Hardware Devices Output

Hardware Devices Output

1. Visual Display Unit

   • Plasma Screen
     • each pixel is controlled by a fluorescent light
       • gas is heated to plasma by a control current
         • plasma emmits ultraviolet light
           • UV causes phosphors to fluoresce
     • large viewing angle, colours similar to a CRT
     • Poor contrast as constant low current must be applied
       • poor pixel response because of lack of priming
         • ..so pixels in off state still emmit some light
     • Hot enough to injure
     • Pixel size limited to 0.81mm, poor quality in modern age
   • Flat Screen
     • Liquid Crystal Display (LCD)
       • Matrix of Liquid crystals in cells, Light Source, electric current,
     • polarisation of light is affected by current
   • Cathode-ray Tube
     • vacuum tube, cathode ray gun, electromagnets and phorphosecent sceen
     • beam of electrons is dragged across the screen in hoizontals from top to bottom
     • retinal image of phosphor dot is held for 0.05s so whole screen must be swept faster than 0.05 to avoid flicker
     • Colour CRT
       • red gun, Blue gun, Green gun
       • phosphor dots arranged in triplets
         • the eye mixes the three primary colours into a single colour
   • Mositor to show data, results and processing

2. Speech Output

   • speech synthesis software, sound card, speakers
   • text is read out by a computer

3. Electronic Paper

   • flexible paper thin displays
   • an electrically charged pencil alters the position of coloured beads
   • can erased and written to by machine

4. Impact Printer

   • used when printing must be done through carbonised paper to leave a mark or physically print into the surface e.g. credit cards
   • dot matrix
     • 24 pins determine shape of character and are stamped in to surface
     • Loud

5. Non-Impact Printer

   • Laser Printer
     • prints whole pages at a time
     • high quality
     • page description reads lines, arcs and polygons
     • Bitmap of page stored in raster memory
     • negative charge is applied to the whole photosensitive drum
       • light/laser beams neutralise/reverse charge on the drum where black ink is to be printed
         • negatively charged toner particles are sprayed onto drum
           • high voltage applied forces toner onto drum
             • the drum rolled onto paper to transfer the image
               • Hot rollers fuse toner and paper
   • Bitmap and TrueType Fonts
     • fonts of characters can be as bitmaps or vectors
     • bitmap characters suffer pixelation and distortion when scaled
     • TrueType/vector fonts do not suffer distortion when scaled
       • TrueType fonts are independent os screen/printer resolution

6. Plotter

   • moves a pen across paper
   • holders contain multiple pens for use
   • special drawing paper prevents wet ink smudging
   • flatbed plotter
     • paper placed flat on platen
   • drum plotter
     • rotating drum with paper skin
     • roll of paper with perforated top/bootom edge grips to spokes on drum
     • to draw a circle the paper must be wound back and forth