Pc Open Del3

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  • 1. Chapter 15: Monitors and Displays399 Figure 15-14: Examples of numerals displayed on an LCD. LCD LIGHTING An LCD uses one of two light source types: reflective and transmissive. A reflective LCD reflects only the light that enters through its polarized filters from its environ- ment. In a well-lighted room or in sunlight, there is usually enough light to see the display. Transmissive LCDs, used in portable computers and flat-panel monitors, incorporate lighting elements to backlight the display. Typically, built-in fluores- cent tubes located around the edges of the display and sometimes behind the LCD provide the lighting.Categorizing LCDs The three different types of LCDs used in various devices are common-plane, pas- sive matrix, and active matrix.COMMON-PLANE LCD Common-plane LCDs arent used for PC displays but rather in watches, handheld games, and microwaves, where the same numbers or objects are displayed repeatedly.PASSIVE MATRIX LCD A passive matrix LCD has pixels, like a CRT, instead of electrodes. However, its operating principles are the same that are used in a common-plane LCD. A grid organized in rows and columns is used to energize the pixels, which are located at the intersections of the rows and columns. Integrated circuits control the rows and columns to ensure that a charge sent over the grid gets to the specific pixel that it was intended to activate. The grids rows and columns are on separate layers of a transparent conductive material that sandwich a layer of liquid crystal. A layer of polarizing film is added to the top and bottom substrates of the sandwich.A pixel is energized when an electrical charge is sent down the appropriate col- umn on one substrate, and a grounding charge is sent over the appropriate row on the other substrate. The two charges converge at the pixel located at the intersec- tion of the row and column and cause the pixels liquid crystal to untwist and block the light source and darken the pixel.

2. 400 Part IV: Sight and Sound Systems Another Passive Matrix ApplicationAn application of passive matrix LCD technology is the portable stylus-based computer,also known as the personal digital assistant (PDA) or palmtop computer. Although thesecomputers might have a keyboard, commands and data are typically entered throughthe screen via a special non-writing pen or stylus. The display is covered by aprotective, plastic covering; beneath the display is a wire grid that recognizes themovements of the stylus. The wire grid records the movements of the pen over thegrids intersections, which is similar to the technology behind touch-screens. A passive matrix LCD has its disadvantages. Its refresh speed (the response time) is slow, and the grid delivers electricity imprecisely to specific pixels. This latter prob- lem can affect nearby pixels and create a fuzzy image or create contrast problems.A passive matrix display uses one of two types of liquid crystal: N Twisted nematic (TN): TN liquid crystal has a 90 twist and is used in low- cost displays. It produces a black on gray or silver background. TN liquid crystal is used primarily on consumer electronics and appliances.N Supertwisted nematic (STN): Although its name sounds a bit like your wacky brother-in-law, STN is the type of liquid crystal found on a portable or handheld PC or a PDA. It is made with either a 180 or a 270 twist, which gives it a much wider range of motion, making it more toler- ant against any energy radiating from nearby pixels and allowing it to provide more steps of color shadings. STN is used in both monochrome and color displays.Although not a type of liquid crystal, dual-scan STN (DSTN) is a process usedin some LCDs to double the number of lines refreshed and to cut the time torefresh the display in half. This is accomplished by dividing the LCD into twoequal halves that are scanned simultaneously.ACTIVE MATRIX LCD The pixels on an active matrix LCD use thin-film transistors (TFTs), which is why this type of LCD is often called a TFT display. TFTs are switching transistors and capacitors etched in a matrix pattern on a glass substrate that forms one of the lay- ers of the active matrix LCD. Each pixel consists of three TFTs, one for each of the 3. Chapter 15: Monitors and Displays 401RGB colors, which can add up to quite a few transistors in the display. For example, a VGA 640 x 480 color display uses 921,600 transistors; comparatively, a 1024 x 768 UVGA color display uses 2,359,296 transistors, all of which are etched into the substrate glass. If a transistor is defective, it creates a bad pixel. TFT displays com- monly have at least a few bad pixels.An active matrix LCD addresses its pixels somewhat like a passive matrix. However, when one row is addressed on the active matrix display, all the other rows are switched off, and the charge is sent down the appropriate column. Because only the addressed row is active, just the pixel at the intersection of the active row and column is affected. The TFTs capacitor holds the energy used to charge the pixel until the next refresh cycle.The color of the pixel is provided by color filters that lay over the areas con- trolled by the pixels three TFTs. Colors are created by the amount of light allowed to pass through the filters by each of the TFTs, which are controlled by the inten- sity of the charge sent to them by the image control circuits.As illustrated in Figure 15-15, the TFTs control how much the liquid crystal ele- ments open (untwist) to block the light passing through the color filters. In the sit- uation shown in Figure 15-15, a small amount of the light source is being allowed to pass through the red filter along with a wide open blue, but no light is being passed through the green filter. Controlling the amount of electricity that flows to the pixel manages the action of the liquid crystal and the amount of light allowed to pass through the color filters. By controlling the light, active matrix screens are able to display 256 levels of color brightness per pixel. Comparing Viewing AnglesA displays viewing angle measures how far above, below, and (more importantly) tothe side of the display that images on the screen can be accurately viewed. Thefollowing table compares the viewing angles of the two LCD displays with a CRT. Thefigure here illustrates the relative differences of the viewing angles of these displays.Display Type Viewing Angle Passive matrix LCD49100Active matrix LCD 90120CRT120180Continued 4. 402 Part IV: Sight and Sound SystemsComparing Viewing Angles (Continued)Cathode ray tubeActive matrix LCD Passive matrix LCDThe curvature of the screen has a lot to do with a displays viewing angle, but next on the list is the amount of contrast in the displayed image. An active matrix (TFT) display has deeper color, clarity, and contrast over a passive matrix display. In the eye of the viewer, LCD displays begin to lose their picture quality as the angle of view increases because less of the displays light (image) is able to reach the viewer. Obviously, the viewing angle champion is the conventional CRT. However, a flat-screen CRT might have a much lower viewing angle. 5. Chapter 15: Monitors and Displays403 RedGreenBlue Figure 15-15: Controlling the color in an active matrix LCD.Powering the Display Monitors dont run off the PCs power supply, even if they are plugged into the back of the PC power supply. Plugging a monitor into the back of the PCs power supply is the same as getting AC power straight from a wall outlet. The plug on the back of the power supply is an AC power pass-through plug. A PCs monitor uses more power than all the other components of the PC added together. And because of how it works, several power issues exist on monitors that dont exist on a PC or its power supply.Managing power In an effort to reduce the tremendous amount of energy being consumed by moni- tors in active mode, governments and industry organizations have developed ini- tiatives to reduce the amount of power consumed by PC monitors in general but especially when theyre idle. The U.S. Environmental Protection Agency (EPA) has the Energy Star program that certifies monitors and personal computers that meet a guideline for reduced energy consumption. This program certifies monitors that use less than 30 watts of power in all power modes and reduce their power consumption by 99 percent when in sleep or suspended mode. Most PCs sold today meet this standard, and youll see the Energy Star logo displayed on the monitor during the boot sequence on these PCs. Virtually all monitors on the market today are also compliant with VESAs Display Power Management System (DPMS) protocol. DPMS is used to power down parts of the monitor and PC after theyve been idle for a certain period of time. DPMS is a Basic Input/Output System- (BIOS) supported protocol that can be enabled in the Complementary Metal-Oxide Semiconductor (CMOS) settings of the PC. 6. 404 Part IV: Sight and Sound Systems Degaussing the screen The internal components of a CRT can become magnetized over time. If they do, it can have a negative affect on the quality of the image produced by the monitor. If the CRT becomes overly magnetized, color blotches can appear on the screen near the edges and in the corners. A CRT can be magnetized in lots of ways, including setting stereo speakers or other forms of magnets too close to the monitor (which can also distort the display), bumping the monitor very hard, or moving the moni- tor so that its positioned over a PCs power supply.The cure for magnetization of the CRT is degaussing. This term is derived from the word gauss, which is a measure of magnetic force. Most better monitors have built-in degaussing circuits that neutralize the CRTs magnetization through a coil of wire inside the monitor. The degaussing circuit is activated by either a manual switch or automatically through the monitors controls.On monitors with a