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

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

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Horizontal deflection plates

Y

ZX

+

-

Light Spot

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’ .

horizontal flyback

vertical flyback

Raster lines

Y signal

Ty=20m sec

F=1/20m = 50 Hz

t

X signal

64µ sec

Tx= Ty/Number of lines = 20m/312 = 64µ sec

t

Exact Y signalY signal

20m sec

t

Z modulation= beam intensity control Z signal

64µ sec

t

‘1’ or ‘0’ duration time = Tx /number of pixels per line

1 0 0 1 1 1 0 1 1 1 1 1

Example:

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

25 lines

80 characters

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

Y

X

• 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.

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.