Display Hardware

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Display Hardware

  1. 1. DISPLAY HARDWARE
  2. 2. Cathode Ray Tubes  Basic operation  Refresh CRT  Primary component – heated metal cathode control grid heating filament
  3. 3.  Intensity control – voltage on control grid  Focusing System force the electron beam small spot of light  Otherwise, electrons repel and spread Accomplished with electric or magnetic fields  Deflection Plates  horizontal and vertical  Spots on screen by transfer of CRT beam energy to phosphor
  4. 4.  Kinds of phosphor based on  Color  Persistence : time it takes the emitted light to decay one-tenth of original intensity  Resolution  Aspect Ratio
  5. 5. Raster Scan Displays  Image created as Grid of Pixels – “Picture Elements” – Scan Conversion  Picture definition is stored in a frame buffer memory or refresh buffer • one storage cell per pixel
  6. 6.  Example: assume a display of 16 x 16 pixels, each of which is either “on” or “off”  The electron beam traces a path across the first row, then returns to starting side and moves across the second row, and so on  The beam is turned on or off as it passes over a pixel that should be on or off  Here, the beam is OFF for the entire first row, OFF during the retrace, and then ON for the two pixels that are ON
  7. 7.  Black and White (Bilevel) systems  Bit Value 0: Beam turned ON  Bit Value 1: Beam turned OFF  Frame buffer : Bitmap  For Color and intensity variations : multiple bits per pixel  Frame buffer : Pixmap  Refreshing : 60 – 80 frames per second  Horizontal and Vertical Retrace  Interlacing : with slower refreshing rates
  8. 8. Random Scan Display  Also called as Vector or Stroke or Calligraphic Displays  Draw a picture one line at a time – each image was expressed in the form of a series of {move-to, draw-to} commands – the beam could be moved to a specific x,y location while turned off, then turned-on, and a draw-to command would move the beam (while on) to a second x,y location – the result, a line drawn from the first location to the second
  9. 9.  Example: – move to (a, b) (c, d) – draw to (c, d) – draw to (e, f) (a, b) – draw to (g, h) (e, f) – draw to (a, b) (g, h) – move to (i, j) – draw to (k,l) (i, j) (k, l) Note: Arrows just indicate the flow  Refresh Display file contains the Line drawing commands
  10. 10.  For Line Drawing applications  Higher resolution  Problems with random-scan – as complexity of display grows, time to draw increases – if time between refreshes grows too long, display begins to flicker
  11. 11. Color CRT  Color CRTs are much more complicated – Phosphors on the face are laid out in a precise geometric pattern – Uses a pattern of color phosphors on the screen: Delta electron gun arrangement In-line electron gun arrangement
  12. 12.  Color CRTs have – Three electron guns (RGB) – A metal shadow mask to differentiate the beams
  13. 13.  General case: – Simple color display has N x M pixels and each pixel is one of 8 = 23 colors – color depth = k bits – each pixel can be one of 2k colors – if k = 3b, then there are 2b intensity levels for Red, Green, and Blue – if k ≠ 3b, then typically a color indexing scheme is used • the value stored in the frame buffer is not used to directly control the drive intensity • frame buffer value is an index into a color palette
  14. 14.  Direct drive case – frame buffer has 3b bits/pixel (example b = 6) 3b bits / pixel Red Green Blue pixel address 000000 000111 000000 0 000100 000100 000100 1 111111 111111 111111 2 … … … … NM pixels … … … … … … … … … … … … directly controls red, green, and blue intensity
  15. 15.  Indexed color palette case – frame buffer has k bits/pixel (example k = 8) k bits/pixel Frame Buffer pixel address 00000000 0 3b bits / row 00000010 1 Color Palette 11111111 2 Red Green Blue … … 000000 000111 000000 000100 000100 000100 … … 111111 111111 111111 … … … … … 2k … … … … … rows … … … … … …
  16. 16. Display Technology: LCDs  Liquid Crystal Displays (LCDs) – Used in Calculators, laptop etc – Light is passed through the liquid crystal that is aligned to either block or transmit light
  17. 17. Display Technology: LCDs  Liquid Crystal Displays (LCDs) – Used in Calculators, laptop etc – Light is passed through the liquid crystal that is aligned to either block or transmit light
  18. 18. Display Technology: Plasma  Plasma display panels – Similar in principle to fluorescent light tubes – Small gas-filled capsules are excited by electric field, emits UV light – UV excites phosphor – Phosphor relaxes, emits some other color

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