Cathode ray tubes use an electron gun and phosphors to display pixels on a screen. The electron beam is focused and deflected horizontally and vertically to scan across the screen. Raster scan displays refresh the screen by scanning across rows of pixels stored in a frame buffer. Random scan displays draw images by moving and drawing the electron beam between points. Color CRTs use three electron guns and a shadow mask to display red, green, and blue pixels. LCDs use liquid crystals that can be aligned to block or transmit light, while plasma displays excite gas-filled capsules that emit UV light to illuminate phosphors.

1. DISPLAY
HARDWARE

2. Cathode Ray Tubes
 Basic operation
 Refresh CRT
 Primary component –
heated metal cathode
control grid
heating filament

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.  Kinds of phosphor based on
 Color
 Persistence : time it takes the
emitted light to decay one-tenth of original
intensity
 Resolution
 Aspect Ratio

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.  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.  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. 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.  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.  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. 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.  Color CRTs have
– Three electron guns (RGB)
– A metal shadow mask to differentiate the
beams

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.  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.  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. 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. 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. 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