Raster Scan Display
The term ‘raster’ is a synonym for the term ‘matrix’. Therefore, a raster scan CRT scans a matrix with an electron beam. Raster scan CRT display is most commonly used in TVs.
The various components of raster scan CRT are explained below:
- Electron Gun
- Control Electrode
- Focusing Electrode
- Deflection Yoke
- Phosphor Coated Screen
1. Electron Gun
It consists of series of components (primarily a heater and a cathode). The electrons are generated from cathode when heat is supplied to cathode. These electron are accelerated by the use of an electric field.
2. Control Electrode
It regulates the flow of electrons i.e. controls the electron gun. The control electrode is connected to an amplifier, which in turn, is connected to the output circuitry of the computer, thus allowing the computer to control the electron beam. Thus, control electrode controls brightness of image by controlling flow of electrons.
3. Focusing Electrode
It is used to create a clear picture by focusing the electrons into a narrow beam. The focusing electrode serves this purpose by applying an electromagnetic force on the electrons in electron beam.
4. Deflection Yoke
It is used to control the directions of electron beam. The deflection yoke creates an electric or magnetic field which will bend the electron beam as it passes through the field.
5. Phosphor Coated Screen
The phosphor crystal are coated on the inside of the CRT screen. These crystals glow when they are hit by a high energy electron beam. The picture is produced on the screen by combined effect of various spot of light on the screen.
Working of Raster Scan Display
- In raster scan display the electron beam scans the entire surface of CRT, one row at a time. It starts from the top left corner of the screen then moves horizontally from left to right. It then jump to left side of screen and scans another line until it reaches bottom right corner of screen.
- The rate at which the electron beam scans the surface of the CRT is directly proportion to the frequency of local line voltage. Therefore, in areas such as united states where 60-cycle line voltage is used, the electron beam will usually scan across the entire face of the CRT 60 times in one second.
- As the electron beam is moved across each row of matrix or raster the electron beam is turned off and on to create a picture in form of illuminated spots. Picture to be displayed (i.e. picture definition) is stored in a memory area called refresh buffer or frame buffer.
- Refresh buffer or frame buffer is a refresh storage area in the digital memory in which the intensity values and other parameters (called attributes) of all the pixels making up the image are stored in binary form. This area is organized in form of 2D table. Every row-column entry stores information such as brightness and/or color value of the corresponding pixel on the screen.
- When the electron beam scan the various lines the various intensity values that are stored in the frame buffer are then read out and displayed on the screen.
- The frame buffer used in black and white system is different from the one used in colored system.
Working pattern of Raster Scan CRT
The frame buffer in black and white system is known as bitmap. Such a frame buffer contains one bit per pixel i.e. just one bit is used for each pixel on the screen. This is because, in black and white system the screen point is cither on or off. So only one bit is needed to control the intensity of screen position. Such a system is bilevel system where a bit value 0 indicates that the beam intensity is off and a bit value of 1 indicates that the electron beam is turned on.
In colored systems, more than one bit is required to show various colors and intensity values. Therefore multiple bits are used per pixel in the frame buffer. Such a frame buffer with multiple bits per pixel is known as pixmap. In high quality system upto 24 bits per pixel can be used. Therefore the size of frame buffer is usually large in colored systems. The size of frame buffer depends on two factors: colors to be displayed and resolution to be displayed. For example, a system with 24 bits per pixel and a screen resolution of 1024 x 1024 requires frame buffer of 3 MB size.
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