CRT Display Devices Video Lecture - Computer Science Engineering (CSE)

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welcome back everybody to the video
lecture on computer graphics today we
are going to discuss about CRT display
devices or cathode ray tube based
display devices in the lecture on
introduction to computer graphics of
course we have seen various types of i/o
devices which are used in the field of
computer graphics and a few of them are
used for output as well as a few are
used for input the output devices are
the monitors let us say and input
devices at the keyboard or Mouse let us
do look at a few examples of computer
graphics devices especially the input
and output devices the CRTs are the
cathode ray tubes as they are called
then we have the EGA CGA vga or s VGA
monitors enhanced graphics adapter
stands for EGA
CGA we have the colored graphics adapter
we have the video graphics adapter vga
and super video graphics adapter SVGA
plotters various types data matrix for
displays laser printers films flat panel
devices video digitizers scanners LCD
panels keyboard joystick mouse touch
screen trackball etc so these are
various examples of computer graphics
devices out of these we are essentially
going to discuss in the lecture is going
to follow from here on is the most
commonly used display device called the
CRT monitors of course there are other
types of better display devices which
are coming out based on solid-state
technology it could be the flat panel
devices of the plasma devices the
organic LEDs and other devices but most
commonly in the world and specifically
in our country most of the display
devices are based on CRT monitors
so what are the different types of CRT
display devices which have been there or
available okay the first one of them the
most simplest one is the direct view
storage tube or dvst okay the second one
is the calligraphic or random scan
display system a third one is the
refresh and raster scan display system
all these three devices we will discuss
in details but the common part of these
display devices are is the concept of
the cathode ray tube or CRT we will
first look at the very basic and a
simple one which is the dvst or direct
view storage tube the storage tube it is
a CRT with a long persistence phosphorus
or phosphor we will see what this means
it provides flicker free display there
is no flicker in the display and it does
not need to refresh the contents of the
screen so these are the three
fundamental properties of direct view
storage tube or dvst it essentially the
dvst consists of a CRT where we have a
slow-moving electron beam which draws a
line on the screen we will move to the
picture of the diagram of a CIT and see
how this electron beam it is generated
and how it is guided on to a particular
point on a screen but this electron beam
which is essentially the basic component
of the CRT which is responsible for
drawing a line on this screen so how it
is controlled we will see later on the
screen for a DV iced tea or dvst has a
storage mesh in which the phosphor is
embedded image is stored as a
distribution of charges on the inside
surface of the screen and
the dvst has very limited interactive
support well this is what is a typical
diagram which illustrates the operation
of a CRT or a CRT based monitor okay it
is an operation which shows how an
electron gun with an accelerating anode
is used to generate an electron beam
which is used to display points or
pictures in this screen on the left hand
side of the CRT you have a heating
filament which is responsible to heat up
the cathode element of the CRT and that
is what generates the electrons the
electrons I could say after the heating
element is heating this cathode the
electrons simply boil off simply boil
off from the cathode and it is guided by
a set of devices which are essentially
all cylindrical in nature and it helps
to guide this electronic beam path
towards this screen we have three
functions here a control grid a focusing
anode and an accelerating anode these
are three essentially cylindrical
devices which are stuck inside the
cylindrical CRT device and three of
these have three independent tasks what
does the control grid do well when you
observe a picture on a screen some parts
of the picture may be bright or some
pictures may be dark this brightness or
darkness or the illumination or
intensity on the screen is basically
controlled by the intensity of the beam
which strikes a particular point on this
screen this intensity of this screen is
controlled by controlling the intensity
of the electron beam which is coming out
of the cathode if you look at this
picture once again the electron beam is
coming out of the cathode and the
intensity of that be
is controlled by the control grid now
this control grid typically is November
the electron beam country's consists of
electrons which are negatively charged
the control grid is a cylindrical device
which also is negatively charged it is a
high negative voltage which is applied
to the control grid now if the if the
electrons are also negatively charged
and the control grid is also negatively
charged both of them repel each other so
the amount of the voltage of the control
grain will essentially allow a certain
amount of electrons of the electron beam
to pass through it okay and the amount
of electron beam or the amount of
electrons which will pass through that
cylindrical control grain will be
controlled by the negative voltage in
the control grid so if you reduce the
amount of voltage in the control grid or
that means the negative voltage is
reduced you are allowing more electrons
to pass through it and the intensity of
the beam will be higher and the amount
of intensity on the screen also will be
higher whereas if you increase the
velocity of the control grain increase
the voltage of the control grain you are
allowing less amount of electrons to
pass through the intensity of the
electron beam which is passing through
the control grid will now be less and
you will have less brightness on this
screen so that is the function of the
control grid which has negative voltage
the other two parts of the CRT are the
focusing anode and the accelerating
anode well the structure and the design
of the CRT varies from one type of
device to another sometimes you have the
but essentially you typically have
cylindrical structures of the focusing
anode or the accelerating anode and both
of these devices Americal devices the
focusing anode and the accelerating
anode are actually unlike the control
grid these are positively charged they
will have positive voltage the control
grid are a negative voltage because it
had to repel or stop a few electrons
from going through if necessary
depending upon the intensity of the beam
which is required to strike on this
screen the focusing anode and the
accelerating anode have to
different tasks but essentially they
have what I will call as positive
voltage because essentially the focusing
anode is responsible to the focus the
beam on to a particular point on the
screen it is similar to a lens focusing
a beam of light on a particular point on
the screen instead of focusing light we
are actually focusing the electrons onto
a particular spot or point on the on the
screen okay so that is the job of the
focusing anode again it is we can say it
is an electrostatic lens that means it
is focusing the electron beams unlike an
optical lens which focuses focuses the
beams on the N and a light beam onto a
surface in this case an electron beam is
focused on to this screen and
accelerating anode is necessary because
we want the electrons to strike the
screen at a very high speed to emit
light so the acceleration of the beam is
provided to also by the ellipse
accelerating anode which also as a
positive voltage it allows the electrons
to accelerate and then leaves it towards
the screen so till now we have seen that
the electron beam path in this picture
is going straight after it is passed
through these three stages the first
stage controls the intensity the second
stage controls the focusing the third
stage gives it the speed or the
acceleration now this will go and strike
the center of the screen this will go
and strike the center of the screen as
it is going straight what happens or
what how will you implement the
deflections of the beam because this
screen is basically a square matrix so
you need not only the beam to go and hit
the center of the screen to create a
point but you also need this beam to
move all around this image if you want
to create a picture okay so you need
horizontal deflection as well as
vertical deflections of this electron
beam so that you can cover the entire
screen all the points are entire
phosphor coating of this screen should
be covered okay so you need to deflect
this reflection of the beam is
implemented by another stage which we
will see in the next like basic design
of a magnetic deflection CRT this is a
diagram where we only show that there
exists an electron gun with a focusing
system at the base of the CRT it does
not of course show the other components
like the accelerating and the control
systems which is part of what we will
commonly term as a part of the electron
gun at the base of the CRT in fact the
electron gun terminologies typically
used which consists of the stages of
accelerating anodes and also the stages
of the control grid to control the
intensity but after the beam is focused
it comes out almost in a straight path
and it has to be reflected so if you
look at the diagram this beam is
deflected by magnetic deflection coils
which actually are on the top bottom and
on the two opposite sides also of the
CRT and this magnetic deflection coil
generates a magnetic field which helps
to deviate or divert this magnetic or
this electron beam onto any portion on
this screen okay on the phosphor coated
screen so this is one mechanism by which
the deflection could be obtained for an
electron beam we move on to the next
picture which shows that this deflection
is also possible with the help of
electrostatic fields the previous one
was a electromagnetic field generated by
magnetic coils in this case
electrostatic fields generated by
capacitance deflection plates we are
talking of electrostatic deflection of
the CRT electrostatic deflection of the
electron beam in a CRT and we have the
same base electron gun focusing system
and then the beam comes out straight and
passes through two plates you can assume
these to be capacitive plates generating
an electrostatic field and the first
stage as seen in the picture we have the
horizontal deflection
plates the horizontal deflection plates
which is responsible for horizontal
deflection of the electron beam and then
it passes through the vertical
deflection plates which is responsible
for vertical deflection of the electron
beam if you look now the horizontal
deflection plates essentially are
vertical plates through which the
electron beam is passing assume these to
be horizontal deflection plates which
are essentially vertical the aligned or
positioned and the electron the
electrostatic field helps the beam to
rotate or deviate or divert in
horizontal direction the vertical
deflection plates are physically
horizontal but it generates an
electrostatic field which is vertically
oriented and that is the reason why this
is responsible for vertical deflection
of the electron beam so these two
together helps you to position or divert
the beam at any point on the phosphor
coated screen on the right hand side
which you can see so essentially there
are two types of deflection mechanisms
one is based on the capacitive
electrostatic mechanism another by
electromagnetic using magnetic coils so
do two types of deflection mechanisms
are possible in a CRT and these are
diagrams which illustrate what is
essentially in it the CRTs which are
used in an oscilloscope for electronic
experiments typically we will have the
deflection plates whereas the TV
monitors typically will use magnetic
coils for deflection you can see that
almost for yourself of course you have
to open it and see but that is typically
the standard which is used okay coming
back to dvst our direct view storage
tube in which the CRT is a very
essential part of course almost all of
these display devices will have some
sort of mechanisms of electron gun with
deflection plates or magnetic coils to
deflect the plates of the CRT is usually
a common part of all the three different
type of storage three different types of
display devices we are discussing about
direct view storage tubes we have seen a
few of the essential part
but dvst or direct view storage tubes
have very limited applications that is
because they have various serious
serious drawbacks and the first drawback
is that modifying any part of the image
it requires redrawing the entire
modifying image if you want to modify
any part you have to modify the entire
image so the animation is almost ruled
out when you are using direct view
storage tubes that is the first drawback
the number two changing in the image
requires to generate a new charge
distribution in the dvst that means we
are saying that we have a phosphor
coated screen on the front and the beam
is allowed to go and strike at any point
the deflection is attended is attained
by the reflection coils or electrostatic
fields and whenever the phosphor is
struck by the electron beam it emits
light that is the mechanism by which you
draw the picture or the picture is
visible okay and now we need to modify
some part this beam has to go and do
that on the entire screen and that is
why the change in the image requires you
to generate a new charge distribution in
the phosphor coated screen in the case
of a T vs T and the process usually very
slow that is my next point it is a slow
process of drawing it takes typically a
few seconds to draw a very complex
picture dvst requires I repeat again a
few seconds to draw quite a good amount
of complex picture you can erase a
picture erasing takes about 0.5 seconds
and all lines and characters must be
raised in the screen remember characters
also drawn by drawing short strokes or
short lines in the T vs T and you can
say virtually no animation is possible
when you are using a T vs T mechanism
it's it's something like highly static
picture which is visible you can erase
and redraw it again something like what
you see in a cathode ray oscilloscope
SCA CRO for electronic experiments
you almost get a static picture of
course with variations but it is drawn
it is redrawn okay we move to the next
type of device which is probably closer
to the TV monitors which we use today
not exactly same they are called the
calligraphic or random scan display
system
I repeat calligraphic or random scan
display system and we see how this is
different from the dvst it is much
improved it is also called a vector
vectored stroke or line drawing displays
characters for the random scan we will
henceforth call this is a random scan
display or a line drawing display
calligraphic term is also used but we
will call it as a random scan characters
are also made up of sequence of strokes
or short lines you cannot draw a
complete character without drawing short
lines that is what it basically means it
is also called a vector because the
electron beam is deflected from one
endpoint to another and those sequence
of commands which helps the electron
move from electron beam to move from one
point to another in order to draw short
strokes or lines are called sort of
vector type of movement and hence it is
called
a due to the vector type of movement or
definitions of the picture in terms of
lines characters made of lines and so on
it is called also called a vector system
it is also called a random scan because
there is no strict order the order of
deflection of the beam is basically
dictated by the arbitrary order of the
display commands so since there is no
strict order the order could be any in
any random form that is why it is also
called a random scan because the order
of deflection there is no strict rule
and it is dictated by the order of the
display commands based on which the beam
is deflected the phosphor is different
the phosphor on the screen in the case
of a random scan is different from the
that of a direct view storage tube or
dvst because dvst had a long persistence
phosphor again in this case the phosphor
has a short persistence
and the illumination of the phosphor the
light which it emits once in electron
beam strikes a phosphor we know it emits
light okay the amount of time over which
the phosphor image slide after the
electron beam is withdrawn is called a
short persistence we will come to the
definition but that decay of the amount
of light which it emits in the case of a
random scan the decay is very fast
whereas in the case of a dvst which had
long persistence the light was on for a
very long time and that is why the once
it is drawn it was there used to be
there for a quite a long time and the
decay in the case of a random scan is
the order of about 10 200 microsecond
typically in the order of about fraction
of a millisecond is what you can say
that the phosphor has a short
persistence so that means the picture
must be redrawn the beam must come back
to that point
otherwise the that point we will stop
emitting light okay so there is a need
for refresh this is the first thing
which we are getting that the display
must be refreshed each point the beam
must come back to that point after a
very short time otherwise the picture
will simply vanish on the screen and it
is true for all the points in the screen
for a random scan display okay the
display must be refreshed at regular
intervals of time and minimum
recommendation is as you see on the
screen 30 Hertz or 30 frames per second
FPS we will talk of this frame in a very
short while we will see what is this
frame but you can say you can you can
safely assume that the frame is just one
picture this one full picture is one
frame so this frame won't be revisited
or recreated at about thirty times per
second the 30 frames per second or a
frequency of 30 Hertz is the frequency
of refreshing this skin that is minimum
recommended for what I will call as a
flicker free display this this unit of
30 Hertz or 30 frames per second comes
out from the concept of persistence of
the vision of the eye not to do with the
screen parameters or display parameters
there is a persistence of vision for
which if a picture is presented very at
at a higher rate with very short gaps in
terms of even milliseconds you will see
a consistent at a constant picture or a
flicker free display if you reduce it to
below 30 or 25 Hertz this short will
start to the screen will start to
flicker in front of it front of your eye
so to provide flicker free display you
must provide the the refreshing
mechanism must operate at T Hertz or
more okay and since you talk of a the
Refresh the first time we are
introducing the concept of graphics
memory which comes into play in a big
way we will call it as a Refresh buffer
okay it's part of a system memory buffer
or the system memory which is used
basically for refreshing or the mixture
contents are the commands for the random
scan are kept there in the Refresh
buffer it is defined as a memory space
as you can see in the screen a memory
space allocated to store the display
list or display program the program
which creates the display list that
means the commands which are to be used
to draw the picture strokes lines
etcetera and which are used by the CPU
or the display processor to draw the
picture is is kept in the Refresh buffer
of course I have not given you the
architecture of a simple or a
general-purpose computer graphics system
we'll come to that in a moment but
refresh buffer is a very important
component or part component or part of
the graphic system where the entire
display program the entire display
program is basically kept the display
processor the CPU itself could act as a
display processor but you can have a
dedicated display processor inside your
computer graphic system which could do
the task of computer graphics or run
your programs or algorithms of computer
graphics without overloading or
disturbing the CPU correct because if
the computer graphics CPU does the
computer graphics job
which are highly computationally
intensive or lower loaded then the
system performance may go down it is
better to have a disturbed processor
which can do the computer graphics
operations for you correct so the
computer so the computer graphics system
will have a display processor in general
and it interprets these commands which
are then the Refresh buffer display
program or the display list and it also
interprets these commands the Refresh
buffer for plotting ok the display
processor must also cycle through the
display list to refresh the phosphor ok
that is under tasks and the display
program has commands for point plotting
lines plotting and also character
plotting will I think go through now the
conceptual block diagram is probably not
an architecture truly but a block
diagram we will give you the
architecture in a very short time the
conceptual block diagram of a
calligraphic or random scan refresh
display calligraphic afresh or random
refresh is what you can talk about so on
the left hand side you can see we have a
host CPU here which drives your graphics
processor or picture processor which in
turn loads the contents of your graphics
screen or the picture on a display
buffer or the Refresh buffer which we
talked about and the contents of the
display buffer or passed on to a display
controller ok the display controller has
some specific jobs we'll come to that in
a moment and it interprets this display
buffer contents and passes at it onto a
vector corrected generator which is
basically the interface to the CRT it
will generate all the analog voltages
which are necessary for the deflection
of the beam deflection of the beam and
the intensity of the beam are all
generated by the display controller and
also by the vector character generator
and that is passed on the CRT for
display this is one model I put this is
a display one because this is one
conceptual bad block diagram for the
display device you can have a slight
modified diagram of the respective
Refresh display I will say it is to a
conceptual block diagram which is
slightly modified as you can
see the modification is that the the
picture processor here which is actually
just after the host CPU in the previous
figure has come down after the display
buffer or after the display controller
that is possible it depends upon what
sort of framework you want to have
sometimes it is necessary to put the
picture processor after the display
buffer or display controller which will
interpret the commands and do some
processing very fast I do not want the
host CPU to be overloaded with computer
graphics commands or computer graphics
algorithms too much it'll just say I won
one these pictures with certain output
primitives we talked about output
primitives in the introduction lecture
where the primitives consists of lines
filled regions characters and so on so I
want the picture processor the graphics
processor to do certain operations on
these output primitives and I do not
want the CPU to do that so the CPU might
just send that I want these output
primitives drawn at this this point with
these attributes or features and the
picture processor has to interpret the
picture processor has to interpret these
commands from the display buffer and
generate correspondingly short short
descriptions for the vector character
generator if you see this screen now the
picture processor interprets from the
controller and send signals to the
vector character generated to generate
the corresponding signals of the CRT if
we revisit the block diagram 1 back
again and the block diagram 2 if you
compare this as we go back and forth the
position of the picture processor has
changed basically in the case of the
block diagram 1 as you see here now it
was next to the CPU and it was driving
the display buffer in the case of block
diagram to the CPU drive the display
buffer and the picture project takes
input from the display controller and
gives it to the vector generator it is
question of an implementation which one
suits once requirement in terms of
designing a computer graphic system you
can put the processor just after the CPU
or after the Refresh buffer ok so go
back and continue on random scan display
systems the display processor we
typically assume one of the two models
of the block diagram it sends digital
and point coordinate values to a vector
generator we will see the meanings of
what do you mean by digital and point
coordinate values well it
something like this the vector generator
needs to know what line you need to draw
from watch point x1y1 to point x2y2 on
the screen and it also needs to know
what should be the intensity of this
line
I repeat it needs coordinate values x1
y1 x2 y2 from which point to the end
point first point to the second point is
to draw a line and also the intensity of
that line so if you come back to this
screen that means you need to say not
only point coordinate values but a
digital value for the intensity that is
what is given to the vector generator by
the display processor after interpreting
the command ok the vector generator
converts the digital coordinate values
to analog voltage for the beam
deflection circuits because it must know
from which point x1y1 to which point
x2y2 it must draw so the beam has to
reflect at first switched off and
deflected to the point x1 y1 then
switched on with a particular intensity
and reflection voltage must be shaped in
such a manner that the beam is deflected
slowly in a linear manner to another
point x2 y2 so the vector generator does
all such tasks generates in fact this
analog voltages from digital coordinate
values and digital intensity values ok
the beam deflection circuits displays
the electron beam for writing on the
CRTs phosphor coating that is - of
course when the beam moves and is
switched on the phosphor gets the beam
and it starts emitting light at all
those points recommended rate was 30
Hertz usually is the minimum requirement
was 30 Hertz but the fresh rate
recommended is 40 to 50 Hertz to
absolutely have complete flicker free
display typically about 50 even 60 Hertz
is were to go beyond that okay scope of
animation is possible in a random scan
or calligraphic display it was not
possible in the case of a dvst but it
had lee it has limited support it has
limited support and immensely metered
support animation we will say that the
scope of animation lies with
segmentation what do you mean by
segmentation here you have a mixture of
static parts and dynamic parts of a
picture I will say I have two regions or
two segments of a picture some part it
could be static some part could be
dynamic where I will create the
animation that is possible in the case
of random
the static part I will draw once I need
to refresh that alright but the contents
of that static part in my display list
or display program in the Refresh buffer
which is the sequence of comment that is
never going to change because when i
refresh I keep on refreshing the display
list or display program in terms of the
short short lines I draw on this screen
there is another region which I will
call as a dynamic part of my of the
scene as a small part may not be though
the whole part that small part is
responsible for the dynamics or the
animation of the screen and there the
contents could change how do I change
that contents typically I go back to the
Refresh buffer and say I have a dynamic
part in my refresh buffer where the
contents could change I could right
there from the CPU or could write that
contents from the picture processor
those contents of drawing the lines
instead of drawing a line from x1y1 to
x2y2
I say I will change what do i implement
by animation I will say next I will draw
a line from a point X 3 by 3 to X or Y 4
so I had a line from x1y1 to x2y2 and
after some time if I draw a line from X
3 y 3 to X - 4 y 4 so I will first see a
line like this and I will see another
line like this that will create
animation for me so I can implement that
in a short part of the screen for a
random or calligraphic display system
and I need to update the Refresh buffer
this is the animated part the dynamic
part of the Refresh buffer which will
create animation for me the rest part
could be static I do not change the
contents but I keep refreshing the
entire display list or entire display
program that I need to update it because
the static part also has to be refreshed
along with the dynamic part the dynamic
part changes where the static part is
all static there is absolutely no change
the random scan display system basically
draws a set of lines in any order we
talked about that there is no strict
order that is why it is called random
but it essentially draws lines ok how
does it draw it just look at this
mechanism ok
we assume we have a screen here and then
an electron gun and the blue line is the
electron beam which is you know hitting
at one point of the phosphor on this
screen ok and assume that this beam is
switched on and it starts to move as you
start to move wherever it hits the
phosphor screen all that will startle
light let us say the boom have the beam
has moved a linear manner and it has
also turned so you see all that part
where ever the beam has scan so far it
start eliminating air emitting light and
of course you can dictate what sort of
color you want and all that okay so this
is the second stage you keep on moving
the beam more and let us say this
picture shows where it has moved now so
you can almost see what I am trying to
draw I have finished one line I have
finished the second line and I am moving
towards the third line so I probably
going to draw a triangle okay and it is
all these points on the triangle where
the beam has moved they are all emitting
still emitting light short persistence
but still emitting light because it is
happening very fast at about 30 40 Hertz
okay and when the entire picture is
completed the team comes back to that
particular point a full triangle is
drawn essentially what I have drawn I
made the beam to travel from vertex x1
y1 from a vertex x one y one to another
what is x2 y2 to a third vertex x3 y3
and come back to x1 y1 I have done in
this figure basically I have drawn a
triangle by making them we move from
first vertex to second vertex to third
vertex and come back I repeat again x1y1
to x2y2 to x3 y3 and then come back
again to x11 I finish the cycle and then
I am able to draw a triangle that is
what I have drawn these four shots of
the figure shows that essentially I have
finished a triangle but a job does not
end there the job does not end there
because you have to refresh the display
and the same thing keeps happening again
and again that the that the the electron
beam moves again from vertex one to
vertex 2 to verse 16 comes back to x1
and this cycle keeps on going at around
40 to 50 Hertz and you see only a
triangle with a certain intensity so and
this order of drawing that is also very
important I can move from point x1y1 to
point x2 y2 - x3 y3 and come back or
nobody stops me from traversing are
making the move making the electron we
move from point x1y1 to point x2 y2 to
point x3 y3 and then come back to point
x1 y1 I can move in any order I can
start from a third vertex move to first
vertex and move to
a number it is only basically an order
which is important the order is not that
important but the points are important
so that you create these three lines in
any order and you and since you're
refreshing at 30 40 Hertz you get a
flicker free static display of this
triangle okay let us see how this line
drawing can be generated in a random
scan or calligraphic scan display this
is an ideal line drawing forget the part
of the filling part we will see how
region filling can be done with the help
of again line drawing but let us worry
about only the lines drawn we have a
house with a window and a door and there
is a moon do not worry about the filling
part I have done the filling myself to
give it slightly better appearance but
do not worry and do not think about how
to fill those regions I am bothered
about the lines being drawn and again
just keep in mind the region filling is
nothing but again a set of line drawings
for the time being okay
so how do you do that is an ideal output
I want and you see it on the screen when
you are watching this TV monitor
actually something is getting refreshed
over this over and above this lines okay
so how do you start we do it by a vector
scan method or random scan method and
this is the screen which is blank so
what I do is the beam is somewhere maybe
somewhere and the Skinit is switched off
and it is switched off but it must be
diverted to one of the points starting
point somewhere let us say this is my
trace this is not my actual beam okay
this is the this is the point this
dashed line shows that the beam is
switched off and it is moved from one
point to another and that point here
with the arrow points to is nothing but
the top left vertex of the rectangle
structure on the left hand side this is
the point the ideal line this vertex is
where the beam has come and stopped and
it is switched off now what I do is now
watch I draw this small rectangular
field region and the arrow mark show in
what order I draw the lines okay I hope
it is clear that I basically draw four
lines there and the beam is switched on
at this point the beam is switched on
and Traverse it first moves horizontally
then vertically down then horizontally
left and then vertically up so it
basically finishes a try a rectangle a
small rectangular strip is drawn by
switching on this beam and traversed in
this version when it when it moves in
the horizontal direction there is on
unit horizontal deflection voltages to
be switched on when it is moving in the
vertical direction you need the vertical
beams to be vertical deflection voltage
to be switched on the other voltages can
be switched off so the beam was off that
blue dashed line shows that the beam was
off and it has been brought to that
vertex the vertex here this vertex it
has come and then it has moved like this
horizontally right vertically down
horizontally left and then horizontally
vertically up again and just come back
to this point let us say and that
triangle down of course the filling also
will be done next we need to draw the
other figure what do you do you switch
off the beam you switch off the whim and
move it you switch off the beam and move
it the path is shown again by the dashed
blue line where the beam is switched off
and it moves and it is going to draw the
next rectangle here this is how it is
drawn remember I am trying to keep an
arbitrary order here you can start with
the house draw the moon come back to
this rectangle absolutely no problem I
have drawn the reverse way I am drawn
the two rectangles first and we will see
what is drawn next so that is what I
mean by random order
not only that not only in terms of what
type of structures I am drawing in
sequence to implement the particular
rectangle also as you can see the second
rectangle is drawn in a different order
the first rectangle was drawn in an
order where I move horizontally right
vertically down horizontally left
vertically up in this case is completely
different just the reverse track or
trajectory is what I do I make the
switch on the beam here bring it
vertically down take it horizontally
right take it vertically up and then
finally horizontal left just the reverse
order so I can follow any order of
drawing lines that is the most basic
thing which you must understand in terms
of random scan as long as the pictures
are drawn does not matter in what order
you are drawing okay so these two
triangles are drawn now I need to
complete two other figures the moon and
the outside structure of the building
so what do I do I move to some other
point I am probably moving here I move
here to draw the moon first the beam is
again switched off I keep repeating
dashed discontinuous blue lines are the
trajectory for when the beam is switched
off continuous black lines the beam is
switched on so the beam is switched off
and it has gone to the place where it
has drawn it has to draw the moon it is
it draws it like this now it is
essentially drawing two curves these
curves are also implemented using short
lines okay so it draws the right-hand
trajectory goes up goes to the top
vertex of the moon and then comes back
the arrows indicate the trajectory by
which the lines move and you go here and
come back and the moon is drawn the PM
is again has to be switched off because
it has to be taken to one of the points
where it will start to draw the house
okay so it is a vector scan and you have
to implement the ideal line rung and
left hand side the part which is now
left is the house so you switch off the
beam take it to one of the points so we
have come back to the point which is the
left vertex left top vertex of the house
and we start to draw let us see how it
is run the beam is has been switched off
and it has Traverse from right to left
along the dashed blue line you switch it
on now and then start moving it
diagonally across to the top vertex
diagonally down right to one of the
vertices there then vertically down
horizontally left beam is on beam is on
and then finally vertically up that
completes the entire diagram as you can
see if I take off the blue lines from
the ideal from the diagram on the right
hand side which is a vector scan you are
basically and the arrow marks of course
then you get the ideal line drawing okay
if you take out I repeat the arrow marks
and the blue lines blue lines on the
screen which typically I repeat again
indicates the trajectory of the beam
movement when it is switched off the
blue lines
- discontinuous lines indicate the
trajectory when it is switched off and
continuous black lines indicate the
trajectory unit is switched on if you
remove those markers and the arrow
markers you basically get back the ideal
line drawing so this is a mechanism by
which a drawing is done in a vector scan
or random scan mode and of course that
doesn't stop the entire thing is
repeated again and again at a rate of 40
frames per second at least 30 frames per
second so that you do not have flicker
if you if it is drawn at a rate of about
10 or 20 frames per second or as low as
about 5 frames per second you will start
to having flicker on the screen to avoid
the flicker on the screen you must
present this entire cycle which I showed
you through the animation just now must
be repeated at around 40 to 50 frames
per second or 40 Hertz frequency of
refresh rate so that you have a flicker
free display otherwise you will start to
have flicker on the screen minimum
recommended rate is 30 Hertz okay so
continuing with the random scan display
system we come to these terms which you
have been using in terms of the
phosphorous material properties of a are
emitting light when an electron beam
strikes the screen the first term which
you must know in terms of the CRT is
display device and the phosphorous
coating is the phosphors fluorescence I
repeat phosphors fluorescence is the
light emitted you can read is the light
emitted as electrons or unstable
electrons the unstable means they are
they are flying through the vacuum the
CRT inside is a vacuum and the electrons
simply fly in a straight line they can
be diverted by the magnetic coils
electrostatic plates obviously paths are
unstable and these unstable electrons
they fly and they lose their excess
energy in terms of a kinetic energy
while it strikes the phosphor okay and
so we are talking of the fluorescence is
the light emitted as these electrons
lose their excess energy while the
phosphor is being struck by those
electrons so the phosphor extract struck
by an electron the electrons lose their
energy from one stable state they are
moving towards four state and that
energy is all emitted by it by means
means of two energies one is the heat
energy disappear at that point the
number one the number two is the light
energy so wherever it's very short time
the light will be emitted and go off the
beam must come back otherwise that if
you want to lie to enter at that
particular point where the beam is
talked continuously you have to refresh
it again by the beam at that point
because the light it emitted are a very
short time fraction of a millisecond and
it again goes team but the fluorescence
definition talks about that light
emitted as electrons lose their excess
energy while the phosphor is being
struck by electrons
phosphorescence is another term
phosphorescence 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 fluorescence was
the light emitted at the point when it
was struck and the excess energy is
being dissipated phosphorescence is the
decay basically the light given off by
the return of the relatively more stable
excited electrons to their own excited
state wants the electron beam excitation
is removed so I take off the beam as the
beam is struck I have a fluorescence
coming out as I take out this is a
phosphorescence which talks about the
light which is coming out but it will
decay with time okay that small amount
of time for which the light energy is
emitted is dictated by the
phosphorescence property of the phosphor
phosphorous persistence is defined as
the time from the removal of excitation
to the moment when phosphorescence has
decayed to 10% of the initial light
output I did say that the decay is
exponential the phosphorescence with the
term which we coined number two in this
slide is the decay which is exponential
but we didn't talk about the time we
didn't talk about the time and the time
over which this decay takes place and it
has almost gone down to if not zero
about 10% of the original light output
that is the fluorescence which was
coming out
when the beam was being struck the beam
is being removed phosphorescence takes
place and the persistence talks about
the trying the time duration over which
the time duration over with the the
phosphorus phosphorescence starts from
an initial value exponential decay mind
you and reaches about 10% of the
original value so that is the
exponential path by which the decay
takes place and that amount of time
typically if you have for those with
electronics electrical background can
compare this decay time with an RC
discharge time decay in electrical
electronic circuits which is used in
most digital analog circuits as well
that amount of time is called the
phosphorous persistence this is a decay
it is exponential the time is
persistence and the concept is
phosphorescence long persistence
remember dvst direct view storage tube
has a long persistence which could run
for several seconds so you can draw it
and wait and redraw it again after a
long time refresh may not be there or if
it is there it could be sluggish short
persistence for the random scan display
system takes about ten to sixty
microseconds which is common in most
modern displays the short persistence
exists with random scan display the time
which we have for this recording I will
just introduce one slide of refresh and
raster scan display systems and wind up
the lecture and the next lecture will
continue on the refresh and raster scan
we have discussed two types of display
devices I think in this lecture I must
introduce the third display device
before winding up but we will talk about
more details about the third display
device later on in the next lecture I
have introduced a dvst direct view
storage tube I have introduced a cally
graph or random scan display system and
we talked about architecture of display
systems we talked about the diagram of a
CRT and the third display device which
is much better than the previous two is
called a refresh or a raster scan
display system the refresh and the
raster scan display system is used in
most modern display systems including
television screens how it is different
the dvst and the random scan display
systems were essentially line drawing
devices we had seen how the line drawing
was used to draw the line draw the
picture of the house with draw and a
window and the moon there were all
drawing lines the Refresh reality or the
raster scan CRT remember it is different
from random scan forget dvst
comparing random scan and refresh scan
or raster scan these terms are used
interchangeably I can use the term
refresh and raster they are meaning the
same or I can use refresh raster they
basically mean the same thing the third
type of display device which I am
talking about it is different from
random scan in the sense that the random
scan was a line drawing device or a line
drawing system like the device dvst or
direct view storage tube the Refresh CRT
as given here in this line is basically
a point plotting device things appear to
be very tough but it gives you a lot of
advantages you draw points here unless
like drawing lines in the case of random
scan that is the essential difference
you must keep in mind the raster display
stores the display primitives like lines
characters shaded areas or patterned
areas in a flash buffer remember the
raster display stores the display
primitives we talked about
discrimination like lines characters and
areas in a Refresh buffer the randoms
can also store this the randoms can also
store this in Refresh buffer but it is
told with the help of storing lines in
this case it stores also lines but with
the help of points the Refresh buffer
also called the frame buffer we will use
these terms interchangeably the Refresh
buffer is also called the frame buffer
which stores the drawing primitives in
terms of points and pixel coordinates
rather than storing line commands draw
line or long drawing commands are
essentially the tools or the basic
utilities for random scan or dvst in
this case we are talking of points or
pixel components to store the drawing
primitives in the case of the Refresh or
after scan this is the architecture of
the typical raster graphic system a
typical computer system will have all of
these except maybe the video controller
where this is the monitor could be
directly driven from the system memory
but essentially any sort of video
controller is essential we have this
jumba's and input output devices like
the keyboard or the mouse or some other
i/o devices we have the central
processing unit which typically does all
the task of monitoring the system as
well as computer graphics commands if
necessary if you do not have a separate
graphics processor we have a system
memory the Refresh buffer could be a
part of the system memory the video
controller could take commands through
the CPU through the system bus from the
CPU the commands are all in the system
memory or frame buffer and the video
control is the one which converts the
line drawing primitives and draws it on
the screen or the monitor ok so that is
the typical architecture of a simple
austere graphic system which does not
have a graphics processor we will see in
the next lecture a slightly more complex
architecture of a raster graphic system
where you have a graphics processor and
a separate frame buffer identified from
the system memory and then we will see
how these components are used to design
a refresh or a raster scan system and
then how we will see how it is going to
be different from the way random scan
display systems were used to draw
pictures with the help of lines in this
case using the Refresh buffer we use
points to draw random scan raster scan
as given in this slide raster scan are
drawn with the help of points randoms
cannot drawn with the help of lines and
we also need a few more functionalities
or a modified advanced architecture of a
graphic system with the use of frame
buffer with use of picture processor
separately to draw a screen so we wind
up and we move over within the next
lecture when we discuss more details
most specifics about raster or refresh
raster scan display systems thank you
very much
[Music]