DISPLAY DEVICES

CHAPTER 4: DISPLAY DEVICES

UNDERSTANDING DISPLAY TYPES AND SETTINGS

 VIDEO DISPLAY TYPES

 Cathode Ray Tube (CRT)

  •  Electron gun shoots beam of electrons toward the back of monitor screen
  • It is coated with chemical dots called phosphors glow when electrons strike them
  • Beam of electrons scans the monitor from left to right, and top to bottom in a raster pattern to create the image.
  • Trio of dot phosphors is grouped in triangle for each hardware picture element
  • Electron beam returns regular to each phosphor to sustain the glow.
  • More dots better quality
  • Dot pitch
    • Measurement between the same spot in two vertically adjacent dot trios
    • Expressed in millimeters or dots per inch
    • Dot pitch tells “sharpness”
    • Software-pixel placement is limited to hardware’s transistor placement
    • Resolution
      • Number of pixelsused to draw the screen.
      • Higher resolutions = more information in the same screen area.
      • indicate rows and columns of pixels on screen

 Liquid Crystal Display (LCD)

  • Crystals align themselves with the current when passed through a semi-crystalline liquid.
  • Combining transistorswith liquid crystals, pattern formed.
  • Patterns combinedto represent numbers or letters.
  • First used in watches, now in monitor, even TV
  • Available in
    • analog (like VGA) :PCs digital signals are converted to analog by video card
    • digital interface: no analog modulation required, generally sharper
  • Backlight makes easier to view
  • ACTIVE MATRIX:
    • Transistor at each pixel, switches to different level changing alignment and producing 1000s of shades
    • crisp, wide viewing angles
    • pixels hold value thus, no refreshing
    • Large power needed
  • PASSIVE MATRIX:
    • pixels in x-y matrix and transistors on the axis
    • circuits controlling rows fire in series to refresh/newly activate pixels on each row in succession
    • circuits controlling the columns are synchronized to fire when that row’s transistor is active
    • refresh required
    • can be muddy
    • DUAL SCAN:
      • Passive matrix split in half and each half refreshed separately
      • quality improved
    • Slower response leading to submarining
    • Lesser viewing angle
  • Antiglare filters brighten, clarifying displays

  LED Displays

  • LCD displays with LED as light sources
  • Each LED can be intelligently controlled for better quality
  • Work on DC power

 Plasma Displays

  • Plasma: Cloud of ionized particles
  • These produce light when electrons change energy levels
  • Electrodes in front and back of sealed chambers with neon gas and mercury vapors
  • Not suitable in high altitude
  • No backlight, produce deeper black
  • 600Hz is refresh rate

 Organic Light Emitting Diode (OLED)

  • Organic light emitting compound placed b/w anode-cathode emits light when excited by current
  • No backlight, deeper black
  • Thin, light weight, flexible
  • Better contrast ratio
  • Reflective thus filters required
  • Active matrix (AMOLED)
  • Passive matrix (PMOLED)
  • Lesser power than LCDs
  • Super AMOLED use one cell TSP thus thinner touch layer over the display
  • Super AMOLED plus has subpixels for clearer display

 Projection Systems

  • condensed video display units with a lighting system that projectsthe VDU’s image onto a screen
  • interactive whiteboards allow presenters to project an image ontothe board as they use virtual markers to electronically draw on the displayed image
  • focusing mechanism is included on the lens

REAR PROJECTION

  • projector is built into a cabinet behind a screen onto which the image is
  • projected in reverse so that an observer in front of the TV can view the image correctly
  • earlier CRT technology used
  • now 3 LCDs used for separate RGB display
  • Digital light processing optical semiconductors, have roughly as many rotatable mirrors on their surface as pixels in the display resolution


 BRIGHTNESS

  • Measure of lumens (total visible light that the projector gives off)
  • Image to be projected few feet away
  • Focused on a larger area, the lux—derivative of lumens measuring how much the projector lights up the surface—decreases
  • As we train the projector on a larger surface, the same lumens produce fewer lux.
  • Cannot use light bulbs as light is not constant
  • Chamber contains a metal halide gas that glows bright white when the tungsten filament lights up.
  • Soot inside of the projector bulb is avoided by using a chemical, extending its life and reducing changes in light output.But more expensive

 COOLING DOWN

  • Residual heat generated bythe projector bulb can damage the electronics or the bulb itself
  • Occurs if the fan is not allowed to remove enough heat before it stopsrunning
  • Without connection to electrical outlet, fan stops immediately

  DISPLAY SETTINGS

 Refresh Rate

  • Vertical scan frequency and specifies how many timesin one second the scanning beam of electrons redraws the screen in CRTs
  • Measured in screen draws per second, or Hertz,
  • Refresh rate on smaller monitors (14-16 in) 60-72Hz.
  • LCD TV support 60, 120, 240, 480hzrefresh rates
  • Higher rates = more fluid videomotion.
  • For higher refresh rate, compromise bychoosing a lower resolution.

  Resolution

  • Number of horizontal dots by vertical dots that make up the rows and columns
  • Adjusting display settings that are recommendedfor monitor can alleviate scrolling effect
  • Adapters come with utilities for changing settings like refresh rate, resolution

 Multiple Displays

  • Sometimes need touse two monitors on same computer simultaneously
  • We can have same display on all the screens or we can extend one screen on multiple displays
  • Might need to change settings for external device, like resolution or the device’s orientation with respect to the built-in display
  • Microsoft calls its multimonitor feature Dual View

 Degauss

  • Reduction of the magnetic field of an object
  • Degaussing the drive makes previously savedinformation all but unrecoverable
  • CRTs use magnetic fields to guide the electron beams, LCDs do not, thusstrictly CRT-related practice.
  • Monitor can be damaged by degaussing it morethan once in short period
  • Constant bombardment of the metallic shadow results image discolorationand rainbow effects
  • Later-generation CRTs have an internal degaussing coil
  • Activate coil each time unit is turned on.
  • External degaussing devices exist
  • Excessive electromagnetic energy can permanentlydamage CRT

   UNDERSTANDING DISPLAY TYPES AND SETTINGS

VIDEO DISPLAY TYPES

  • difference: highest resolution and maximum number of colors
  • amount of memory of pre-VGA adapters was fixed, thus resolution and number of colors was fixed
  • Newerwith expandable memory
  • 24-bit color palettes, Truecolor made of 17 million colors, number of colors the human eye can distinguish
  • Latest commercial video standards continue to grow in resolution

 MONOCHROME

  • first video technology for PCs (black-and-white for DOS)
  • First adapter Monochrome Display Adapter (MDA) displayed text, no graphics with resolution  720×350
  • Hercules Graphics Card (HGC), resolution of 720×350,could display graphics and text
  • Text Mode: displayingpredawn characters from onboard library
  • Graphics Mode:drawing individual pixels for graphics.
  • Modes switchable
  • These modes of operation used today

 Color Graphics Adapter (CGA)

  • displays 16-color text
  • Displays 320x200graphics with only four colors per mode.
  • 3 fixed and a selectable 4th
  • CGA’s 640x200graphics resolution has 2 colors—black and one from palette-of-16.

 Enhanced Graphics Adapter (EGA)

  • display 16 colors from palette-of-64
  • With CGA resolutions and 640n350 mode.
  • digital data pins on the 9-pin D-subminiatureconnector accounted for 6 of 9 pins

Video Graphics Array (VGA)

  • 256KB of video memory on board
  • display16 colors at 640×480, 640×350, and 320×200 pixels
  • It became base standard for color PC video.
  • Analog technology, graphics adapters output and monitors receive an analog signal
  • palette-of-256 colors chosen from 18-bit palette of 262,114 colors
  • more pins needed in digital for every 2-to-the-power increase in color
  • VGA requires only 3 pins, for RGB
  • Video card must support the monitor

ADVANCED VIDEO RESOLUTIONS AND CONCEPTS

 Resolutions

 Advancements after VGA adapter occurred in memory and firmware of adapter, not the connector or analog functionality.

 Super VGA

  • Introduced to advance VGA
  • Support 16 colors at resolution of 800×600, but expanded to 1024×768 pixels with 256 colors
  • VGA monitorscan advance along with graphics adapter and color palette

 Extended Graphics Array (XGA)

  • Available only as Micro Channel Architecture (MCA) expansionboard
  • XGA supports 256 colors at 1024n768 pixelsor 65,536 colors at 800n600 pixels
  • Optimized for GUIs of theday like Windows or OS/2.
  • It scans every other line on each pass, using the phenomenon knownas “persistence of vision” appears to our eyes as continuous image
  • Refresh rate specifies frequency with which all odd/even rows arescanned
  • Refresh rateof 120Hz would result in refresh rate of 60Hz

 More Recent Video Standards

  • Preceded by the letter W :same vertical but a wider horizontal resolution toaccommodate 16:10 wide-screen formats
  • Letter Q :horizontal and vertical resolutions eachdoubled for 4 times (quadruple)
  • Letter H:4 times each for (16 times) hexadecatuple is used.
  • Therefore, if XGA has a resolution of 1024×768, then QXGA will have a resolutionof 2048×1536.
  • Needbetter technology, more video memory to display even slightly higher resolutions
  • Starting with SXGA, advanced resolutions can be paired with 32-bit graphics,with the 24-bit Truecolor palette and other 8 bitsfor noncolor features

 Nonadjustable Characteristics

  Native Resolution

  • Flat-panel displays have a single fixed resolution:native resolution.
  • Resolution other than native resolution can distort picture like blurred text, elliptical circles,etc
  • LCD computer monitors before use of wide-screen monitors became pervasive.

 Contrast Ratio

  • Ratio of luminance of brightest color to darkest color the screen is capable of producing
  • Different fromcontrast.
  • Contrast is an adjustable setting
  • LCDs have low contrast ratios.
  • A display with low contrastratio won’t show “true black” very well, and the other colors will look washed out
  • Lower contrast ratios mean difficult viewing images fromthe side
  • Ratios for smaller LCD monitors around 500:1
  • Today’s LED backlights controlled in zones or individually per pixel, resulting in high dynamic contrast ratios
  • In darker areas, a high contrast ratiowill be more noticeable.
  • In brighter surroundings, varying contrast ratios do not makea difference

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