(LCD) Technology

A LCD Monitor

Introduction

LCD or flat panel computer displays are the latest and greatest offerings in the desktop computer industry. They have been used for years in the portable and notebook computing markets, but recent developments have increase performance and size while reducing costs making them viable in the desktop environment. LCD displays are lightweight, extremely thin and use much less power than CRT based monitors.

What is Liquid Crystal and How It Works?

Liquid crystal was discovered by the Austrian botanist named Fredreich Rheinizer in 1888. Liquid crystal is an unusual organic material and it is neither solid nor liquid. That means although it is liquid in form and appearance, Liquid Crystal exhibits a crystalline molecular structure that resembles a solid. Liquid crystals are rod-shaped molecules whose molecules can be aligned precisely when subjected to electrical fields. As a liquid they are able to flow over and around small grooves and can change their position depending on applied voltage. When properly aligned, the liquid crystals allow light to pass through makes the desired images appear.

Molecules are When coming Molecules line arranged in a into contact with up parallel along loosely ordered a finely grooved grooves. fashion with surface their long axes (alignment layer). parallel.

Crystal Molecules in Natural state

An LCD monitor consists of six layers: a backlight, a sheet of polarized glass (polarizer), TFT glass, a layer of liquid crystal solution, colour filter/glass and a second polarized sheet of glass.

Photo source (courtesy of Samsung Electronics)

A fluorescent light source, known as the backlight, makes up the rearmost slice of bread. Light is shined from behind the panels. This light passes through the first of two polarizing filters. The polarized light then passes through a layer that contains thousands of liquid crystal blobs arrayed in tiny containers called cells. The cells are, in turn, arrayed in rows across the screen; one or more cells make up one pixel (the smallest discernible dot on a display). Electric leads around the edge of the LCD create an electric field that twists the crystal molecule, which lines the light up with the second polarizing filter and allows it to pass through. Each crystal either allows light to pass through or blocks the light. The configuration of the crystals forms the image.

Types of LCD’s

There are two basic kinds of LCD colour displays: passive-matrix and active-matrix.

In a colour LCD panel, each pixel is made up of three liquid crystal cells. Pixels do not actually generate the colours that you see. It is the white light (backlight) passing through each pixel, which filtered to form the intended colour. The front glass is coated with colour filter material in front of each red, green and blue dot (cell). Light passing through the filtered cells creates the colours you see on the LCD.

Each cell or subpixel, can be individually addressed with a control voltage. This means, for example, that a 15” LCD Monitor screen that have the resolution of 1024 x768 contains 2,359296 subpixels (1024 x 768 x 3). Occasionally the mechanism that sends the electrical current to one or more pixels fails; in those instances you'll see a completely dark cell (bad cell) or a "bad" pixel. Read more information about bad pixel in the chapter of “Stuck and Dead Pixel in LCD Monitors”.

Passive Matrix LCD

Passive-matrix LCD Monitors use a simple grid to supply the voltage to a particular pixel on the display. Creating the grid is quite a process! It starts with two glass layers called substrates. One substrate is given columns and the other is given rows made from a transparent conductive material. The rows or columns are connected to integrated circuits that control when a charge is sent down a particular column or row. The liquid crystal (LC) material is sandwiched between the two glass substrates, and a polarizing film is added to the outer side of each substrate. To turn on a pixel, the integrated circuit sends a charge down the correct column of one substrate and a ground activated on the correct row of the other. The row and column intersect at the designated pixel, and that delivers the voltage to untwist the liquid crystals at that pixel.

For example, if the dot at row 0, column 0 is supposed to be red, the green and blue dots turn “On” at that point to block white light through all but the red filter. White light travels through the red filter on the front glass where it emerges as red. When the red, green and blue dots are all on, all light is blocked and the pixel appears black. If all three dots are off, all light passes through and the pixel appears white.

There are disadvantages although the simplicity of the passive-matrix system is beautiful. First, the response time is slow. Response time refers to the LCD's ability to refresh the image displayed. The easiest way to observe slow response time in a passive-matrix LCD is to move the mouse pointer quickly from one side of the screen to the other. You will notice a series of "ghosts" following the pointer. Such slow update times make passive displays poor choices for fast graphic operations (like games), animation and motion video. Second, their contrast ratio is poor which generally results in washed out or hazy pictures. Third, the viewing angles for colour passive matrix LCD’s also are poor at around 45 degrees. That means your clearest view of the display will be to look at it straight on.

Active-matrix or TFT (thin film transistor) technology

TFT stand for thin film transistor (or active-matrix) produces colour images that are as sharp as traditional CRT displays. Basically, TFTs are tiny switching transistors and capacitors. The three elements provide the red, green and blue light source for each pixel that your eye perceives. They are arranged in a matrix on a glass substrate. To address a particular pixel, the proper row is switched on, and then a charge is sent down the correct column. Since all of the other rows that the column intersects are turned off, only the capacitor at the designated pixel receives a charge. The capacitor is able to hold the charge until the next refresh cycle. And if we carefully control the amount of voltage supplied to a crystal, we can make it untwist only enough to allow some light through. This means that the switching occurs right at the cell turning the white light on or off and the result is faster response times, and less crosstalk between cells.

When the red, green and blue elements are all off, white light shines through the three elements, and the pixel appears white. If the red, green and blue elements are all on, all light is blocked, and the pixel appears black.

Active-matrix LCD’S response time is very fast-approximately 16ms and better. Such fast response time provides excellent performance for graphics or animation applications. The active matrix screen also provides a comfortable viewing angle of 90 degrees and above. Additionally, higher drive signals can be used which creates much brighter and higher contrast images. The disadvantage of active matrix LCD’S is that the price is still high due to the high cost of building TFT factories and expensive technology used to fabricate all the tiny transistors (FET) onto the glass plate. 

Nearly all modern colour LCDs--both in notebooks and for desktop monitors is using the active matrix LCD (TFT).

Screen Size

When you purchase a 17-inch CRT monitor, you usually get 16.1 inches or a bit more of actual viewing area, depending on the brand and manufacturer of a specific CRT. The difference between the "monitor size" and the "view area" is due to the large bulky frame of a CRT. Unlike CRT monitors, LCD displays are marketed by the actual screen dimensions. That means if you purchase a 17" LCD monitor, you actually get a full 17" viewable area, or very close to a 17". This is the measurement of the displayable area of the screen from the lower corner to the opposite upper corner of the display. Below is the rough guide for the screen size:

17” CRT = 15” TFT 19” CRT = 17”-18.4” TFT 21” CRT = 19”-20” TFT

Obviously these are not always exact, but it is a good rough guide to the sizes. For instance a 21” CRT may offer a viewable area of more like 20”. Nowadays, 15” and 17” LCD Monitors are fairly rare in the market because manufacturers are focusing in making 19” model and above and they also has shifted to producing Widescreen format monitors too.

A diagonal view (screen size) of  a widescreen LCD Monitor

What is Response Time?

Response Time is the specification which many people, especially gamers, have come to regard as the most important. It translates to the amount of time it takes for a liquid crystal cell (pixel) to go from active (black) to inactive (white) and back to active (black) again. In practical terms, it refers to the speed of the pixels and how fast they can change from one colour to another, and therefore how fast the picture can be redrawn. The faster this transition can change the better. This helps reduce the effects of ghosting/ blurring in games and movies which can result if the response time is too slow.

The response time is measured in milliseconds or (ms). Lower numbers mean faster is the transitions time (e.g. 16 ms is faster than 25 ms.). If you visit any computer dealers and get the brochure from them you could see a small word (ms) printed besides the LCD Monitor price list. This is to tell you that the particular LCD Monitor is running on what milliseconds. Generally the lower the milliseconds (response time) the more expensive is the LCD Monitor price.

Native Resolutions

The physical structure of some types of displays, including LCD Monitors/TVs and plasma panels, defines how many pixels can be displayed at once. The display produces the sharpest picture when used at its so-called native resolution. This is the physically number of horizontal and vertical pixels that make up the LCD matrix of the display.

Setting a computer display to a resolution lower than this resolution will either cause the monitor to use a reduced visible area of the screen or it will have to do extrapolation. This extrapolation attempts to blend multiple pixels together to produce a similar image to what you would see if the monitor were to display it at the given resolution but it can result in fuzzy images.

Below are some of the common native resolutions found in LCD monitors: 

14-15": 1024x768 (XGA) 17-19": 1280x1024 (SXGA) 20"+: 1600x1200 (UXGA) 19” (Widescreen): 1440x900 (WXGA+) 20” (Widescreen): 1680x1050 (WSXGA+) 24” (Widescreen): 1920x1200 (WUXGA) 30” (Widescreen): 2560x1600 

Contrast Ratio

Contrast ratio is a big marketing tool by the manufacturers and one that is not easy for consumers to grasp. Contrast ratio relates to the display's comparative difference between its brightest white values and its darkest black values. As a rule of thumb, the higher the contrast ratio, the better. A higher contrast ratio will have truer colours with less "wash out." The standard offering for lower end models is commonly 700:1. Many experts recommend a contrast ratio of 1000:1 or better.

Be wary of quoted specs however, as sometimes they can be exaggerated. Some technologies boast the ability to dynamically control contrast and offer contrast ratios of 3000:1 and above!

Brightness

Brightness is a measure of the brightest white the LCD Monitor can display. Typically LCD Monitors are far too bright for comfortable use, and the On Screen Display (OSD) is used to adjust the brightness setting down. Higher brightness is good as it leads to a better contrast ratio and can be useful for dark scenes in games / movies where it might be difficult to distinguish between shades of grey.

Viewing Angles

A CRT monitor can be viewed from almost any angle, but with an LCD this is often a problem. The viewing angle is an especially important consideration if you plan to have multiple people viewing the LCD monitor at any given time. When you use an LCD, your view changes as you move different angles and distances away from the monitor. At some odd angles, you may notice the picture fade (wash out), and possibly look as if it will disappear from view. The reason for this is because LCD's produce their image by having a film that when a current runs through the pixel, it turns on that shade of colour. The problem with the LCD film is that this colour can only be accurately represented when viewed straight on.

The LCD monitors are generally rated for their visible viewing angle for both horizontal and vertical which refers to the degree you can stray from dead centre before the picture starts to wash out. A theoretical viewing angle of 180 degrees would mean that it is fully visible from any angle in front of the screen. Many recommend a viewing angle of at least 140 degrees horizontal and 120 degrees vertical. The wider the viewing angles, the better. High contrast levels usually go hand-in-hand with wider viewing angles.

Digital and analogue connections

LCD Monitors are digital devices and thus have to convert analogue (VGA) signals before they can be displayed. A graphics card with a digital video interface (DVI) can send the signal straight to the display in digital format and no conversion required. Many LCD Monitors come with an analogue input (featuring a D-shaped connector that has 15 pins arranged in three rows, sometimes labelled D-Sub), some come with both, and only a very few come with just a digital input.

This is a digital interface that is supposed to allow for a cleaner and brighter picture compared to standard VGA connectors. Nevertheless, at this point, many LCD Monitors do such a good job of signal conversion that digital connections are not as important as they used to be. 

Digital input

Portrait/Landscape modes

Some LCD Monitors pivot so that the longer edge can go horizontal (Landscape mode) or vertical (Portrait mode). This feature can be very useful for desktop publishing, Web surfing, and viewing large spreadsheets, but don't pay extra for it if you won't use it.

Portrait mode

LCD Monitor Life span

Life span, this is typically the time taken (viewing hours) for the average backlight to dim to 50% of their original brightness. Generally, LCD monitors last longer than CRTs. A typical LCD lifespan is 50,000 hours of use compared to 15,000 to 25,000 for a CRT. A longer monitor lifespan can provide a better return on investment.

LCD application

LCD panels are used in various applications ranging from smaller portable electronic equipment to larger fixed location units. Applications such as the display device for digital watches, portable calculators, LCD Monitor and TV, laptop and notebook, arcade game machines, automobile navigation systems, industrial machine, video and digital cameras. 

Overview of LCD Monitor Circuits

Most LCD Monitors can be broken down into 6 major circuits. Each circuit have its own function and in this page I will just briefly explain to you the overview of LCD Monitor and more throughout explanation on each circuit function will be clearly explain in the following chapters.

Power Supply Circuit

As it name suggests, the role of the power supply is to provide power to the rest of the circuits in the LCD Monitor. Normally the output voltages are 12V and 5 Volts and the 5 Volts were brought it down again to 3.3 V and 2.5 Volts through voltage regulator. However in some LCD Monitor designs, the output voltages may not be the values I’ve mentioned above. You have to test it with your digital multimeter

Inverter Circuit

Provide high voltages and current required by the backlight (lamps). Inverter generates from 600 up to 1000 plus VAC from one, two or even four high voltage transformer depending on how many backlights were used.

Internal view of LCD Monitor

Backlight (lamps)

Generate a consistent, uniform light source. The light generated from the backlight focused through the LCD.

Main board/AD board

Convert the RGB analogue signal into digital signal and channel it to the LCD driver/controller board.

LCD Driver/Controller board

Accepts additional display information from the Main Board and drive the transistors in the LCD panel.

LCD Panel

Controls light throughout using the liquid crystal material.

LCD Monitor Block Diagram

Understanding Power Supply Board

The switch mode power supply used to power up LCD Monitor can be either the external or internal type. The function of the power supply is to convert the main supply AC 230 volts into DC output voltages to supply to the necessary boards in LCD Monitor.

The internal type power supply

230 Volts AC supply enters the power supply and to the bridge rectifier ac pins (normally is the 2^nd and the 3^rd leg). The AC supply is then converted into DC output voltage (about 300 VDC-in USA about 155 VDC) where the big filter capacitor filtered off the ripple so that the power supply will have a nice constant of DC voltage. This high voltage DC supply is then given to a switching power FET Transistor. This switching FET transistor circuit is switched on and off at a very high speed by a control circuit (power IC) which generates very high frequency square wave pulses. 

The power FET and power IC (UC3842B) are separated

The switching FET transistor circuit switches the given high voltage DC, on and off at the same high frequency and gives square wave pulses as the output. These square wave pulses are then given to the primary winding of Switch Mode Power Transformer. These pulses induce a voltage at the primary winding of the transformer which will generate voltage at the secondary winding. This voltage at the secondary winding is then rectified and filtered to produce the required output. 

The build in power supply have output of usually 12 volts and 5 volts where the 12 volts enters the inverter IC and also audio power amp IC.

The 5 volts will go through one or two voltage regulators to get the 3.3 and 2.5 volts to power the Scalar IC, MCU, EEprom and even the LCD driver/controller board.

The power FET transistor already integrated into the power IC

Please take note that many latest designs of LCD Monitor power supply designs have the switching power FET transistor already integrated into the power IC thus you will not find the power FET in the power supply board.

The External type power supply

Internal view of external power supply