Two types of display technology are commonly used monitors. The CRT which has been around for a long time is mostly used with desktop computers. On the other hand, laptops and portables require a more compact display of the flat screen type. Both of these can be either of the monochrome or color types:-
1. Cathode Ray tube Display
2. Flat screen/panel display
- Cathode Ray Tube:
– The main component of the Cathode ray terminal is the electron gun. The electron beam controlled by an electron magnetic field and a display screen which is phosphor coated two types of CRT.
- 1. Vector CRT Display: –
In which the electron beam is directed only to places where image is to be created.
2. Raster scan display:-
In which image is projected on the screen by directing the electron bean across each row of the picture elements from top to bottom on the screen.
CRT is similar to big vacuums glass battle. It contains three electron guns.
The primary components of an electron gun in a CRT are the heated metal cathode and a contrail grid. Heat is supplied to cathode by directing a current through a coil of wire called the filament, inside the cylindrical cathode structure. This causes electron to be “boiled off” the hat cathode surface.
A heated cathode emits a high speed electron into he phosphor-coated glass screen. The electron gives the energy to the phosphor coated causing it to glow at the point where beam make contact by focusing the electron beam changing its intensity & controlling its point of contact against the phosphorus coating through the use of deflector system, the beam can be made to generate a picture on the CRT screen.
Components of CRT
Main components of CRT are:-
1. Electron Gun:-
electron gun consisting of a series of components. Primarily a heating filament (heater) and cathode. The electron gun creates a source of electrons which are focused into a narrow beam directed at the face of the CRT.
2. Control electrode:-
it is used to turn the electron beam on and off.
3. Focusing System:-
it is used to create focusing the electron into a narrow beam.
4.Deflection Yoke:- i
t 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 field. In a conventional CRT the yoke is connected to the sweep or scan generator. The deflection yoke which is connected to the sweep generator creates a fluctuating electric or magnetic potential. The change on potential causes the electron beam to be deflected.
the inside front surface of every CRT is coated with phosphorus. Phosphorus glow when they are hit by a high energy electron beam. All phosphorus has a limited life which is a function of both the electron beam and intensity of the electron beam. Phosphorus is the term used to describe the light gives off. By a phosphor after it has been exposed to an electron beam.
Properties of Video Monitor
is the duration of phosphors sense. Different kinds of phosphors are available for use in a CRT. Beside color, a major difference between phosphors is their persistence, how long they continue to emit light after the electron beam is removed lower phosphors require higher refresh rate to maintain a picture on the screen without flicker. A phosphor with low persistence is useful for animation and a high persistence phosphor is useful for display highly complex, static picture.
3. Aspect Ratio
There are two types of technique used in current computer graphics terminals for generating the image on the CRT screen. They are:-
1. Random scan
2. Raster scan
Raster Scan Display
In Raster scan display the beam is moved all over the screen one scan line at a time from top to bottom and then back to top. In this the refresh process is independent. Raster Scan Display mathematically smooths lines, polygon and boundaries. Its cost is low. Raster scan has ability to display areas filled with sailed colors or patterns.
Raster display stores the display printers (such as line, characters sailed) in a refresh buffer in the form of pixel.
Architecture of a raster display
It consists of display controller, central processing unit, refresh buffer, keyboard, mouse and the CRT.
In raster scan display a special area of memory is dedicated to graphics only. This memory is called frame buffer. It holds a set of values for all the screen points. The store values are retrieving from frame buffer and display on the screen one row at a time.
Each screen point referred to as a pixel. Each pixel on the screen can be specified by its row and column number. Thus by specifying row and column number. We can specify the pixel position on the screen.
The raster screen display system is the most common method of displaying images on the CRT screen. In this method, the horizontal & vertical deflection signals are generate to move the beam all over the screen in a pattern shown in figure.
Picture def. in a memory area hold the set of intensity values for all screen point stored intensity value are then retrieve from the refresh buffer and “pointed” on the screen one row at a time. Each screen point is referred to as a pixel. Home television sets and printers are examples of other system using raster scan methods.
1. Realistic image
2Million Different colors to be generated
3. Shadow scenes are possible
1. Low resolution
2. Electron beam directed to entire screen and only to that part of the screen where picture is to be draw.
In the raster scan approach the viewing screen is divided into a large no. of discrete phosphor picture elements, called pixels. The matrix of pixels constitutes the raster the no. of separate pixels in the raster display might typically range from 256*256 (total 65000) to 1024*1024 (total 1,000,000) each pixel on the screen can be made to glow with a different brightness. During operation an electron beam creates the image by sweeping along a horizontal line on the screen from left to right and give the energy to the pixels in that line during the sweep.
When the sweep of one line is completed, the electron beam moves to the next line below & proceeds in affixed pattern as in directed.
DIRECT VIEW STORAGE TUBE
D V S T is used today as part of a display system. DVST marks a technology change in usual refresh type display. Both in raster scan & random scan system the screen image is maintained by redrawing or refreshing the screen many lines. The DVST give the alternative method of maintaining the screen image.DVST use the storage grid which stores the picture information as a charge distribution just behind the phosphor-coated screen.
In DVST there is no refresh buffer the images are created by drawing vector and line segments with a slow moving electron beam.
It consists of two electron guns, a primary gun and a flood gun. A primary gun stores the pictures pattern and the flood guns maintain the picture display. A primary gun produces high speed electron which stripe on the storage grid to draw the picture pattern. As electron beam strikes on the storage grid with high speeds
It knocked out the electron from the storage grid keeping the not positive change. The knocked out electron are attached the collector. The net positive charge on the storage grid is nothing but picture pattern. The continuous low speed electron from flood gun pass through the control grid and are attached to the positive charged areas of the storage grid. The low speed electron then penetrates the storage grid and strike the phosphor the collector just behind the storage grid smooth out the flow of flood electron.
1. It has flat screen.
2. Refreshing of screen is not require.
3. Because no refreshing is required, very complex pictures can be displayed.
4. DVST is much cheaper than random scan display.
5. Very complex picture can be displayed at very high resolution without flicker.
1.They does not display colors.
2. Selecting or part erasing of screen is not possible.
3. It has poor contrast.
FLAT PANEL DISPLAY
To need of the compact portable monitor, modern technology has gifted us with LCD panel, plasma-display panel, LED panel and this CRT. These displays are smaller, lighter & thinner than CRT are termed as FLAT PANEL DISPLAY.
The FPD are currently used in TV monitor, calculator, pocket game, laptop, computer, FPD categorized into:-
E MISSIVE- Are that display that convert electrical energy into light are plasma panels.
Non-e missive:- use optical effect to convert sunlight or light from other source into graphics patterns. The most important example of these displays is LCD.
LCD is made up of 6 layers as shown
The front layer is a vertical polarizes plate. The next layers contain thin vertical grid wires than we have a layer with horizontal grid wires on the surface followed by a horizontal polarize and finally a reflector.
The liquid crystal material is made up of long crystal line molecules
Light .entering through out the front layer is polarized vertically. As the light passes through the crystal, the polarizes is rotated 90. to horizontal, so the light now passes through the horizontal polarizer is reflected and return through the line polarizer and crystal.
IS a flat display technology used in laptops, calculators and flat screen displays.LCD allows displays to be much thinner than CRT technology. LCD consumes much less power than LED and gas-display because they work on the principal of blocking light rather than emitting it.
It is known as liquid crystal display. It is a thin, flat electronic visual display that uses the light modulating properties of liquid crystal. LCD does not emit light directly. They are used in wide range of application including computer monitor, TV, panels, aircraft. They are common in consumer device video players, gamming device, clocks, watches, telephone.
LCD displays are addressed in a matrix fashion.
1. Low power consumption
2. Small size
3. Low cost
4. Physical aspect: – It takes up about 40% of less desk space.
5. Screen shape: – It is completely flat screen.
6. Brightness: – It produces very bright images due to high peak intensity.
1. The resolution is not as good as that of a CRT.
2. LCD does not emit light as a result. The image has very little contrast.
3. They have no color capability.
4. It has fixed aspect ratio and resolution.
5. Cost: – it is more expensive than CRT.
6. It works only at the native resolution. The native resolution can not be change.
IS an array of neon bulbs which are a part of a single panel made up of three front layer of glass? The inside surface of he front layer has thin vertical strip of electrical conductor.
The central layer has a no. of hales and bulbs.
The next layer has thin horizontal strips of an electrical conductor. We use matrix addressing to turn the bulbs on and off.
The plasma panel are flat transparent and rugged and do not need refresh buffer.
1. Refreshing is not required.
2. Less bulky then a CRT.
3. Produce a very steady image.
4. It has flat screen.
5. Allows selective writing and selective erasing.
6. High resolution
7. Less weight
8. Low volume
9. Large screen size is possible.
10. Images are much brighter than CRT.
1. Costlier than the CRT’S.
2. It requires complex addressing and wiring.
3. Relatively poor resolution of about 60 dates per inch.
4 .Technology is expensive.
5. It uses a lot of power.
6. Does not show sharp contrast.
Plasma panel displays are also called gas-discharge display. It has an array of tiny neon bulbs. Each bulbs can be put into an “on” an “off” state and remains in the state until explicitly changed to the other. These memory properties mean that plasma panel needs not to refresh. Plasma panels have 50 to 125 cells per inch. The neon bulbs are not discrete units but rather than are part of a single integrated panel made of three layer of.
The inside surface of the front layer has thin vertical strips of an electrical conductor, the center layer has a number of hales and the inside surface of the rear layer has thin horizontal strips of an electrical conductor. A cell is made to glow placing a firing voltage across it by means of the conductor. Once he glow starts, a lower voltage is applied to sustain it to turn off a bulbs, the system decrease the voltage on the appropriate lines to less than the sustains.
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