DISPLAY DEVICES - MASTER GUIDE
DISPLAY DEVICES
A display device is a device for presentation of information, such as image or a text, for visual or tactile reception, acquired, stored, or transmitted in various forms. When the input information is supplied as an electrical signal, the display is called electronic display. Electronic displays are available for presentation of visual and tactile information.
CATHODE RAY TUBES:
CRT (Cathode Ray Tube) is one of the mostly used display technology. A CRT is a specialized vacuum tube in which images are produced when an electron beam strikes a phosphorescent surface. Most desktop computer displays make use of CRTs. A CRT consists of several basic components as illustrated in Figure 1.7.
In CRT, a beam of electrons emitted by an electron gun strikes on specified positions on phosphor coated screen after passing through focusing and deflecting systems. Heat is supplied to the cathode by passing current through a heater element.
The cathode is a cylindrical metallic structure that is rich in electrons. On heating, electrons are released from cathode surface. The control grid is the next element that follows the cathode. It almost covers the cathode leaving small opening for electrons to come out. Intensity of the electron beam is controlled by setting voltage levels on the control grid.
A high negative voltage applied to the control grid shuts off the beam by repelling electrons and stopping them from passing through the small hole at the end of the control grid structure. A smaller negative voltage on the control grid simply decreases the number of electrons passing through the cathode. Thus, we can control the brightness of display by varying the voltage on the control grid. Positively charged anodes, in the sequence accelerating anodes, accelerate the electrons towards phosphor screen.
Focusing and deflection coils are together needed to force the electron beam to converge into a small spot, as it strikes the screen, otherwise the electrons would repel each other and the beam would spread out as it approaches the screen.
Deflecting coils produce an extremely low frequency electromagnetic field that allows for constant adjustment of the direction of the electron beam. There are two sets of deflecting coils: horizontal and vertical. Electrostatic focusing is commonly used in television and computer graphics monitors.
Finally, when the accelerating electron beam collides on the phosphor coating, a part of kinetic energy is converted into light and heat. When the electrons in the beam collide with the phosphor coating, they are stopped and their kinetic energy is absorbed by the phosphor, resulting in screen display.
Graphical displays for early computers used vector monitors, a type of CRT, that uses magnetic (rather than electrostatic) deflection. Magnetic deflection allows the construction of much shorter tubes for a given viewable image size. Here, the beam traces straight lines between arbitrary points, repeatedly, refreshing the display as quickly as possible.
CRTs produce crisp, vibrant images. But they do have a serious drawback:
• They are bulky.
• In order to increase the screen width in a CRT set, you also have to increase the length of the tube.
• The electromagnetic fields emitted by CRT monitors constitute a health hazard and can affect the functioning of living cells.
Raster-scan display
A raster-scan display is the most common method of drawing images on a CRT screen. In this method, horizontal and vertical deflection signals are generated to move a beam all over the screen in a pattern for displaying any image (Figure 1.8). The electron beam is swept across the screen one row at a time from top to bottom.
The electron beam sweeps back and forth from left to right across the screen. The beam is on, while it moves from left to right. The beam is off, when it moves back from right to left. This phenomenon is called the horizontal retrace, as shown by dotted lines in the figure.
As soon as the beam reaches the bottom of the screen, it is turned off and is rapidly retraced back to the top to start again. This is called the vertical retrace.
Raster-scan displays maintain the steady image on the screen by repeating scanning of the same image. This process is known as refreshing of screen.
A graphics display consists of three components: frame buffer, display controller, and a TV screen or monitor.
The frame buffer stores an image as a matrix of intensity values. In a personal computer, the frame buffer is located on the graphics card that manages the video subsystem of the computer. Stored intensity values are then retrieved from the refresh buffer and displayed on the screen one row at a time. Each intensity value is represented by bit zero (0) or one (l) in the frame buffer.
The video or display controller has direct access to memory locations in the frame buffer. It is responsible for retrieving data from the frame buffer and passing it to the display device. It reads each successive bytes of data from frame buffer and converts this 0's and 1's in one line into a corresponding video signals, and this line is called a scan line. If the intensity is one (l) then the controller sends a signal to display a dot in the corresponding position on the screen. If the intensity is zero (0) then no dot is displayed.
Frame buffer is digital and the raster CRT is an analog device, it requires a device to convert from a digital representation to an analog signal. This is done by a Digital-to-Analog Converter (DAC) implemented as part of display controller. Figure 1.9 illustrates the raster graphics device with a single bit plane having a black and white frame buffer
The use of additional bit planes allows color or gray levels implementation into the frame buffer. In an N-bit plane frame buffer, the intensity at the location of each pixel on the CRT is decided by a corresponding pixel location in each of the N-bit planes. The bit from each of the N bit planes is loaded into corresponding positions in a register. The resulting binary number is interpreted as an intensity level between 0 and 2N-1. The value 0 represents dark and 2N-1 represents the full intensity level. This is converted into an analog voltage between 0 and the maximum voltage of the electron gun by the DAC, resulting in 2N intensity levels or colors.
A simple color frame buffer is implemented using three bit planes corresponding to three primary colors (red, green, and blue) with one bit plane for each of the three primary colors being used. Each bit plane drives one color gun for each primary color, thus yielding 2 power 2 = 8 colors.
Random-scan display
A CRT, as a random-scan display unit, has an electron beam directed only to the parts of the screen where a picture is to be drawn. Random-scan monitors draw a picture one line at a time. These are also referred to as vector displays (or stroke-writing or calligraphic displays). The component of a picture (lines and curves) can be drawn and refreshed by a random-scan system in any specified order. A pen plotter operates in a similar way and is an example of a random scan, hard-copy device.
The refresh rate, on a random-scan system, depends on the number of primitives like lines to be displayed. A picture definition is stored as a set of line-drawing commands in an area of memory called a refresh display file (or a refresh buffer). To display a specified picture, the system cycles through the set of commands in the display file, drawing each component line one by one. After all line-drawing commands have been processed, the system cycles back to the first line command in the list and repeat the procedure of scan, display, and retrace, Random-scan displays are designed to draw all the component lines of a picture 30 to 60 times each second. High-quality vector systems are capable of handling approximately 100,000 short lines at this refresh rate. It is important to note that the faster refreshing of the set of lines could burn out the phosphor. Therefore, when a small set of lines are to be displayed, each refresh cycle is delayed to avoid greater refresh rates, typically 60 frames per second.
Random-scan systems are designed for line-drawing applications; hence cannot display realistic shaded scenes. Vector displays generally have a higher resolution than raster systems, as picture definition is stored as a set of line-drawing instructions instead of a set of intensity values for all screen points. These vector displays produce smooth line drawings, because the CRT beam directly follows the line path. A raster system, in contrast, produces jagged lines that are plotted as discrete point sets. Table 1.1 presents the differences between a raster-scan display and a random-scan display.
Difference between a raster-scan display and a random-scan display
FLAT PANEL DISPLAY
Volatile Displays
Volatile displays require constant power output to refresh the image on screen many times a second. Steady images are maintained by refreshing the images more often than the human eye can perceive. Examples of volatile flat panel displays include plasma displays, liquid crystal displays (LCDs), light-emitting diode displays, and organic light-emitting diode displays (OLEDs).
A plasma display panel (PDP) is a kind of a flat panel display, now commonly used for large television displays.
A plasma display panel is composed of many tiny fluorescent-type lamps, each a few tenths of a millimeter in size located between two panels of glass that holds a mixture of inert gases (neon and xenon). The gas in the cells is electrically turned into plasma, which then excites phosphors to emit light. The basic idea of a plasma display is to illuminate tiny colored fluorescent lights to form an image. Each pixel is made up of three fluorescent lights—red light, green light, and blue light.
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. In plasma 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. Xenon and neon atoms, the atoms used in plasma screens, release light photons when they are excited Mostly, these atoms release UV light photons that are invisible to the human eye.
The released UV photons interact with phosphor material coated on the inside wall of the cell. 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 gets heated. When the electrons come back to their normal level, they release energy in the form of a visible light photon.
On excitation, the phosphors in plasma display produces colored light. Each pixel in the display is made up of three separate sub pixel cells with different colored phosphors, namely red, green, and blue light phosphor. By varying the pulses of current flowing through the different cells, the control system can increase or decrease the intensity of each sub pixel color to create hundreds of different combinations of red, green, and blue.
The main advantage of plasma display technology:
• It can produce a very wide screen using extremely thin materials.
• Each individual cell in PDPs can be turned on and off rapidly enough to produce a high-quality moving picture.
• And because each pixel is lit individually, the image is very bright and looks good from almost every angle.
• The image quality is not quite up to the standards of the best CRT sets, but it certainly meets most people's expectations.
The biggest drawback of this technology is the price. But as prices fall and technology advances, they may start to edge out the old CRT sets.
Active matrix liquid crystal displays
Active matrix liquid crystal display (AMLCD) are a type of flat panel displays, at present overpowering choice of notebook and computer display manufacturers due to its light weight, extremely good image quality, wide color range, and response time.
The active matrix displays contain polarizing sheets, cells of liquid crystal, a matrix of thinfilm transistors (TFTs) known as TFT LCD. These devices store the electrical state of each pixel on the display, while all the other pixels are being updated. This technique provides a much brighter, sharper display than a passive matrix of the similar size. Thin film transistors are generally used for constructing an active matrix.
A passive matrix display uses a simple conductive grid to deliver current to the liquid crystals in the target area.
An active matrix display uses a grid of transistors and capacitors (which are called thin-film transistors) with the ability to hold a charge for a limited period of time.
Because of the switching action of transistors, only the desired pixel receives a charge, and the pixel acts as a capacitor to hold the charge until the next refresh cycle, improving image quality over a passive matrix.
The display resolution signifies the number of dots (pixels) on the entire screen. The total number of pixels in an active matrix display is fixed. The higher is the resolution, the more dots or pixels you find on your display device.
The following are different types of LCD display technologies:
1. Twisted nematic: This type of display makes use of twisted nematic (TN). It is a phase crystals that have a natural helical structure and can be Untwisted by an applied voltage to allow light to pass through. These displays have low production costs and fast response times. It has limited viewing angles, and many have a limited color gamut.
2. In-plane switching: The two electrodes corresponding to a pixel are both on the same glass plate and are parallel to each other. Liquid crystal molecules do not form a helical structure and, instead, are also parallel to each other. In its natural or off state, the molecule structure is arranged parallel to the glass plates and electrodes. In an inplane switching (IPS) display, the angle at which light leaves a pixel is not as restricted, and therefore viewing angles and color reproduction are much improved. However, IPS displays have slower response times. IPS displays also initially suffered from poor contrast ratios but has been significantly improved with the development of advanced super IPS (AS-IPS).
3. Multi-domain vertical alignment: In this type of display, liquid crystals are naturally arranged perpendicular to glass plates and can be rotated to control light passing through it. There are also pyramid-like protrusions in the glass substrates to control the rotation of the liquid crystals. This technology results in wide viewing angles while boasting good contrast ratios and faster response times. The major drawback is a reduction in brightness.
4. Patterned vertical alignment: This type of display is a variation of MVA and performs very similarly but with much higher contrast ratios.
5. A thin film transistor liquid crystal display (TFT LCD) is a variant of an LCD that uses the thin film transistor (TFT) technology to improve image quality. A TFT LCD is one type of active matrix LCD, though it is usually synonymous with LCD. It is used in TV, flat panel displays, and projectors. Sri Bhuvanendra College, Karkala IV SEMESTER BCA COMPUTER GRAPHICS AND ANIMATION – UNI
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