Hardware for creating and displaying multimedia

Screens (CRT displays, LCD displays, plasma displays, touchscreens)

CRT (Cathode Ray Tube) Displays → Works by firing a stream of electrons on the inside of a screen coated with a RGB phosphor. Each phosphor will glow red, green or blue when hit

Screen → Display surface which provides feedback on what computer is doing. Displays text, images, videos/animations

• Monochrome CRT displays use one beam (either black or white) while colour CRT displays use multiple.

• Refreshing → process of maintaining an image by firing the electron beam at each individual pixel in a zigzag pattern. (Left to right, top to bottom)

  • This involves redrawing the screen many times a second.
  • The refresh rate of a VDU (video display unit) such as a monitor is represented in frequency
    • E.g. 144Hz (144 hertz/times a second).

• CRT was used before plasma and LCD screens came around. Not used anymore

  

  Pros	Cons
  Relatively cheap (compared to LCD)	Large size/weight
  Good colour quality	High power consumption
  Can refresh screen quickly. (High hertz)	Emits radiation (X-rays). Long exposure to a display can cause brain tumors, etc
  Support multiple resolutions	Not as sharp of an image as LCD
  Viewable from any angle	Will have annoying flickering line due to refreshing

LCD (Liquid Crystal Display) → 2 layers of liquid crystal material between 2 polarising sheets. Current is applied at particular points to block/allow light to pass (different colours)

• LCD can be active matrix or passive matrix

  • Passive matrix → Uses simple grid of horizontal/vertical wires arranged in checkerboard pattern. Intersection of wires is a pixel. Voltages can be applied at these intersections
  • Active matrix → Uses transistors to control every pixel. Transistors act as switch, allowing for pixel to be on or off. Faster and more precise control (better quality).

• Used in laptops, computers, phones, desktop PCs

  

  Pros	Cons
  Smaller size	More expensive
  Light weight	Fragile
  Lower power consumption	Only one resolution
  No glare	Slower pixel response time resulting in an effect called “ghosting” Shown below
  No radiation	Only can be viewed from a straight angle
  Produces no heat	Not as much colour detail as CRT

  

Plasma Displays → Hundreds and thousands of cells receive constant low pressure of xenon/neon gas

• one pixel is made up of 3 cells (1 red, 1 blue, 1 green)

• When a cell is supplied with an electric current, gas is changed to plasma emitting invisible UV rays

  • UV rays hit phosphor coating on cell, emitting light

• Used in television displays

  

  Pros	Cons
  High quality, better contrast and colour compared to LCD and CRT	Expensive
  Good motion (no motion blur)	Generates excessive heat
  Large screen size availability	Relatively short lifespan
  Very bright	Uses more electricity than LCD
  Thin, lightweight	Heavier than LCD
  Free from Electromagnetic radiation	-
  Wider viewing angle than LCD	-

Touchscreens → Screen allowing data entry via detection of user’s finger/stylus

• There are resistive screens, and capacitive screens

  • Resistive screens → simplest form of touch screen

    • 2 layers of electrically conductive material. Layers separated by insulating dots. When person touches screen, 2 layers touch completing circuit. Position sent through complete circuit, and software registers click

      

  • Capacitive screens → Layer of material that creates small electric field

    • Human body conducts electricity, so when human body comes into contact with screen, charge is altered. Circuit detects it, and pinpoints it