Display devices – Cathode Ray Tubes
- Display devices are also known as output devices.
- Most commonly used output device in a graphics system is a video monitor.
Cathode-ray-tubes [ Cathode Ray Tubes ]
It an evacuated glass tube.
An electron gun at the rear of the tube produce a beam of electrons which directed towards the screen of the tube by a high voltage typically 15000 to 20000 volts
Inner side screen coated with phosphor substance which gives light when it stroked by electrons.
Control grid controls a velocity of electrons before they hit the phosphor.
The control grid voltage determines how many electrons are actually in the electron beam. The negative the control voltage is the fewer the electrons that pass through the grid.
Thus control grid controls Intensity of the spot where the beam strikes the screen.
The focusing system concentrates the electron beam so it converges to a small point when hits the phosphor coating.
Deflection system directs beam which decides the point where the beam strikes the screen.
Deflection system of the Cathode Ray Tubes consists of two pairs of parallel plates which are vertical and horizontal deflection plates.
The voltage applied to vertical and horizontal deflection plates is control vertical and horizontal deflection respectively.
There are two techniques used for producing images on the CRT screen:
- Vector scan/Random scan display.
- Raster scan display.
Vector scan/Random scan display
[ Cathode Ray Tubes ]
Vector scan display directly traces out only the desired lines on Cathode Ray Tubes – CRT.
If we want the line between point p1 & p2 then we directly drive the beam deflection circuitry which focuses beam directly from point p1 to p2.
If we do not want to display line from p1 to p2 and just move then we can blank the beam as we move it.
To move the beam across the Cathode Ray Tubes – CRT, the information about both magnitude and direction required. This information generated with the help of vector graphics generator.
1.2 shows the architecture of vector display. It consists of a display controller, CPU, display buffer memory and Cathode Ray Tubes.
A display controller connected as an I/O peripheral to the CPU.
Display buffer stores computer produced display list or display program.
The Program contains point & line plotting commands with endpoint coordinates as well as character plotting commands.
Display controller interprets the command and sends digital and point coordinates to a vector generator.
Vector generator then converts the digital coordinate value to analog voltages for beam deflection circuits that displace an electron beam which points on the CRT’s screen.
In this technique, a beam is deflected from end point to end point hence these techniques also called random scan.
We know as beam strikes phosphors coated screen it emits light but that light decays after few milliseconds and therefore it is necessary to repeat through the display list to refresh the screen at least 30 times per second to avoid flicker.
As display buffer used to store display list and used for refreshing, it also called refresh buffer.