| What types of flat-panel displays are available? |
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| ichael Scott (scott@bme.ri.ccf.org) and some from Bill Nott (BNott@bangate.compaq.com)] Flat-Panel Display (FPD) technology is evolving rapidly, so I will only touch on the most common current types of displays. There are other types of displays still in use, though the most common ones are based on LCD (Liquid Crystal Display) or PDP (Plasma Display Panels) technology. Now, FPD's are expensive due to the difficulty in manufacturing (typically ~65% yield - ~4 in 10 are discarded) and relatively small number of units sold. As manufacturing techniques improve and volume increases, prices will drop. In fact, in 1995, yields are up, volumes are up, _and_ factory capacity has expanded to the point where prices are dropping significantly this year. It appears there will be an oversupply of panels this year. However, the prices are still not down to the point where they can compete with CRT monitors in desktop applications. [From: Michael Scott (scott@bme.ri.ccf.org)] The vast majority of FPD's are addressed in a matrix fashion, such that a given pixel is activated by powering the corresponding row and column. This means that an individual LCD element is required for each display pixel, unlike a CRT which may have several dot triads for each pixel. LCD displays consist of a layer of liquid crystal, sandwiched between two polarizing plates. The polarizers are aligned perpendicular to each other, so that light incident on the first polarizer will be completely blocked by the second one. The liquid crystal is a conducting matrix with cyanobiphenyls (long rod-like molecules) that are polar and will align themselves with an electric current. The neat feature of these molecules is that they will shift incoming light out of phase when at rest. Light exiting the first polarizer passes through the liquid crystal matrix and is rotated out of phase by 90 degrees, then it passes through the second polarizer. Thus, unpowered LCD pixels appear bright. When an electric current is passed through the crystal matrix, the cyanobiphenyls align themselves parallel to the direction of light, and thus don't shift the light out of phase, the light is blocked by the second polarizer and the LCD appears black. So, basic LCD technology can generate bright or dark pixels, like a monochrome (not grayscale!) monitor. In order for the eye to see shades of gray, the LC activation time is modulated. i.e. a pixel that is activated 50% of the time will appear as 50% gray. The number of shades that can be generated without visible flicker is limited by the response time of a LC element - typically 16 shades, although some display manufacturers claim 64 or more shades. Most colour LCD's use red, green and blue sub-pixels, similar to the way that CRT's use coloured dots of phosphor. The concept is the same; that when viewed from a distance, the human eye will perceive the three sub-pixels as a single colour. Obviously, this requires three times as many discrete elements as would a monochrome display of the same resolution. A second method of implementing colour uses a subtractive CYM (Cyan Yellow Magenta) system where white light is generated at the back plane. The light then passes through each of three LC layers, each one blocking one of the three colours. By activating the LC layers in different combinations, a variety of colours can be produced. Common to all LCD displays is the requirement for either high ambient light levels, or bright backlighting since liquid crystals don't generate light - they can only block it. Typically, LCD's allow 5-25% of incoming light (i.e. from the backlight source) to pass through. The result of this is that LCD technology requires a significant amount of energy, and this is an important consideration in light- weight laptop design. Specific type of LCD's Passive Matrix (twisted-nematic) LCD's PM LCD's come in several types including; supertwisted nematic, double supertwisted nematic and triple supertwisted nematic. The original PM LCD's had a very limited viewing angle and poor contrast. Super and double supertwisted nematic designs provide an increased viewing angle and better contrast. The triple supertwisted design implements the subtractive CYM colour model mentioned above. PM designs are addressed in matrix fashion, so a VGA PM display would require 640 transistors horizontally and 480 vertically. Rows of pixels are activated sequentially by activating the row transistors while the appropriate column transistors are activated. This means that a given row is activated for only a short time during a screen refresh, resulting in poor contrast. Some implementations of PM technology break the screen into two parts, top and bottom, and refresh them independently, resulting in better contrast. These are called Dual Scan PM LCD's. In addition, PM displays suffer from very slow response times (40-200 ms) which is inadequate for many applications. Aside from their performance shortcomings, PM displays are inexpensive - their relatively low number of discrete components reduces manufacturing complexity and increases yields. Note that while dual scan displays are better than the original PM LCD's, they still don't have the high refresh rates and brightness of active matrix LCD's. Active Matrix LCD's Instead of using one switch (transistor) for each row and column, AM LCD's dedicate one switch for each pixel. This results in a more complex display which requires a larger number of discrete components, and therefore costs more to manufacture. An AM display is basically a large integrated circuit (IC). The benefits are significant over the PM design. Pixels can be activated more frequently, giving better contrast and control over modulation. AM technology can produce higher resolution displays that can generate more, and brighter colours. The main types of AM LCD's are; TFT (Thin-Film Transistors), MIM (Metal- Insulator-Metal) and PALC (Plasma Addressed Liquid Crystal). Ferroelectric LCD's FE LCD's use a special type of LC which holds its polarization after being charged. This reduces the required refresh rate and flicker. Also, FE LCD's have a fast response time of 100ns. Although they are very difficult to manufacture, and therefore expensive, FE LCD's may provide AM quality at PM prices in future. Plasma Display Panels PDP's have been under development for many years, and provide rugged display technology. A layer of gas is sandwiched between two glass plates. Row electrodes run across one plate, while column electrodes run up and down the other. By activating a given row and column, the gas at the intersection is ionized, giving off light. The type of gas determines the colour of the display. Because it has excellent brightness and contrast and can easily be scaled to larger sizes, PDP's are an attractive technology. However, their high cost and lack of grayscale or colour have limited applications of PDP's. However, advancements in colouring technology have allowed some manufacturers to produce large full-colour PDP's. In future, large colour PDP's will be more common in workstation and HDTV applications. |
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