Mobile Display Types

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• For a device to be considered mobile
  • its form factor must closely integrate both the system components and peripheral devices for video, sound, and input control

Liquid Crystal Display

• Most mobile devices use a flat-panel screen technology based on a type of liquid crystal display (LCD)
  • liquid crystal is a compound whose properties change with the application of voltage
  • each pixel in a color LCD comprises subpixels with filters to generate the primary red, green, and blue (RGB) colors
  • Each pixel is addressed by a transistor to vary the intensity of each cell, thereby creating the gamut (range of hues, tints, shades and tones) that the display can generate
  • the liquid crystal elements and transistors are placed on a thin film transistor (TFT)
    • such LCD panels are often just referred to as TFTs
    • 3 main types of TFT technology:
      • Twisted nematic (TN)
        • crystals twist or untwist in response to the voltage level
        • earliest type of TFT technology
        • found in budget displays
        • supports faster response times than other TFT technologies
          • reduce ghosting and motion trail artifacts when the input source uses a high frame rate
      • In-plane switching (IPS)
        • uses crystals that rotate rather than twist
        • main benefit
          • is to deliver better color reproduction at a wider range of viewing angles
          • support 178/178 degree horizontal and vertical viewing angles
        • main drawback
          • of early and cheaper IPS screens is slightly worse response times
        • best TFT option for both gaming and graphics/design work
          • similar response times to TN while retaining better color reproduction and viewing angles
      • Vertical alignment (VA)
        • uses crystals that tilt rather than twist or rotate
        • benefit:
          • supports a wide color gamut and the best contrast-ratio performance
            • Contrast ratio is the difference in shade between a pixel set to black and one set to white
            • E.g.,
              • high-end IPS panel might support a 1200:1 contrast ratio
              • a VA panel would be 2000:1 or 3000:1
        • drawback:
          • viewing angles are generally not quite as good as IPS
          • response times are worse than TN, so more prone to motion blur and ghosting

Important

You need to distinguish between refresh rate and response time when evaluating displays:

• Refresh rate is the speed at which the whole image is redrawn
  • The refresh rate should be a multiple of the video source frame rate
  • measured in Hz
• Response time is the time taken for a pixel to change color
  • measured in milliseconds (ms)

LED Backlit Displays

• LCD must be illuminated to produce a clear image
  • In a TFT, the illumination is provided by an array of light-emitting diodes (LED)
  • Most smartphone and tablet screens use edge lighting where the LEDs are arranged at the top or bottom of the screen, and a diffuser makes the light evenly bright across the whole of the screen

Info

• Early types of laptop display used a cold cathode fluorescent (CCFL) bulb as a backlight
• The bulb requires AC power
  • inverter component is used to convert from the DC power supplied by the motherboard to the AC power for the bulb
• This type of panel is no longer in mainstream production, but you might come across older laptop models that use it.

Organic LED Displays

In an organic LED (OLED) display each pixel is generated by a separate LED.

• technically an active-matrix OLED (AMOLED)
• the panel does not require a separate backlight
  • allows much better contrast ratios and allows the display to be thinner, lighter, and consume less power
• can be made from plastic with no requirement for a layer of glass
  • can be curved to different shapes
• drawbacks
  • maximum brightness may be lower than with LCDs
  • more susceptible to burn-in
    • displaying the same static image for many hours causes the LEDs to retain the image persistently