Output Devices

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Output Devices

• Semester 2, Week 2

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Hardware Devices for Output

• Any hardware device that the computer uses to
  present information to the user can be
  categorised as an output device. Kinds of
  information might be sound, data, memory, images
• Obvious examples are the monitor and the printer.

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Monitors / Visual Display Units

• The monitor that shows a user what is going on
  with a computer. Older ones look a bit like a
  television screen modern monitors look flat
  these are Liquid Crystal Display (LCD) or Thin
  Film Transistor (TFT) displays.

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CRT Monitors

• The screens that look like small televisions are
  'CRT' monitors. Cathode Ray Tubes (CRT) consist
  of a vacuum tube enclosed in glass. One end of
  the tube contains an electron gun the other end
  contains a screen with a phosphorous coating.
• When heated, the electron gun emits a stream of
  high-speed electrons that are attracted to the
  other end of the tube.

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CRT Monitors (2)

• The light-streams will hit a point on the
  phosphor coating momentarily and cause the
  phosphorous molecules to vibrate, giving off
  light.
• The phosphor chemical has a quality called
  persistence and the rate of persistence indicates
  how long this glow will remain on-screen.

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CRT Monitors (3)

• In colour monitors the mix of red, green and blue
  light will determine the colour emitted in the
  light of the phosphor glow.
• The electron beam sweeps the screen from left to
  right in lines from top to bottom, in a pattern
  called a raster.

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CRT Monitors (4)

• The horizontal scan rate refers to the speed at
  which the electron beam moves across the screen.
• The electron beam must continue to sweep the
  screen to maintain an image - a practice called
  redrawing or refreshing the screen.
• You should have a good match between persistence
  and scanning frequency so that the image has less
  flicker (if the persistence is too low) and no
  ghosts (if the persistence is too high).

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CRT Monitors (5)

• Most CRT displays have a refresh rate of about 70
  hertz (Hz), refreshed 70 times a second.

A basic depiction of a CRT 'raster'
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CRT Monitors (6)

• The scan rates expected by your monitor should
  match those produced by your video card.
• Most CRT monitors are multiple-frequency
  monitors, with a variety of popular video signal
  standards.

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CRT Monitors (7)
Pixels and raster lines for a Cathode Ray Tube
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LCD Screens

• LCD (liquid-crystal display) displays have
  low-glare flat screens and low power requirements
  than CRTs. (5 watts versus nearly 100 watts for
  an ordinary CRT monitor.)
• There are three basic LCD choices passive-matrix
  monochrome, passive-matrix colour, and
  active-matrix colour.

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LCD Screens (2)

• In an LCD, a polarising filter creates two
  separate light waves. In a colour LCD, there is
  an additional filter that has three cells per
  each pixel - one each for displaying red, green,
  and blue.
• The light wave passes through a liquid-crystal
  cell, with each colour segment having its own
  cell.

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LCD Screens (3)

• The liquid crystals are rod-shaped molecules that
  flow like a liquid. They enable light to pass
  straight through, but an electrical charge alters
  their orientation, changing the orientation of
  the light passing through them.

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LCD Screens (4)

• The diagrams below are simplified 3-D
  cross-sections of a LCD display cell

Light coming out
See them animated at http//plc.cwru.edu/tutorial/
enhanced/files/lcd/tn/tn.HTM
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LCD Screens (5)

• In a passive-matrix LCD, each cell is controlled
  by electrical charges transmitted by transistors
  according to row and column positions on the
  screen's edge.
• In an active-matrix LCD, each cell has its own
  transistor to charge it and twist the light wave.
• This provides a brighter image than
  passive-matrix displays because the cell can
  maintain a constant, rather than momentary,
  charge.

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LCD Screens (6)

• The best colour displays are active-matrix or
  thin-film transistor (TFT) panels, in which each
  pixel is controlled by three transistors (for
  red, green, and blue).
• Active-matrix-screen refreshes and redraws are
  immediate and accurate, with much less ghosting
  and blurring than in passive-matrix LCDs (which
  control pixels via rows and columns of
  transistors along the edges of the screen).

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LCD Screens (7)

• Active-matrix displays are also much brighter and
  can easily be read at an angle.
• An alternative to LCD screens is gas-plasma
  technology, typically known for its black and
  orange screens in some of the older Toshiba
  notebook computers but have developed as clear
  colour monitors since then.

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The Light of an LCD Monitor

• Most computer Liquid Crystal Displays are lit
  with built-in fluorescent tubes above, beside and
  sometimes behind the LCD. A white diffusion panel
  behind the LCD redirects and scatters the light
  evenly to ensure a uniform display. This is known
  as a backlight.

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The Light of an LCD Monitor (2)

• A typical laptop display uses a tiny Cold Cathode
  Fluorescent Lamp (CCFL) for the backlight. One of
  these small tubes is able to provide a bright
  white light source that can be diffused by the
  panel behind the LCD.

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The Light of an LCD Monitor (3)

• The fluorescent light lights up the cells of an
  LCD display. The moving molecules of the cells
  are aligned by electrical current applied to the
  cells' transistors to filter or bend light so
  that colours and brightness are detected by the
  human eye.

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Video Cards

• A video card is a circuit board that fits to the
  motherboard to support the monitor by providing
  signals that operate your monitor. Most video
  cards follow one of several industry standards
• MDA (Monochrome Display Adapter)
• VGA (Video Graphics Array)
• CGA (Colour Graphics Adapter)
• SVGA (Super VGA)
• EGA (Enhanced Graphics Adapter)
• XGA (eXtended Graphics Array)

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VGA

• IBMs PS/2 systems, appeared first around 1987.
• It introduced the Video Graphics Array (VGA)
  display.
• The technology for the very popular VGA had/has
  particular hardware to ensure reliability and
  versatility.

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VGA (2)

• On a motherboard VGA is implemented by a single
  custom VLSI chip. (Very Large Scale Integration.)
  
• The VGA BIOS (Basic Input/Output System) is the
  control software residing in the system ROM for
  controlling VGA circuits. (NOT related to the
  BIOS chip associated with the startup procedure
  of a personal computer.)

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VGA (3)

• This software can initiate commands and functions
  able to display up to 256 colours on screen,
  from a palette of 262,144 (256K) colours.
• Because the VGA outputs an analog signal, you
  must have a monitor that accepts an analog input.

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SVGA

• SVGA offers different resolutions but 1,024 x 768
  resolution is described as a resolution capable
  of detailed work. (If working with graphics.)
• Most microcomputer monitors support at least one
  video standard.

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EGA

• The IBM EGA (Enhanced Graphics Array), appeared
  first around 1984.
• It was a colour monitor displaying 16 colours in
  a range of either 320 x 200 or 640 x 200 pixels.

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Back to VGA, Briefly

• Unlike earlier video standards, which are
  digital, the VGA is an analog system.
• Why are displays going from digital to analog
  when most other electronic systems are going
  digital?
• Why, then, did IBM decide to change the video to
  analog? The answer is colour.
• Digital display generates different colours by
  firing the red, green, blue (RGB) electron beams
  in on-or-off mode.

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Back to VGA, Briefly(2)

• You can display up to eight colours (2 to the
  third power).
• Analog displays, as digital, use RGB (Red, Green,
  Blue) electron beams to construct various
  colours, but each colour in the analog display
  system can be displayed at varying levels of
  intensity - 64 levels, in the case of the VGA.

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Back to VGA, Briefly(3)

• This versatility provides 262,144 possible
  colours.
• For realistic computer graphics colour is often
  more important than high resolution - because the
  human eye perceives a picture that has more
  colours as being more realistic.

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Printers

• A printer is a device that accepts text and
  graphic output from a computer and transfers the
  information to paper, card, plastic or
  photo-sensitive paper.
• Printers vary in size, speed, sophistication, and
  cost. In general, more expensive printers are
  used for higher-resolution colour printing.

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Printers (2)

• Computer printers are usually of two types
• impact printers or
• non-impact printers.

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Impact Printers

• Impact printers typically have a hammer or a
  dot-matrix that strikes paper through a ribbon
  containing ink.
• These two examples are old technology though
  dot-matrix is still often used by many
  organisations for such things as sample data
  output or computer program prints.

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Impact Printers (2)

• Dot-matrix printers were cheap to buy and operate
  for banks and hospitals so that is where you
  often still see them.

An OKI Dot Matrix Printer (_at_1980s(?))
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Non-Impact Printers

• Non-impact printers examples are the laser
  printer and the inkjet printer.
• The laser allows toner (an ink powder) to stick
  to paper as it rolls past a drum.
• The inkjet sprays ink from an ink cartridge at
  very close range to the paper as it rolls by.

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Printing on a Laser Printer

• When a computer sends the data of a print job to
  a laser printer it is routed through a central
  controller, a small computer inside the printer
  which manages the printer.
• The controller may place several jobs at once
  into a queue, then printing them.
• When the controller has determined what is going
  to be printed, the process of preparing the
  printing drum begins.

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Printing on a Laser Printer (2)

• The drum inside a laser printer can hold an
  electric charge. Close to the drum is a transfer
  corona roller or wire which can negatively or
  positively charge the drum. In most laser
  printers the drum starts out positively charged,
  although this process can also work in reverse.
  The controller manipulates a small laser
  reflected from a mirror to write on the drum
  with a negative charge, creating an electrostatic
  image. This charge causes powdery ink (toner) to
  be attacted to selected paper areas as a sheet
  rolls over the drum.

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Printing on a Laser Printer (3)

• The paper that is fed through the printer is
  given an even stronger negative charge by the
  transfer corona wire before being rolled past the
  drum. The electrostatic image on the drum will
  transfer to the paper.
• Then it is fed through a fuser which heats the
  toner and causes it to bind with the fibres in
  the paper.

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Printing on a Laser Printer (4)

• The laser receives the page data - the tiny dots
  that make up the text and images - one horizontal
  line at a time. As the beam moves across the
  drum, the laser emits a pulse of light for every
  dot to be printed, and no pulse for every dot of
  empty space.
• The laser does not actually move the beam itself.
  It bounces the beam off a movable mirror instead.

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Printing on a Laser Printer (5)

• Meanwhile, the drum passes a discharge lamp,
  which will expose the entire surface of the drum
  and erase the electrostatic image.
• The transfer corona wire applies another positive
  charge, and the printer is ready for the next
  page or job.

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Inkjet Printers (Bubblejet)

• With the thermal bubble, or bubblejet, resistors
  create heat, which then creates a bubble in the
  ink. The bubble expands and forces ink out from
  the nozzle. Eventually, it will collapse, drawing
  more ink into the cartridge. On average, a
  bubblejet printer will have a range of three
  hundred to six hundred nozzles.

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Inkjet Printers (Piezoelectric)

• The piezoelectric utilizes small crystals in the
  nozzles which will vibrate under the influence of
  an electric current this in turn pushes ink out
  and draws more ink into the cartridge. The drops
  of ink that come from the piezoelectric type
  printer are significantly smaller than those of
  the bubblejet printers, allowing for greater
  control over the image quality.

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Next

• Storage Devices