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Background In 1981, IBM introduced the Color Graphics Adapter (CGA), which was capable of rendering four colors and had a maximum resolution of 320 pixels horizontally by 200 pixels vertically. IBM introduced the Enhanced Graphics Adapter (EGA) display in EGA allowed up to 16 different colors and increased the resolution to 640x350 pixels, improving the appearance of the display and making it easier to read text. In 1987, IBM introduced the Video Graphics Array (VGA) display system. Most computers today support the VGA standard and many VGA monitors are still in use. Copyright © Texas Education Agency, All rights reserved. UNT in partnership with TEA, Copyright © . All rights reserved

Background (cont.) IBM introduced the Extended Graphics Array (XGA) display in 1990, offering 800x600 pixel resolution in true color (16.8 million colors) and 1024x768 resolution in 65,536 colors. Ultra Extended Graphics Array (UXGA) can support a palette of up to 16.8 million colors and resolutions of up to 1600x1200 pixels, depending on the video memory of the graphics card in your computer. The maximum resolution normally depends on the number of colors displayed. For example, your card might require that you choose between 16.8 million colors at 800x600, or 65,536 colors at 1600x1200. Copyright © Texas Education Agency, All rights reserved.

Background (cont.) Most desktop displays use a cathode ray tube (CRT), while portable computing devices such as laptops incorporate liquid crystal display (LCD), light-emitting diode (LED), gas plasma, or other image projection technology. Because of their slimmer design and smaller energy consumption, monitors using LCD technologies are beginning to replace the venerable CRT on many desktops. Copyright © Texas Education Agency, All rights reserved.

What to buy? When purchasing a display, you have a number of decisions to make. These decisions affect how well your display will perform for you, how much it will cost, and how much information you will be able to view with it. Your decisions include: Cable technology – VGA and DVI are the two most common Refresh rate Color depth Viewable area (usually measured diagonally) Aspect ratio and orientation (landscape or portrait) Maximum resolution Dot pitch Display technology – currently, the choices are mainly between CRT and LCD technologies. Copyright © Texas Education Agency, All rights reserved.

Cable Technology You can see that a VGA connector like this has three separate lines for the red, green, and blue color signals, and two lines for horizontal and vertical sync signals. In a normal television, all of these signals are combined into a single composite video signal. The separation of the signals is one reason why a computer monitor can have so many more pixels than a TV set. A standard VGA connector has 15 pins (3 rows of 5). The cable has the male end and the computer has the female end. Copyright © Texas Education Agency, All rights reserved.

Cable Technology Since today's VGA adapters do not fully support the use of digital monitors, a new standard, Digital Video Interface (DVI), has been designed for this purpose. Because VGA technology requires that the signal be converted from digital to analog for transmission to the monitor, a certain amount of degradation occurs. DVI keeps data in digital form from the computer to the monitor, virtually eliminating signal loss. The DVI specification is based on Silicon Image's Transition Minimized Differential Signaling (TMDS), and provides a high-speed digital interface. TMDS takes the signal from the graphics adapter, determines the resolution and refresh rate that the monitor is using, and spreads the signal out over the available bandwidth to optimize the data transfer from computer to monitor. Copyright © Texas Education Agency, All rights reserved.

Refresh Rate In monitors based on CRT technology, the refresh rate is the number of times that the image on the display is drawn each second. If your CRT monitor has a refresh rate of 72 Hertz (Hz), then it cycles through all the pixels from top to bottom 72 times a second. Refresh rates are very important because they control flicker, and you want the refresh rate to be as high as possible. Too few cycles per second and you will notice a flickering, which can lead to headaches and eye strain. Televisions have a lower refresh rate than most computer monitors. To help adjust for the lower rate, they use a method called interlacing. This means that the electron gun in the television's CRT will scan through all the odd rows from top to bottom, then start again with the even rows. The phosphors hold the light long enough that your eyes are tricked into thinking that all the lines are being drawn together. Copyright © Texas Education Agency, All rights reserved.

Color Depth The combination of the display modes supported by your graphics adapter and the color capability of your monitor determines how many colors can be displayed. For example, a display that can operate in SuperVGA (SVGA) mode can display up to 16,777,216 (usually rounded to 16.8 million) colors because it can process a 24-bit-long description of a pixel (dot). Displays use bits to describe color and how many colors can be displayed. The number of bits used to describe a pixel is known as its bit depth. With a 24-bit bit depth, 8 bits are dedicated to each of the three additive primary colors -- red, green, and blue. This bit depth is also called true color because it can produce the 10,000,000 colors discernible to the human eye, while a 16-bit display is only capable of producing 65,536 colors. Displays jumped from 16-bit color to 24-bit color because working in 8-bit increments makes things a whole lot easier for developers and programmers. Copyright © Texas Education Agency, All rights reserved.

Dot Pitch Dot pitch is the diagonal distance between the same color phosphor dots. The smaller the dot pitch, the greater the potential image sharpness. Today’s monitors have a dot pitch of .28mm or smaller. The actual sharpness of a display image is measured in dots-per-inch (dpi). The dots-per-inch is determined by a combination of the screen resolution and the physical screen size. Copyright © Texas Education Agency, All rights reserved.

Aspect Ratio On desktop computers, the display screen width relative to height, known as the aspect ratio, is generally standardized at 4 to 3 (usually indicated as "4:3"). Screen sizes are measured in either millimeters or inches, diagonally from one corner to the opposite corner. Common desktop screen sizes are 15, 17, and 19 inches. Notebook screen sizes are somewhat smaller. The other aspect ratio in common use is 16:9. Used in cinematic film, 16:9 was not adopted when the television was first developed, but has always been common in the manufacture of alternative display technologies such as LCD. With widescreen DVD movies steadily increasing in popularity, most TV manufacturers now offer 16:9 displays. Copyright © Texas Education Agency, All rights reserved.

Display Technology There are three basic types of monitors used with today’s technology: Cathode Ray Tube (CTR) Liquid Crystal Display (LCD) Plasma Copyright © Texas Education Agency, All rights reserved.

Cathode Ray Tube Monitors (CRTs)
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Display Technology The projection technology used by most displays is Cathode Ray Tube (CRT) technology, which is similar to that used in most television sets. CRT technology requires a certain distance from the beam projection device to the screen in order to function and tend to be very bulky. Using other technologies, displays can be much thinner, and are known as flat-panel displays. Copyright © Texas Education Agency, All rights reserved.

Cathode Ray Tube Monitors
The terms anode and cathode are used in electronics as synonyms for positive and negative terminals. For example, you could refer to the positive terminal of a battery as the anode and the negative terminal as the cathode. In a cathode ray tube, the "cathode" is a heated filament (not unlike the filament in a normal light bulb). The heated filament is in a vacuum created inside a glass "tube." The "ray" is a stream of electrons that naturally pour off of a heated cathode into the vacuum. Electrons are negative. The anode is positive, so it attracts the electrons pouring off the cathode. In a monitor's cathode ray tube, the stream of electrons is focused by a focusing anode into a tight beam, and then accelerated by an accelerating anode. This tight, high-speed beam of electrons flies through the vacuum in the tube and hits the flat screen at the other end of the tube. This screen is coated with phosphor, which glows when struck by the beam. Copyright © Texas Education Agency, All rights reserved.

Cathode Ray Tube Monitors
There is a cathode and a pair (or more) of anodes. There is the phosphor-coated screen. There is a conductive coating inside the tube to soak up the electrons that pile up at the screen-end of the tube. However, in this diagram you can see no way to "steer" the beam – the beam will always land in a tiny dot right in the center of the screen. Copyright © Texas Education Agency, All rights reserved.

Steering Coils The steering coils are simply copper windings. These coils are able to create magnetic fields inside the tube, and the electron beam responds to the fields. One set of coils creates a magnetic field that moves the electron beam vertically, while another set moves the beam horizontally. By controlling the voltages in the coils, you can position the electron beam at any point on the screen. Copyright © Texas Education Agency, All rights reserved.

Phosphor A phosphor is any material that, when exposed to radiation, emits visible light. The radiation might be ultraviolet light or a beam of electrons. Any fluorescent color is really a phosphor – fluorescent colors absorb invisible ultraviolet light and emit visible light at a characteristic color. In a CRT, phosphor coats the inside of the screen. When the electron beam strikes the phosphor, it makes the screen glow. In a black-and-white screen, there is one phosphor that glows white when struck. In a color screen, there are three phosphors arranged as dots or stripes that emit red, green and blue light. There are also three electron beams to illuminate the three different colors together. Copyright © Texas Education Agency, All rights reserved.

Painting the Picture Standard TVs use an interlacing technique when painting the screen. In this technique, the screen is painted 60 times per second, but only half of the lines are painted per frame. The beam paints every other line as it moves down the screen – for example, every odd-numbered line. Then, the next time it moves down the screen, it paints the even-numbered lines, alternating back and forth between even-numbered and odd-numbered lines on each pass. The entire screen, in two passes, is painted 30 times every second. The alternative to interlacing is called progressive scanning, which paints every line on the screen 60 times per second. Most computer monitors use progressive scanning because it significantly reduces flicker. Copyright © Texas Education Agency, All rights reserved.

Liquid Crystals Displays (LCDs)
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Display Technology Liquid crystal display (LCD) technology works by blocking light rather than creating it, while light-emitting diode (LED) and gas plasma work by lighting up the display screen positions based on the voltages at the different grid intersections. LCDs require far less energy than LED and gas plasma technologies, and are currently the primary technology for notebooks and other mobile computers. As flat-panel displays continue to grow in screen size and improve in resolution and affordability, they will gradually replace CRT-based displays. Copyright © Texas Education Agency, All rights reserved.

Liquid Crystals We know that there are three common states of matter: solid, liquid, and gaseous. Solids act the way they do because their molecules always maintain their orientation and stay in the same position with respect to each other. The molecules in liquids are just the opposite: they can change their orientation and move anywhere in the liquid. But there are some substances that can exist in an odd state that are sort of like a liquid and sort of like a solid. When they are in this state, their molecules tend to maintain their orientation, like the molecules in a solid, but also move around to different positions, like the molecules in a liquid. This means that liquid crystals are neither a solid nor a liquid. That's how they ended up with their seemingly contradictory name. Copyright © Texas Education Agency, All rights reserved.

Liquid Crystals It turns out that liquid crystals are closer to a liquid state than a solid. It takes a fair amount of heat to change a suitable substance from a solid into a liquid crystal, and it only takes a little more heat to turn that same liquid crystal into a real liquid. This explains why liquid crystals are very sensitive to temperature. It also explains why a laptop computer display may act funny in cold weather or during a hot day. Copyright © Texas Education Agency, All rights reserved.

Liquid Crystals Just as there are many varieties of solids and liquids, there is also a variety of liquid crystal substances. Depending on the temperature and particular nature of a substance, liquid crystals can be in one of several distinct phases. Liquid crystals in the nematic phase make LCDs possible. One feature of liquid crystals is that they're affected by electric current. A particular sort of nematic liquid crystal, called twisted nematics, is naturally twisted. Applying an electric current to these liquid crystals will untwist them to varying degrees depending on the current's voltage. LCDs use these liquid crystals because they react predictably to electric current in such a way as to control light passage. Copyright © Texas Education Agency, All rights reserved.

Liquid Crystals Displays
There are two main types of LCDs used in computers: passive matrix and active matrix. Copyright © Texas Education Agency, All rights reserved.

Passive Matrix Passive matrix LCDs use a simple grid to supply the charge to a particular pixel on the display. 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. Copyright © Texas Education Agency, All rights reserved.

Passive Matrix The passive matrix system has significant drawbacks, notably slow response time and imprecise voltage control. Response time refers to the LCD's ability to refresh the displayed image. 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. Imprecise voltage control hinders the passive matrix's ability to influence only one pixel at a time. When voltage is applied to untwist one pixel, the pixels around it also partially untwist, which makes images appear fuzzy and lacking in contrast. Copyright © Texas Education Agency, All rights reserved.

Active Matrix Active-matrix LCDs depend on thin film transistors. Basically, TFTs are tiny switching transistors and capacitors. 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. By doing this in very exact, very small increments, LCDs can create a gray scale. Most displays today offer 256 levels of brightness per pixel. Copyright © Texas Education Agency, All rights reserved.

Active Matrix An LCD that can show colors must have three subpixels with red, green, and blue color filters to create each color pixel. Through the careful control and variation of the voltage applied, the intensity of each subpixel can range over 256 shades. Combining the subpixels produces a possible palette of 16.8 million colors (256 shades of red x 256 shades of green x 256 shades of blue), as shown below. These color displays take an enormous number of transistors. For example, a typical laptop computer supports resolutions up to 1,024x768. If we multiply 1,024 columns by 768 rows by 3 subpixels, we get 2,359,296 transistors etched onto the glass! If there is a problem with any of these transistors, it creates a "bad pixel" on the display. Most active matrix displays have a few bad pixels scattered across the screen. Copyright © Texas Education Agency, All rights reserved.

LCD Advances LCD technology is constantly evolving. LCDs today employ several variations of liquid crystal technology, including super twisted nematics (STN), dual scan twisted nematics (DSTN), ferroelectric liquid crystal (FLC) and surface stabilized ferroelectric liquid crystal (SSFLC). Display size is limited by the quality-control problems faced by manufacturers. Simply put, to increase the display size, manufacturers must add more pixels and transistors. As they increase the number of pixels and transistors, they also increase the chance of including a bad transistor in a display. Manufacturers of existing large LCDs often reject about 40% of the panels that come off the assembly line. The level of rejection directly affects LCD prices, since the sales of the good LCDs must cover the cost of manufacturing both the good and bad ones. Only advances in manufacturing can lead to affordable displays in bigger sizes. Copyright © Texas Education Agency, All rights reserved.

What is Plasma? The central element in a fluorescent light is plasma, a gas made up of free-flowing ions (electrically charged atoms) and electrons (negatively charged particles). Under normal conditions, a gas is mainly made up of uncharged particles. That is, the individual gas atoms include equal numbers of protons (positively charged particles in the atom's nucleus) and electrons. The negatively charged electrons perfectly balance the positively charged protons, so the atom has a net charge of zero. If you introduce many free electrons into the gas by establishing an electrical voltage across it, the situation changes very quickly. The free electrons collide with the atoms, knocking loose other electrons. With a missing electron, an atom loses its balance. It has a net positive charge, making it an ion. Copyright © Texas Education Agency, All rights reserved.

What is Plasma? In a plasma environment with an electrical current running through it, negatively charged particles are rushing toward the positively charged area of the plasma, and positively charged particles are rushing toward the negatively charged area. In this mad rush, particles are constantly bumping into each other. These collisions excite the gas atoms in the plasma, causing them to release photons of energy. Copyright © Texas Education Agency, All rights reserved.

Inside the Display: Gas and Electrodes
The xenon and neon gas in a plasma television is contained in hundreds of thousands of tiny cells positioned between two plates of glass. Long electrodes are also sandwiched between the glass plates, on both sides of the cells. The address electrodes sit behind the cells, along the rear glass plate. The transparent display electrodes, which are surrounded by an insulating dielectric material and covered by a magnesium oxide protective layer, are mounted above the cell along the front glass plate. Copyright © Texas Education Agency, All rights reserved.

Inside the Display: Gas and Electrodes
Both sets of electrodes extend across the entire screen. The display electrodes are arranged in horizontal rows along the screen and the address electrodes are arranged in vertical columns. As you can see in the diagram below, the vertical and horizontal electrodes form a basic grid. To ionize the gas in a particular cell, the plasma display's computer charges the electrodes that intersect at that cell. It does this thousands of times in a small fraction of a second, charging each cell in turn. When the intersecting electrodes are charged (with a voltage difference between them), an electric current flows through the gas in the cell. As we saw in the last section, the current creates a rapid flow of charged particles which stimulates the gas atoms to release ultraviolet photons. Copyright © Texas Education Agency, All rights reserved.

Creating a Picture The released ultraviolet photons interact with phosphor material coated on the inside wall of the cell. Phosphors are substances that give off light when they are exposed to other light. When an ultraviolet photon hits a phosphor atom in the cell, one of the phosphor's electrons jumps to a higher energy level and the atom heats up. When the electron falls back to its normal level, it releases energy in the form of a visible light photon. The phosphors in a plasma display give off colored light when they are excited. Copyright © Texas Education Agency, All rights reserved.

Creating a Picture (cont.)
Every pixel is made up of three separate subpixel cells, each with different colored phosphors. One subpixel has a red light phosphor, one subpixel has a green light phosphor, and one subpixel has a blue light phosphor. These colors blend together to create the overall color of the pixel. By varying the pulses of current flowing through the different cells, the control system can increase or decrease the intensity of each subpixel color to create hundreds of different combinations of red, green and blue. In this way, the control system can produce colors across the entire spectrum. UNT in partnership with TEA, Copyright © . All rights reserved

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