Display Technologies

Computer Graphics generated pictures can be displayed on diversified display medium called the video display devices. The primary output device in a graphics system is a video monitor.

Types of Display Technologies

  • Cathode Ray Tube(CRT)
  • Direct View Storage Tube(DVST)
  • Flat Panel Display
Cathode Ray Tube(CRT)
This is a vacuum glass tube capable of converting computers electrical signals into variable images at high speed. CRT is the technology used in traditional computer monitors and televisions.


In CRT, when cathode is heated it emits electrons which after through the focusing and deflection plates
gets strike on the phosphor coated screen and as the electron strike the screen, they give their energy to phosphor, which causes a few photons to be generated and results in our seeing a spot on the CRT's face.

The CRT is also called the Continual Refresh Tube as one has to draw pictures repeatedly by quickly directing the electron beam over the same point as light emitted by phosphor fades very rapidly.

Components of CRT

i) Heating Filament- It is used to heat the Cathode.

ii)|Cathode- It gets heated from the heating filament and emits out electron, which on passing through focusing and deflection system strike phosphor coated screen.

iii) Control Grid- Control Grid controls the intensity of electron beam by setting various voltage levels as negative charge at control grid repel the electrons. The number of electrons reaching the screen reduces and since the light emitted by the screen depends on the number of electrons striking the screen, so we can control the brightness of screen.

iv)Focusing Anode- Focusing Anode is required to converge the electrons together as a beam, as they are negative charge they will repel each other.

v) Accelerating Anode- Accelerating Anode is positive charged plate and is used to accelerate the speed of electron beam towards the screen.

vi) Deflection Plate - Deflection plate are used to locate the electron beam to a particular point on the screen.
There are two deflection plates: Horizontal and Vertical Deflection plates and both are at 90 degree to each other. The Deflection plates can also be generated with the help of the magnetic field.

vii) Phosphor Coated Screen- It is a glass coated with a positive charged materials made up of Zinc and Cadmium.

Terms related to CRT

i) Persistence- It is the time for brightness i.e. how long the phosphor continues to emit light after the CRT beam is removed.

ii) Resolution- Resolution is defined as the maximum number of points that can be displayed without overlapping on any screen.

iii) Interlacing- Interlacing is a technique of improving the picture quality of a video signal without consuming extra bandwidth.
This technique uses two fields to create a frame. One field contains all the odd lines in the image, the other contains all the even lines of the image.






iv)Aspect Ratio- It is the ratio of number of points required to produce equal length line in horizontal and vertical direction.
                                            Aspect Ratio= Width/Height


Advantages of CRT
  • High resolution & fast response
  • Large number of colors
  • Inexpensive Technology
  • Wide angle, high contrast & brightness
Disadvantage of CRT

  • It is physically large and heavy.
  • High power consumption.
  • Geometrical errors at the boundary edges.
  • It has no memory effect, i.e. flickering at 50-80 Hz

Display Techniques
There are two types of display techniques-
*Raster Scan Display
*Random Scan Display

*RASTER SCAN DISPLAY-
It is the common type of  graphics monitor based on television technology. In a raster scan system, the electron beam is swept across the screen, one row at a time from top to bottom. 
When electron beam moves across each row the beam intensity is turned ON & OFF to create a pattern of illuminated spots. Picture definition is stored in a memory called FRAME BUFFER. Which holds the set of intensity values, which are then retrieved from the frame buffer & pointed on the screen one row at a time as shown in figure.








At the end of each line, the beam must be turned off and redirect to the left hand side of the CRT, this is called HORIZONTAL RETRACE. At the end of each frame, the electron beam return to top left corner of the screen to begin , the next frame is called VERTICAL RETRACE.

Advantages
  • Produce realistic images
  • Also produced different colors
  • And Shadow scenes.
Disadvantages
  • Low resolution
  • Expensive
  • Electron beam directed to whole screen


*Random Scan Display
In Random Scan System, an electron beam is directed to only those parts of the screen where a picture is to be drawn. The picture is drawn one line at a time. So, also called vector display or stroke writing or calligraphic displays. 
After drawing the picture the system cycles back to the first line and design all the lines of the picture 30 to 60 time each second.

Random scan displays are designed for line drawing, applications and cannot display realistic shaded scenes.

Vector display have higher resolution.

Vector display produce smooth line drawing because the CRT beam directly follow the line path.


Advantages:-
  • Produced smooth line drawings
  • High resolution

Disadvantages:-
  • Designed only for line drawing application
  • Can not display realistic image
Difference between Raster & Random Scan Display

1) Raster display has ability to display areas filled with solid color and patterns.
    Random display only draw line & character.

2) Using Interlacing in Raster Scan.
    Don't use Interlacing in Random Scan.

3) In Raster Scan Display, the beam is moved all over the screen, one scan line at a time from top     left to bottom right.
    In Vector Display, the beam is moved between the end points of the graphics primitives.

4) Raster has lower resolution.
    Random has higher resolution.

5) Raster produces jagged lines that are plotted as Discrete point sets.
    Random produces smooth line drawing.

6)In Raster, Editing is difficult.
    In Random, Editing is easy.

7) In Raster, refresh rate is independent of picture complexity.
    In Random, refresh rate depends directly on the picture complexity.

8) In Raster Scan, refreshing rate is 60 to 70 frames per second.
    In Random Scan, Refreshing rate is 30 to 60 lines per second.









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