random and raster scan display

1. RASTER & RANDOM SCAN
DISPLAYS

2. Raster Scan Display
• Electron Beam – Swept Across Screen –Top to Bottom –One
at Time
• Beam moves – Each Row – Intensity –Turned On/Off Create Pixels
• Refresh/Frame Buffer: Information of picture stored –
Memory Area
• Pixels value retrieve – memory(buffer) – Plotted - Screen

3. • Horizontal Retrace: Beam Return - left screen
– after refreshing each scan line.
• Vertical Retrace: End of Frame - Beam Return
– top left screen – begin next frame.
• Example: TV Sets

4. RASTER SCAN DISPLAY

5. Advantages of raster scan:
 Decrease memory costs
 High degree realism is achieved in picture advanced shading & hidden surface technique.
 Computer monitors and TVs use this method

6.  Lines produced are zigzag as the plotted values are
discrete.
 Resolution is low.

7. Random Scan Display
• Electron Beam – Directly – Part of the screen
• Vector Display/Stroke Writing/Calligraphic Display:
Picture- Drawn – one at Time
• Example: Pen Plotter
• Refresh Rate –Depends - no. of line to be Displayed

8. • Refresh Buffer: Picture Definition –Stored –
Set of line drawing –Memory area
• Draw – Line in picture – 30-60 times a
second

9. RANDOM SCAN DISPLAY

10. Advantages of random scan:
 Very high resolution, limited only by
monitor.
 Easy animation, just draw at different
position.
 Requires little memory.

11.  Requires “intelligent electron beam, i.e., processor controlled.
 Limited screen density before have flicker, can’t draw a
complex image.
 Limited color capability

12. DIFFERENCE B/W RASTER & RANDOM SCAN
SYSTEM
Random Scan System
Raster Scan System
Cannot draw realistic shaded scenes.
Used in Systems to Display Realistic
Images.
Store - Line drawing instruction
Store – Value related to pixels (Intensity
value)
Higher Resolution
No Support – High Resolution
Smooth line drawings
Capable of Producing Curves Better

13. DIFFERENCE B/W RASTER & RANDOM SCAN
SYSTEM
Random Scan System
Raster Scan System
Cannot Produce Contrast , memory
doesnt store intensity value of pixel
Picture with better contrast
Ex. Pen plotter
Ex. TV sets
Costlier
Cost not too high as compared to
Random Scan
Line Drawing ,Known as Vector Display
Mainly used for Point Plotting

14. Cathode ray tube
• Cathode Ray Tube (CRT) – Vacuum Tube – Contains
 Electron gun (a source of electrons)
 Filament (copper grid)
 Cathode (metal)
 Control Grid (Positively charged)
 Focusing Structure
 Accelerating Anode
 Deflection Coils
 Screen (inside layer covered with phosphor)

16. Cathode Ray Tube

17. Electron gun

18. CATHODE RAY TUBE

19. Properties of CRT
Phosphoresce
Electron Beam Strikes - Glows
Fluorescence
Ability of Phosphoresce glow after beam is shut down
Persistence
Time duration –phosphoresce emit light – after turn off
Aspect Ratio
Resolution
Refresh Rate
Ratio – No. of vertical pixels – horizontal pixels - plotted
Maximum No. of Pixels - plotted
Frames – plotted again – seconds time

20. BEAM PENETRATION
• Used in Random Scan Displays
• Colors Depends on the speed of the beam
• 2 phosphoresce layers : Red & Green
• Not Expensive
• Not Good Quality Picture
• If Speed of Beam High Then Red Color is
visible on screen

22. SHADOW MASK
• Used - raster-scan systems (Color
TV)
• Produce - wider range of color than
beam penetration method
• 3 phosphor color dots at each pixel
position.
• Emits Red ,Green, Blue light
• Expensive
• No light Speed Criteria

24. THANK YOU