Chapter 3 Display Devices and Interfacing
Monitor Details Collection of dots Matrix of dots creates character Monochrome monitor screen is collection of 350 *720 350 rows and each rows having 720 dots 1 2 3 720 1 2 3 350
scanning Scanning is process in which electronic beam inside CRT tube is moving from left to right and from top to bottom. Beam moving left to right,it is scanning all dots in row and it is called Trace Beam moving right to left then it is called Retrace No info is displayed during retrace Dot is illuminated by electron beam during scanning when logic 1 is provided by processor Dots become invisible when logic 0 is given by processor
Character generation on the screen 9 *14 dot matrix is used in monochrome monitor Actually only 9 * 7 dots are used for character generation,to provide space between adjacent charters in the same row and space between characters in adjacent lines. Character,no,picture to be displayed on CRT can constructed by displaying some dots and blanking out the remaining dots according to the pattern
Monitor related terminology Pixels: An individual dot on the screen. Color monitor pixel contain three dots Also called pixels or pel. Resolution:- Resolution is expressed in the number of horizontal and vertical picture elements i.e. pixels More resolution == quality better Resolution= Total no. of Horizontal pixels * Total no. of Vertical pixels
Dot pitch or slot pitch:- Horizontal distance between two like-colored phosphor dots Also called strip pitch Dot pitch is an indicator of sharpness of the display image. It is measured in mm (smaller no means sharper image) Dot pitch
Screen size:- Computer display rated by their size 15 inch,17inch,19 inch monitors Screen Size is measured diagonally
Important terminology related to displays Display technology CRT cathode ray tube LCD- Liquid crystal display Standards and Resolution Common display standard and resolutions standards Resolution Typical use XGA (extended graphics array) 1024 * 768 15, 17 inch monitor 15 inch LCD SXGA(super XGA) 1280 * 1024 15,17 inch CRT monitor 17, 19 inch LCD UXGA(ultra XGA) 1600 * 1200 19,20 inch monitor 20 inch LCD QXGA(Quad XGA) 2048 * 1536 21 inch monitor WXGA(wide XGA) 1280 * 800 15.4 inch laptop LCD WSXGA 1680 * 1050 20 inch LCD WUXGA 1920 * 1200 22 inch LCD moniot
Aspect Ratio and viewable Area Two measures describe the size of your display: the aspect ratio and the screen size. computer displays, like most televisions, have had an aspect ratio of 4:3. This means that the ratio of the width of the display screen to the height is 4 to 3. For wide screen LCD monitors, the aspect ratio is 16:9 (or sometimes 16:10 or 15:9). Wide screen LCD displays are useful for viewing DVD movies in wide screen format, playing games and displaying multiple windows side by side. High definition television (HDTV) also uses a wide screen aspect ratio.
Video Controller interface: It provides horizontal synchronization pulse and vertical synchronization pulse and video info These frequencies are used to synchronize the electron beam vertically and horizontally Dot pattern is provided to generate character on screen
Block diagram on color monitor and function of each block
working K:-cathode G1:-Control grid G2:-Accelerating anode G3:-focusing anode Screen coated with millions of tiny red,green.blue phosphor dots
Ray is stream of electrons generated by electron gun Anode is positive so it attracts the electrons pouring off the cathode Screen is coated with phosphor that glows when struck by the electron beam There are three ways to filter beam in order to obtain correct image shadow mask aperture grill slot mask
CRT Characteristics DOT Pitch Dot pitch is an indicator of the sharpness of the displayed image. It is mean CRT Characteristics used in millimeters (mm), and a smaller number CRT Characteristics means a sharper image.
A shadow mask is a thin metal screen filled with very small holes. Three electron beams pass through the holes to focus on a single point on a CRT displays' phosphor surface. The shadow mask helps to control the electron beams so that the beams strike the correct phosphor at just the right intensity to create the desired colors and image on the display. The unwanted beams are blocked or "shadowed." Shadow-mask
Aperture-grill Monitors based on the Trinitron technology, which was pioneered by Sony, use an aperture-grill instead of a shadow-mask type of tube. The aperture grill consists of tiny vertical wires. Electron beams pass through the aperture grill to illuminate the phosphor on the faceplate. Most aperture-grill monitors have a flat faceplate and tend to represent a less distorted image over the entire surface of the display than the curved faceplate of a shadow-mask CRT. However, aperture-grill displays are normally more expensive.
Shadow mask and Aperture Grill
The dot pitch translates directly to the resolution on the screen. If you were to put a ruler up to the glass and measure an inch, you would see a certain number of dots, depending on the dot pitch. Table that shows the number of dots per square centimeter and per square inch in each of these common dot pitches: Dot Pitch Approx. number of pixels/cm 2 Approx. number of pixels/in 2.25 mm 1,600 10,000.26 mm 1,444 9,025.27 mm 1,369 8,556.28 mm 1,225 7,656.31 mm 1,024 6,400.51 mm 361 2,256 1 mm 100 625
e.g 1280x1024 at 85 Hertz or 1600x1200 at 75 In 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 Rate control flicker.so the refresh rate should be as high as possible. If it is low then the image will flicker which can lead to headaches and eye strain. Refresh Rate
Resolution Resolution is expressed in the number of horizontal and vertical picture elements i.e. pixels More resolution == quality better Resolution= Total no. of Horizontal pixels * Total no. of Vertical pixels
Resolution
Multiple Resolutions Because a CRT uses electron beams to create images on a phosphor screen, it supports the resolution that matches its physical dot (pixel) size as well as several lesser resolutions. For example, a display with a physical grid of 1280 rows by 1024 columns can obviously support a maximum resolution of 1280x1024 pixels. It also supports lower resolutions such as 1024x768, 800x600, and 640x480.
Video Bandwidth It is the highest input frequency a monitor can handle and helps in determining the resolution capabilities of the monitor Unit MHz Higher video bandwidth == image quality better. Bandwidth=Horizontal pixel * vertical pixel * frame rate For 600*400, if frame rate is 25 Video bandwidth=600*400*25=5,760,000 i.e 5.7 MHz video bandwidth (In MHz) MDA 16.3 CGA 14.3
Horizontal Scanning Frequency The frequency at which monitor repaints the horizontal lines that make up an image is called Horizontal Scanning Frequency. Unit := KHz If resolution 800*600, 600 lines per screen
Frame Rate How many times a screenful of information is produced per second is called frame rate. Higher frame rate : less flicker problem
Vertical Scanning Frequency The frequencies at which the monitor repaints the whole screen It is also called as vertical scanning frequency. Unit : Hz (no. of cycles per second) Higher rate associate less flicker 72Hz means screen should e redraw 72 times per second.
Interlace Mode In interlaced mode the image is scanned in two passes even pass and odd pass Gun scans from top to bottom, left to right with each scan displaying a frame. On first pass it will skip every next line. On second pass, it will scan the lines it missed during the first pass, thus creating image in two scan Used in TV.
Non- interlacing All the lines are scanned or displayed in one pass instead of two passes required in the interlaced mode. Used in CRT monitors
Advantages of CRT Monitors Less expensive - Although LCD monitor prices have decreased, comparable CRT displays still cost less. Better color representation - CRT displays have historically represented colors and different gradations of color more accurately than LCD displays. However, LCD displays are gaining ground in this area, especially with higher-end models that include color-calibration technology. More responsive - Historically, CRT monitors have had fewer problems with ghosting and blurring because they redrew the screen image faster than LCD monitors. Again, LCD manufacturers are improving on this with displays that have faster response times than they did in the past. Multiple resolutions - If you need to change your display's resolution for different applications, you are better off with a CRT monitor because LCD monitors don't handle multiple resolutions as well. More rugged - Although they are bigger and heavier than LCD displays, CRT displays are also less fragile and harder to damage.
Advantages of CRT Monitors Contrast ratio allows perception of true black. Darks are darker. Displays full motion video better. Screen images viewed from side angle are as good as when viewed straight-on. (no dilution of clarity or color) Modern versions may include sleep mode for energy efficiency Strong glass screen can withstand routine handling and not easily damaged by casual touch. Many now available with flat viewing screens for less glare, viewing convenience. Strong glass screen can withstand routine handling.
Disadvantages of CRT Monitors Radiation emission. Larger size takes up more desk space, bigger the screen bigger the footprint. Heavy weight. Heavier size limits easy and convenient movement. Expends more energy and throw off heat. Measuring difference. 19 CRT provide 18 usable area
1. Heated cathode emits an electron beam which is accelerated and focused into high speed narrow beam, using control grid, accelerating anode and focusing anode. 2. Beam is deflected horizontally and vertically using H & V deflecting coils. 3. Beam strikes on the phosphor coated tube surface 4. CRT screen is made up of phosphor material dots called pixels. 5. Beam falls on screen and dots are illuminated 6. The high speed scanning of electron beam on the screen called Raster scanning 7. In this type of scanning, in each frame, new screen (timing synchronized by V-sync pulse) starts at the top-left point of the screen,then beam trace top horizontal line, retrace in blanked condition back to the start of the next line(starting of new line is sync by H-sync pulse) and the sequence continues till the bottom line is reached. 8. Then beam retrace in blanked condition to the top-left position and the scanning for the next frame starts
LCD Characteristic Native Resolution LCD monitors display information well at only the resolution they are designed for, which is known as the native resolution. Digital displays address each individual pixel using a fixed matrix of horizontal and vertical dots. If you change the resolution settings, the LCD scales the image and the quality suffers. Native resolutions are typically: 17 inch = 1024x768 19 inch = 1280x1024 20 inch = 1600x1200
Viewing Angle When you look at an LCD monitor from an angle, the image can look dimmer or even disappear. Colors can also be misrepresented. To compensate for this problem, LCD monitor makers have designed wider viewing angles. Manufacturers give a measure of viewing angle in degrees (a greater number of degrees is better).
Brightness or Luminance This is a measurement of the amount of light the LCD monitor produces. It is given in nits or one candelas per square meter (cd/m2). Typical brightness ratings range from 250 to 350 cd/m2 for monitors that perform general-purpose tasks. For displaying movies, a brighter luminance rating such as 500 cd/m2 is desirable. Contrast Ratio The contrast ratio rates the degree of difference of an LCD monitor's ability to produce bright whites and the dark blacks. The figure is usually expressed as a ratio, for example, 500:1. The higher the contrast ratio, the easier it is to see details. Typically, contrast ratios range from 450:1 to 600:1, and they can be rated as high as 1000:1.
Response Rate The response rate indicates how fast the monitor's pixels can change colors. Faster is better because it reduces the ghosting effect when an image moves, leaving a faint trial in such applications as videos or games. Adjustability Unlike CRT monitors, LCD monitors have much more flexibility for positioning the screen the way you want it. LCD monitors can swivel, tilt up and down, and even rotate from landscape (with the horizontal plane longer than the vertical plane) to portrait mode (with the vertical plane longer than the horizontal plane). In addition, because they are lightweight and thin, most LCD monitors have built-in brackets for wall or arm mounting.
Color Depth The combination of the display modes supported by your graphics adapter and the color capability of your monitor determine how many colors it displays. color bit depth refers to the number of bits used to describe the color of a single pixel. The bit depth determines the number of colors that can be displayed at one time. Aspect Ratio Two measures describe the size of your display: the aspect ratio and the screen size. most televisions, have had an aspect ratio of 4:3. This means that the ratio of the width of the display screen to the height is 4 to 3. For wide screen LCD monitors, the aspect ratio is 16:9 High definition television (HDTV) also uses a wide screen aspect ratio.
Viewable Area Screen sizes are normally measured in inches from one corner to the corner diagonally across from it. CRT monitors, screen size is measured diagonally from outside edges of the display casing. In other words, the exterior casing is included in the measurement But For LCD monitors, screen size is measured diagonally from the inside of the beveled edge. The measurement does not include the casing as indicated in the image
Twisted nematic (TN) crystal, which are rod shaped crystals are used in LCD Liquid crystal display technology works by blocking light. LCD is made of two pieces of polarized glass (also called substrate) that contain a liquid crystal material between them. A backlight creates light that passes through the first substrate. At the same time, electrical currents cause the liquid crystal molecules to align to allow varying levels of light to pass through to the second substrate and create the colors and images that you see. LCD monitor
Construction:- LCD is thin,flat display device made up of any number of color or monochrome pixels arranged in front of light source or reflector. It uses very small amounts of electric power. It consist of:- 1.Vertical filter(allows light waves to enter in only vertical direction) 2.Glass plate electrode 3.Liquid crystal 4.Glass plate electrode 5.Horizontal filter
Working Principal 1. When some voltage is applied across the opposite glass electrodes of LCD 2. Direction of liquid crystal molecules is unchanged 3. This unchanged made visible with the help of certain polarizing filters which covers outer surface of the glass electrode Polarization :- Orientation of light rays in space. Polarizer :- Two polarizer's (vertical in the input and horizontal in the output) also called as filters (which changing the direction of light rays in space) When liquid crystal molecules orientation is changed (twisted) due to electric field then they changed vertically polarized waves into horizontal direction.
Working Light entering the display is guided by liquid crystal molecules Molecules of the liquid crystal twist and they rotate incoming light by 90 degree. Incoming light is vertically polarized because of vertical filter and due to twist given by molecules the light waves becomes horizontally polarized. When current is passed through electrodes The liquid crystal molecules straighten out and stop redirecting light. No light can pass through,making this region darker compared to rest of the screen To display characters or graphics, voltage is applied to the desired regions making them darker and visible to the eye.
Passive Matrix Displays This type of LCD display uses a grid of conductive metal to charge each pixel. Two glass layers called substrates. One substrate is given column and other is given rows. It is made up of transparent conductive material (indium-tin-oxide) Rows and columns are connected to integrated circuit that control when a charge is sent down a particular column or row. Liquid Crystal Molecule is sandwiched between 2 glass substrate and polarizing film is added to the outer side of each substrate. Passive matrix is made up of set of multiplexed transparent electrodes. Electrode are made up of ITO, are placed above and below the LC layer in row/column formation.
Passive-matrix LCDs use a simple grid to supply the charge to a particular pixel on the display. Creating the grid is quite a process! It starts with two glass layers called substrates. One substrate is given columns and the other is given rows made from a transparent conductive material. This is usually indium-tin oxide. The rows or columns are connected to integrated circuits that control when a charge is sent down a particular column or row. The liquid crystal material is sandwiched between the two glass substrates, and a polarizing film is added to the outer side of each substrate. 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.
Advantages: 1. Simple to implement 2. Less costly 3. Simple circuitry Disadvantages: 1. Can cause distortion 2. Loss of contrast in bigger array sizes resulting from crosstalk
Active Matrix Displays Most LCD displays use active matrix technology. Active display have transistor built into each pixel. A thin film transistor (TFT) arranges tiny transistors and capacitors in a matrix on the glass of the display, acts as switch controlling the voltage each pixel receives. Electrode placed above the liquid crystal matrix. 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.
Advantages: 1.Sharp display 2.Better viewing angle 3.40:1 contrast Disadvantages: 1. Need better backlight 2. Complex hardware 3. More costly as compared to passive matrix display.
Comparison of Passive and Active Matrix LCDs Parameter Passive Active Contrast 10-20 100+ Viewing scale Limited Wide Gray scale 16 256 Response time 100-200ms <50ms Multiplex ratio 480 >1000 Size Up to 17 <14 Manufacturability Simple Complex Cost Moderate high
Advantages of LCD Monitors Require less power - Power consumption varies greatly with different technologies. The average is about 45 watts for a 19-inch LCD display. LCDs also produce less heat. Smaller and weigh less - An LCD monitor is significantly thinner and lighter than a CRT monitor, typically weighing less than half as much. In addition, you can mount an LCD on an arm or a wall, which also takes up less desktop space. More adjustable - LCD displays are much more adjustable than CRT displays. Less eye strain - Because LCD displays turn each pixel off individually, they do not produce a flicker like CRT displays do. In addition, LCD displays do a better job of displaying text compared with CRT displays. Flicker free screen. More usable display area than on comparably sized CRT. Low frequency radiation is practically eliminated
Disadvantages of LCD Monitors Fragile screen : may result in both screen and backlight lamp damage if touched or handled. Contrast ratio causes darkness to not to be displayed true. Dark may be viewed as dark gray rather black. Designed for only optimum resolution. Can not adjust images. Best view of screen is straight on, limiting clarity and colors for those viewing from angle.
Analog (VGA) Connection Because most CRT monitors require the signal information in analog (continuous electrical signals or waves) form and not digital (pulses equivalent to the binary digits 0 and 1), they typically use an analog connection. The computer and video adapter convert digital data into analog format. A video adapter is an expansion card or component that provides the ability to convert display information into a signal that is sent to the monitor. It can also be called a graphics adapter, video card or graphics card. Once the display information is in analog form, it is sent to the monitor through a VGA cable. The cable connects at the back of the computer to an analog connector (also known as a D-Sub connector) that has 15 pins in three rows. Signals on VGA interface 1:red out 6:red return(ground) 11:monitor ID 0 in 2:green out 7:Green return(ground) 12:monitor ID 1 in 3:blue out 8:Blue return(ground) 13:horizontal sync out 4:unused 9:Unused 14:vertical sync 5:ground 10:sync return (ground) 15:monitor ID 3 in
Video Accelerated Card The video card is middleman working between the processor and monitor. Video card the job of translating what the processor produces into a form that monitor can display. Characteristics that differentiate graphics accelerators Memory:- own memory which is reserved for storing graphical representations. the amount of memory determines how much resolution and how much many colors can be displayed. Video RAM Bus:-Each graphics accelerator is designed for a particular type of video bus. Register width:- the wider the register the more data the processor can manipulate with each instruction VGA OUT (Monitor) AG902(H/W accelerator card) SH3 DSP (Processor) RS1 (Application ) SD Card (File System) Capture card 4 Input channels
Components of video accelerator card:- Graphical processing unit(gpu) GPU is specialized processor with advanced image processing capability, (3D graphics), based on encoding.because of high temp that the graphics processor can reach, a radiator and fan are mounted on it. The video Bios provide a set of video related functions that are used by programs to access the video hardware. contains the graphics card s setting, in particular graphics modes. Video memory(frame buffer) Store images processed by the GPU before they are displayed by the monitor.
RamDac(Random access memory digital analog convertor) Screen information stored in the video memory (RAM) is digital. Every value is stored as o and 1 with intensity signal. The monitor is analog. In order to display the image on screen The information in the video memory must be converted to analog signals and sent to monitor. Interface a kind of bus used to connect the graphics card to the motherboard AGP bus or PCI express bus PCI Express bus is better than AGP bus in performance.
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