RASTER & RANDOM SCAN DISPLAYS

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

• 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

RASTER SCAN DISPLAY

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

Lines produced are zigzag as the plotted values are discrete.

Resolution is low.

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

• Refresh Buffer: Picture Definition –Stored –

Set of line drawing –Memory area

• Draw – Line in picture – 30-60 times a

second

RANDOM SCAN DISPLAY

Advantages of random scan:

Very high resolution, limited only by monitor.

Easy animation, just draw at different position.

Requires little memory.

Requires “intelligent electron beam, i.e., processor controlled.

Limited screen density before have flicker, can’t draw a complex image.

Limited color capability

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

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

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)

Cathode Ray Tube

Electron gun

CATHODE RAY TUBE

Time duration –phosphoresce emit light – after turn off

Properties of CRT Electron Beam Strikes - Glows

Ability of Phosphoresce glow after beam is shut down

Ratio – No. of vertical pixels – horizontal pixels - plotted

Maximum No. of Pixels - plotted

Phosphoresce

Fluorescence

Persistence

Aspect Ratio

Resolution

Refresh Rate Frames – plotted again – seconds time

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

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

THANK YOU