is a difference between the quoted size and the actual viewable area. A monitor's viewable area should be within an inch or so of the quoted size.

Price

The standard monitor size used to be 15 inches, but 17-19 inch monitors have become the norm as prices have decreased. You can get a 17 inch CRT starting at US$250. An adequate 19 inch CRT can be had for US$400 but better quality will cost more. If you need a large screen for group presentations, a 29 inch monitor

Resolution

A factor for both CRTs and LCDs is resolution. The number of pixels horizontally and vertically defines a monitor's resolution in pixels or dots per inch (ppi or dpi). The greater the resolution, the more information or image you'll be able to view at once. The average user will find a resolution of 1024x768 more than sufficient for everyday work. You can achieve this resolution on CRT monitors 17 inches and larger or LCDs 15 inches and larger. Keep in mind that CRTs can display multiple resolutions, but LCDs are optimized at only one resolution.

Extras

Monitors can come with a variety of extras. Some have built-in speakers or jacks for microphones and headphones. Other monitors have dual inputs so you can connect two computers to the same monitor. With the advent of USB (Universal Serial Bus), some monitors have USB hubs at the back, allowing you to connect more peripherals. You can also get accessories like anti-glare filters and specialized mounting stands that help minimize glare and provide a comfortable working position.

Power Conservation

To keep your utility bill down, you should look for a monitor that is Energy Star compliant. Energy Star is a program developed by the US Environmental Protection Agency (EPA) to make energy-saving office equipment like computers and monitors. An Energy Star monitor automatically goes to sleep or powers down after a period of inactivity. This feature can save 60-80% of power during idle times. All you have to do to wake up the monitor is touch the keyboard or mouse.

Emissions

The majority of monitors are certified as "low emission" since they meet standards like MPR II or TCO. These guidelines were developed in Sweden (the acronyms are Swedish too) by a number of organizations to set limits for electric and magnetic field emissions. The newer TCO standards are the strictest. So if you're concerned about emissions look for MPR II or TCO certification, not just the words "low emission

No Picture (Power LED not lit)
Make sure the Power cable is plugged to the wall and the back of the monitor.

First, power button in front of the monitor should be in the OFF position, then press it to ON position again.
  No Picture (Power LED is amber or yellow)
Make sure the computer is turned on.

Make sure the signal cable is properly connected to your computer.

Check to see if the monitor cable has bent pins.

The Energy Saving feature may be activated
  Screen Says - "No Video Input"
Make sure the monitor cable is properly connected to your computer. (Also refer to the Quick Set Up Guide).

Check to see if the monitor cable has bent pins.

Make sure the computer is turned on.
  Screen Says - "This is 85HZ overdrive. Change computer display input to 1024x768@60HZ"
Make sure the vertical sync of input signal is within the range of 56 ~ 75 Hz.

Change the refresh rate to 56~75Hz within 10 minutes.

Re-power on monitor to start over again if you failed to change the refresh rate within 10 minutes.
  AUTO button not working properly
The Auto Function is designed for use on standard Macintosh or IBM-compatible PC running Microsoft Windows.

It may not work properly if using nonstandard PCs or video card.


Imaging Problems
  Display position is incorrect
Push the Auto button.

Adjust the image position using the Horizontal Position &/or Vertical Position in OSD Main Controls.
  Image vibrates on the screen
Check that the signal cable is properly connected to the graphics board or PC.
  Vertical flicker appears
Push the Auto button.

Eliminate the vertical bars using the Clock Adjustment of VIDEO NOISE in OSD Main Controls.
  Horizontal flicker appears
Push the Auto button.

Eliminate the horizontal bars using the Phase Adjustment of VIDEO NOISE in OSD Main Controls.
  The screen is too bright or too dark
Adjust the contrast and brightness in OSD Main Controls. (The backlight of the LCD monitor has a fixed life span. When the screen becomes dark or begins to flicker, please contact your dealer.)
  An after-image appears
If an image remains in the screen for an extended period of time, it may be imprinted in the screen and leave an after-image. This usually disappears after a few hours
  An after-image appears after the power has been turned off
This is characteristic of liquid crystal and is not caused by a malfunction or deterioration of the liquid crystal. The after-image will disappear after a set amount of time
  Green, red, blue, dark, and white dots remain
The remaining dots are normal characteristic of the liquid crystal used in today's technology.

Analog or Digital - The Biggest Difference of the Interfaces

Flat panel displays with an analog VGA interface continue to dominate the market. The reason why digital interfaces have not been able to penetrate the market is basically due to the uncertainty of the buyer. Well, that isn't surprising in view of the sheer number of standards such as LVDS, TDMS, GVIF, P&D, DVI and DFP - just to name a few. It's obvious that there are far too many. A similar situation arose in the 80's when video standards such as VHS, Beta and Video2000 sparred for market domination.

The ultimate winning system for digital TFT interfaces now appears to be fixed, but first let's take a look at a few facts:

Analog flat panel displays have had a justified existence for a long time as hardware to control a digital equivalent did not exist. The lack of suitable standards and the objective of the graphic board manufacturers to sell volume also helped their short-term dominance. From today's point of view, analog TFTs are absolutely unnecessary, but they are still successful on the market. The reason for this is that these devices are mainly sold for specific projects and are often part of large tenders. The buyers, normally large companies and government departments, already have an existing infrastructure, which they do not want to change. The majority of these customers simply wants to exchange their old workspace monitors for new flat panel displays and therefore need to be able to plug the new displays into the analog VGA connector of their existing graphic boards. This approach isn't exactly visionary, because it is then impossible to upgrade to digital interfaces in the future.

At this stage we'd like to touch on the subject of pixel jitter, which is almost certainly one of the most unpleasant effects of analog TFTs. This shimmering effect occurs when the clock and phase are not synchronized 100% with the analog signal. Individual pixels then start to swim, which is most apparent and annoying in characters and lines. Digital TFTs avoid the need to trim the clock and phase completely and therefore don't have this problem. If you own one of these digital displays you only need to change the brightness and contrast according to your needs. The technical frills are dropped entirely, and this makes it much easier for you.

A look at today's turnaround situation allows us to be optimistic. The question of standards has been practically resolved and graphic boards are now available with the corresponding digital outputs. The following table gives you an overview of the most important points:

Advantages and Disadvantages of Digital and Analog Interfaces

    Digital Control    Analog Control
Advantages    

   * No signal losses duet to DA and AD conversion
   * Geometry, clock and phase settings unnecessary, therefore simple to use
   * Lower costs as less electronic circuity required

  

   * Compatible with standard VGA boards on a broad installed PC basis
   * Not necessary to purchase a new graphics board

Disadvantages    

   * Low avilability of models with digital interfaces
   * Requires graphic board with digital output

  

   * Clock and phase of the TFTs must be synchronized with the analog signal to avoid pixel jitter, which is a relatively complex issue
   * Cables sensitive to external influences
   * High cost of signal conversion inside the display
   * Upgrade to digital interface not possible