1. Screen Monitor Visual display unit (VDU)
Cathode Ray Tube (CRT)
Screen
Monitor
Visual display unit (VDU)
Needs high operating voltage (around 16 KV)

2. Horizontal deflection plates+
Light Spot - - Y + X Z

4. Phosphor
It is a semiconductor material which emits visible radiation in response to the impact of electrons.
(i.e. when it absorbs energy from some source such as an electron beam, it releases a portion of this energy in the form of light)
In response to a sudden change in the electron beam(from on to off), the light emission does not fall instantaneously, there is a gradual reduction challed ‘fluorescence’ .

5. horizontal flyback
Raster lines
vertical flyback

6. Y signal t
Ty=20m sec
F=1/20m = 50 Hz

7. Tx= Ty/Number of lines = 20m/312 = 64µ sec
X signal t
64µ sec
Tx= Ty/Number of lines = 20m/312 = 64µ sec

9. Exact Y signal
Y signal t
20m sec

10. Z modulation= beam intensity control
Z signal 1
0 0
1 1 1 t
64µ sec
‘1’ or ‘0’ duration time = Tx /number of pixels per line

11. Example:
Suppose we want to design a CRT with 25 lines of 80 characters per line.
25 lines
80 characters

12. 9 x 7 character format.
Each character occupies 10 x 8 pixels
Thus : we need 10 x 25 = 250 pixels
in the y-axis
& 8 x 80 = 640 pixels in the x-axis Y X

13. Duration of ‘on’ or ‘off’ state is:
250 line in 20 m sec NOT 312 lines
Time required for vertical and horizontal flyback.
Top and bottom of the CRT is not used to avoid image distortion.
Each horizontal sweep takes 64µ sec, only 40 µ sec available for character generation.
Duration of ‘on’ or ‘off’ state is:
40 µ sec/ 640 = 62.5 n sec.  the vedio signal has 16 million bit per second.

14. Coloured Display
Each of these dots has its own hole in the shadow mask, and each dot is formed from a different phosphor, which is chosen to fluoresce with that color.
Three electron guns, one to stimulate the red dots, one for the green, and one for the blue. The three electron beams scan across the screen together, but the intensity of each beam can be varied independently.