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