L2-Video Display Devices-Raster Scan System.pptx

04/17/2025
Video Display Devices
 A Video display device is an output device for presentation of
information in visual way.
 When the input information is supplied as an electrical signal, the
display is called an electronic display.
 Common applications for electronic visual displays are televisions
or computer monitors.
 Different types of Video display devices are Cathode Ray Tube,
Raster Scan displays, Random Scan displays, Color CRT-monitors,
Direct View Storage Tube, Flat-Panel Displays, Light-emitting
Diode(LED), Liquid-crystal Displays(LCDs)
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1. Cathode Ray Tube (CRT)
Figure 1: Cathode Ray Tube (CRT)
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1. Cathode-Ray Tube (CRT)
 Invented by Karl Ferdinand Braun(1897).
 Convert electrical signals to visual signals.
 Fluorescence: It is the light emitted as electrons lose their excess
energy while the Phosphor is being struck by electrons.
 Phosphorescence: It is the light given off by the return of the
relatively more stable excited electrons to their unexcited state,
once the electron beam excitation is removed.
 Persistence: Time from the removal of the excitation to the
moment when Phosphorescence has decayed to 10% of the Initial
Light Output.
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Components of CRT
 Electron Gun: Electron gun consisting of a series of elements,
primarily a heating filament (heater) and a cathode. The electron
gun creates a source of electrons which are focused into a narrow
beam directed at the face of the CRT.
 Control Electrode: It is used to turn the electron beam on and off.
 Focusing System: It is used to create a clear picture by focusing
the electrons into a narrow beam.
 Deflection Yoke: It is used to control the direction of the electron
beam. It creates an electric or magnetic field which will bend the
electron beam as it passes through the area. In a conventional CRT,
the yoke is linked to a sweep or scan generator. The deflection yoke
which is connected to the sweep generator creates a fluctuating
electric or magnetic potential.
 Phosphorus-coated screen: The inside front surface of every CRT is
coated with phosphors. Phosphors glow when a high-energy
electron beam hits them. Phosphorescence is the term used to
characterize the light given off by a phosphor after it has been 4.

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Characteristics of Cathode-Ray Tube (CRT)
 Intensity: It is proportional to the number of electrons repelled in
beam per second (brightness).
 Resolution: It is the maximum number of points that can be
displayed without overlap. It is expressed as number of horizontal
points by number of vertical points. These points are called pixels
(picture elements). Example: resolution 1024 x 768 pixels. Typical
resolution is 1280 x 1024 pixels.
 High-definition systems means high resolution systems.
Figure 2: Resolution
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Brands of CRT
Figure 3: CRT Brands
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2. Random Scan System
 Random scan systems are also called vector, stroke-writing, or
calligraphic displays.
 Random scan system uses an electron beam which operates like a
pencil to create a line image on the CRT. The image is constructed
out of a sequence of straight-line segments.
 Each line segment is drawn on the screen by directing the beam to
move from one point on screen to the next, where each point is
defined by its x and y coordinates.
 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.
 When operated as a random scan display unit, a CRT has the
electron beam directed only to the parts of the screen where a
picture is to be drawn.
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 Refresh rate on a random scan system depends on the number of
lines to be displayed.
 Picture definition is now stored as a set of line drawing commands
in an area of memory referred to as the refresh display file.
 Random scan systems are designed for line drawing applications
and can not display realistic shaded scenes.
 Since picture definition is stored as a set of line drawing
instructions and not as a set of intensity values for all screen points,
vector displays generally have higher resolution than raster
systems.
 The component lines of a picture can be drawn and refreshed.
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 Each line segment is drawn on the screen by directing the beam to
move from one point on the screen to the next, where its x & y
coordinates define each point.
 After drawing the picture. The system cycles back to the first line
and design all the lines of the image 30 to 60 time each second. The
process is shown in Fig 4:
Figure 4: Random Scan System 9.

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 The architecture of Random Scan System is shown in Figure 5.
Figure 5: Random Scan System
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3. Raster Scan System
 In raster scan approach, the viewing screen is divided into a large
number of discrete phosphor picture elements, called pixels/dots.
 Pixel: One dot or picture element of the Raster.
 Scan Line: A row of pixels
Figure 6: Example of Pixel
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 The electron beam is swept across the screen, one row at a time
from top to bottom. As the electron beam moves across each row,
the beam intensity is turned on and off to create a pattern of
illuminated spots.
 Each pixel on the screen can be made to glow with a different
brightness.
 Color screen provide for the pixels to have different colors as well
as brightness.
 Picture Definition is stored in a memory area called the Refresh
Buffer or Frame Buffer.
 This memory area holds the set of intensity values for all the screen
points.
 Stored intensity values are then retrieved from there fresh buffer
and "painted“ on the screen one row (scan line) at a time as shown
in Figure 7.
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Figure 7: Image of Raster Scan System
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Figure 8: Raster Scan System’s Architecture
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 In addition to the Central Processing Unit (CPU), a special
processor, called the Video Controller or Display Controller, is used
to control the operation of the Display Device.
 A fixed area of the system memory is reserved for the frame buffer,
and the video controller is given direct access to the frame buffer
memory.
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Video Controller of Raster Scan System
 Operations performed by Video Controller are given as follows:
 It can retrieve pixel intensities from different memory areas on
different cycles
 In high quality systems, two frame buffers are often provided so
that one buffer can be used for refreshing and other filled with
intensity values. Provide the fast mechanism for generating real
time animation.
 Video controllers often contain a lookup table, so that pixel value
in the frame buffer are used to access the lookup table instead of
controlling the CRT beam intensity directly.
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Video Controller of Raster Scan System
Figure 5: Video Controller’s Operation
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Raster vs Random Scan System
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Difference Raster Scan System Random Scan System
Resolution It has poor or less Resolution
because picture definition is
stored as a intensity value.
It has High Resolution
because it stores picture
definition as a set of line
commands.
Electron
Beam
It is directed from top to bottom
and one row at a time on
screen, but electron beam is
directed to whole screen.
It is directed to only that part
of screen where picture is
required to be drawn, one
line at a time so also called
Vector Display.
Cost It is less expensive It is Costlier than Raster
Scan System.
Refresh Rate Refresh rate is 60 to 80 frame
per second.
Refresh Rate depends on the
number of
lines to be displayed i.e 30 to
60/sec

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Raster vs Random Scan System
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Difference Raster Scan System Random Scan System
Picture
Definition
It stores picture definition in
Refresh Buffer also called
Frame Buffer.
It stores picture definition as
a set of line commands
called Refresh Display File.
Line Drawing Zig–Zag line is produced
because plotted value are
discrete.
Smooth line is produced
because directly the line path
is followed by electron beam
Realism in
Display
It contains shadow, advance
shading and hidden surface
technique so gives the realistic
display of scenes.
It does not contain shadow
and hidden surface technique
so it can not give realistic
display of scenes.
Image
Drawing
It uses Pixels along scan lines
for drawing an image.
It is designed for line
drawing applications and
uses various mathematical
function to draw.

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Numerical on Raster Scan
Question 1
 Consider three different raster systems with resolutions of 640
by 480, 1280 by 1024, and 2560 by 2048. What size is frame
buffer (in bytes) for each of these systems to store 12 bits per
pixel?
Answer 1
 Because eight bits constitute a byte, frame-buffer sizes of the
systems are as follows:
640 * 480 * 12 bits / 8 = 450KB;
1280 * 1024 * 12 bits / 8 = 1920KB;
2560 * 2048 * 12 bits / 8 = 7680KB;
Question 2
 Find out the aspect ratio of the raster system using 8 x 10
inches screen and 100 pixel/inch.
Answer 2
 We know that Aspect ratio = Width/Height
= (8 * 100) / (10*100) = 4 / 5 20.

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Question 3
 How Many k bytes does a frame buffer needs in a 600 by 400
pixel?
Answer 3
 Resolution is 600 x 400
 Suppose 1 pixel can store n bits
 Then, the size of frame buffer = Resolution * bits per pixel
= (600 * 400) * n bits
= 240000*n bits
= 240000*n) /(1024 * 8)
(as 1kb = 1024 bits)
= (29.30*n) KB
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Question 4
 How much time is spent scanning across each row of pixels
during screen refresh on a raster system with resolution of 1280
by 1024 and a refresh rate of 60 frames per second?
Answer 4
 Here, resolution = 1280 X 1024
 That means system contains 1024 scan lines and each scan line
contains 1280 pixels
 refresh rate = 60 frame/sec.
 So, 1 frame takes = 1/60 sec.
 Since resolution = 1280 X 1024
 1 frame buffer consist of 1024 scan lines
 It means then 1024 scan lines takes 1/60 sec
 Therefore, time taken by 1 scan line = 1/(60*1024) = 0.058 seconds
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Question 5
 Suppose RGB raster system is to be designed using on 8 inch by
10 inch screen with a resolution of 100 pixels per inch in each
direction. If we want to store 6 bits per pixel in the frame
buffer, how much storage (in bytes) do we need for frame
buffer?
Answer 5
 Here, resolution = 8 inch by 10 inch = 8 * 100 by 10 * 100 pixel
= 800 by 1000 pixel
 1 pixel can store 6 bits
 So, frame buffer size required = 800 * 100 * 6 bits
= (800 * 100 * 6)/8 B
= 6 x 105
B
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Questions
 Define Raster scan displays?
 Write the properties of Video Display Devices?
 Explain CRT in detail.
 Illustrate the basic Refresh Operation of the Video Controller in
Raster Scan display?
 What does it mean by RGB?
 Define VDU?
 Define persistence in terms of CRT Phosphorous.
 What do you mean by an aspect ratio?
 Differentiate raster and random scan displays.
 Define refresh buffer/Frame buffer?
 Define Pixel.
 Define bitmap
 What do you mean by retracing? Define horizontal as well as
vertical retracing
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References
 D. Hearn, P. Baker, "Computer Graphics - C Version", 2nd Edition,
Pearson Education, 1997
 Heam Donald, Pauline Baker M: "Computer Graphics", PHI 2nd
Edn. 1995.
 Harrington S: "Computer Graphics - A Programming Approach",
2nd Edn. Mc GrawHill.
 Shalini Govil-Pai, Principles of Computer Graphics, Springer, 2004
 Coursera - https://www.coursera.org/learn/fundamentals-of-
graphic-design
 https://www.youtube.com/watch?
v=fwzYuhduME4&list=PLE4D97E3B8DB8A590
 NPTEL - https://nptel.ac.in/courses/106/106/106106090/
 https://www.coursera.org/learn/research-methods
 https://www.coursera.org/browse/physical-science-and-
engineering/research-methods
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L2-Video Display Devices-Raster Scan System.pptx

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