Wesley's Tool-Box

• Assembling the Monitor

Now let's put the components together. We need a casing. It doesn't need to be complex or thick, because the components are lightweight, weight only a couple of hundreds of grams. I used some Plexiglas (pressed acrylic) panels left over from building my new computer (to be introduced in the upcoming article) to accommodate the components in place. First, the back panel:



I put together two panels into one, hence the grey area in the middle. Holes were drilled for placing all the electronic components on it. Next, the side panels:



The LCD panel I used had fastening holes on the left and the right side because it was intended for use in laptop computers, where space must be conserved. Therefore I drilled holes on the side panels to hold the LCD in place.

• Controller / A/D Converter

This component is the 'brain' of the LCD monitor. It drives the monitor to properly display images, and it interprets the computer's video signals to that which can be understood by the LCD panel. LCD's native control signals are in a world of their own: TTL and LVDS to name a few. These are digital signals are significantly different from the analog video signals most video cards use today, and the A/D converter bridges the difference. If the video card uses digital signal like DVI, you'll still need a Digital-Digital converter because DVI and LCD signals are incompatible.



To ensure full compatibility, the A/D converter module usually comes customized for a specific model of the LCD panel. Therefore, if you were to buy an LCD panel, you should buy the A/D converter together as well. I obtain my parts from Eunpa LCD, a direct distributor of LG.Philips LCD panels, the largest laptop LCD producer in the world. Both companies are based in Korea, so take note of this for those living outside the country. The Europeans may want to try Data Display AG, by the way.

For those who are trying to obtain an LCD panel from a used laptop computer, you will probably need to find a company that produces A/D converter module for the model you have. This is because the LCD controller in the laptop computer usually interfaces to the video chip by a proprietary connection instead of a normal video connector, so you won't be able to make a standalone LCD monitor with it. You'll have a good chance of getting the converter module from the companies mentioned above if your panel says it's from LG.Philips, of course.

• OSD Control

The OSD(On-Screen Display) control is not an essential part of the LCD monitor, as it can operate independently without one. However, you will need to have some control over adjustments of the screen, so you will want one if you could.



The OSD control is actually just a set of buttons and LED that connects to the controller. If you have the datasheet for the controller, you can make the OSD control by yourself. By pressing the buttons here, it sends signals to the controller, which in turn adjusts the screen's properties.

• Block Diagram of LCD Monitor

This is how the whole assembly of the LCD monitor is linked. We've already seen the LCD panel part (shown here as LCD Array), but it is apparent that other parts are needed for a properly functioning unit. Let's see what these other components do.

• Inverter Module

LCD does not, and cannot glow like the phosphor in CRT, so some sort light source must always be present. Some of the simple LCD displays use reflective sheet to utilize ambient light to function as a weak backlight. However, complex display like an LCD monitor needs a much stronger light source, and this is why a proper backlight is used. It is actually an array of Cold Cathode Fluorescent Tubes (CCFL) with a light diffusion panel that evenly distributes the light throughout the whole display area. CCFL is a type of a fluorescent lamp known for longevity and evenness in light transmission, and it is often found in scanners and copiers.

An observant reader will know that a fluorescent lamp operates on alternating current (AC). But the rest of the LCD monitor works on direct current (DC) as any normal electronic device would. Therefore, to operate the backlight off the common DC source, we need a module that converts DC into AC. This is the job of the inverter module as seen in the picture above. This component is noticeable by a small transformer that sits on the middle which is usually protected by a plastic cover. The one shown here has 'CAUTION HIGH VOLTAGE' written on it. If you were to take apart an LCD monitor or an old laptop computer, you will discover that the wires coming out of the backlight will lead to this module.

• LCD Panel

Now let's look at the components that make up an LCD monitor. First, the most important and obvious part - the LCD panel. This is where you see the screen displayed through the power of liquid crystals. There are many forms of LCD displays, but we will look at the TFT type, the one mostly commonly used in LCD monitors these days.



The panel itself consists of the core display, and the backlight. The core is made up of protective layer, colour filters, polarizing sheet, liquid crystals, and activating transistors. Let's look at the simplified diagram of this. First, the display drive signals reach from the specific row and column (1). This activates the transistor that controls a certain portion of the liquid crystal layer (2). The amount of voltage applied to the transistor determines the level of 'twist' of  the liquid crystal line-up.



The light from the backlight (3) follows the line-up of the liquid crystals, then hits the polarizing sheet. If the line-up is perpendicular to the polarization of the sheet, the light will be completely blocked, and if the line-up is in the same direction, all the light will pass through (5). Any line-up between these two extremes will results in different levels of brightness for the element. Since the LCD does not have any inherent colour, having one element per pixel will result in greyscale display. The colour display uses three elements per pixel, each with either red, green, or blue colour filters (4).

• Why LCD is Expensive Right Now

While there has been great price reduction of the LCD displays, it still hovers beyond the price range of the comparably-sized (CRT monitors lose about 1 to 1.5 inches of actual display at the edge, so 15" LCD approximately equates to 17" CRT) CRT monitors at the same moment. This is mainly because CRT is a 'mature industry'. CRT had been the primary method of display since the beginning, all the way back to the creation of television in the 1920's. With eight decades of technology build-up, production process has been fully optimized. This results in cheaper production and development costs.

Meanwhile, LCD display technology is a relative newcomer. While the discovery of the principle of LCD dates back to 19th century, utilizing the principle to make a functioning display had been quite difficult. We've only started seeing its use with the laptop computers around late 1980's. Before that, the laptops had small CRT monitor on it. As it is apparent, LCD display production had less time to develop and mature, and its production costs are still relatively high.

Still, the pure material cost of an LCD monitor is generally less than that of the comparably-sized CRT, and this is quite apparent as the size gets bigger. While the LCD only needs to grow two-dimensionally, retaining relatively same thickness regardless of size, CRT grows three-dimensionally, as the electron gun needs to be pulled farther to the back and increases the bulk of the tube. Then why does the price of the LCD monitor get exorbitantly expensive as the size increases? Production yield.



Modern LCD displays are based on TFT (Thin Film Transistor) technology, meaning that the whole display area you will see gets completely covered with tiny transistors. As shown in the diagram above, a casual 15" LCD monitor with 1024x768 resolution has the display area of nearly 70000 square millimetres. This is pretty much the same area covered by the 300mm (12-inch) wafer (70686mm²), which is currently one of  the largest wafer used in the production of semiconductors at the moment.

While it is true that the transistors of the LCD are bigger than that of the ones found in many high-performance semiconductors, asking for the whole wafer to be defect-free is a relatively tall order. There are almost always defective chips coming out of a wafer. Defective transistor on LCD means dead pixel, and customers certainly don't want this, so in order to make top-grade LCD display, there will be a relatively high level of rejects. The probability of the reject increases proportionally to the area, so larger the display, more defects there will be.

Fortunately, process refinements enabled considerable reduction of defects, and this is why the LCD display's price have dropped quickly in the recent years. Hopefully, we will be able to see the convergence of price of LCD displays to that of the CRT in the coming years.