خطوط المسح .. كتاب التلفزيون الملون والعادي

3-1 Scan lines The ejector releases the electronic beam and directs it from the cathode towards the phosphorescent layer on the inner surface of the screen - and passes through the focus electrodes that prevent its dispersion, and make it give a bright spot in the middle of the screen, and the voltage of the governing grid controls its illumination by means of a light switch.

By changing the cathode voltage, the bright spot changes its luminosity between black and white, and between them a gray color gradient, all of this is effected by the image output signal voltage. To form the image, this ray moves horizontally from the left of the screen to its right, then returns to draw another gradual line towards the bottom of the screen, Figure (8-1). The number of lines is 625 horizontal lines in the European system (CCIR). When the ray returns after drawing each horizontal line, its effect does not appear on the screen due to the presence of horizontal erasing or extinguishing pulses, just as the return of the ray from the bottom to the top is erased, and its effect does not appear due to the presence of vertical erasing or extinguishing pulses (Fig. 1-10/1-11 ) It is important that each image consists of two fields. One field scans single lines and the other field scans double lines. Each field consists of 312.5 lines, but they intertwine with each other without the lines superimposing each other - the number of images per second is 25. While the number of fields is 50 fields per second, which is called the vertical frequency, and this system, which forms each image from two fields, helps to make the image stable, and no flickering or vibration appears in it, as if it was formed from one field.
Figure (1) - (8) Electron beam scan lines on the screen. Continuous lines show scan lines, and dashed lines show beam return lines that must be erased and their effect hidden on the screen.

The ray draws and erases the screen so that it appears completely illuminated even when there is no reception of a visible signal, and it is called the linear structure of the image (RASTER).

The number of lines per second:

= 25 25 25 15625 line seconds, which is

Horizontal scan frequency.

deviation files:

The horizontal scan lines, with their regular movement from the left to the right of the screen, then bounce back without leaving a trace, and the formation of two fields for each image: this is what the deflection files do, and these files are located around the neck of the screen and generate the magnetic field that moves the beam (Fig. From - Horizontal deflection files: which move the beam with its horizontal movement from the left of the screen to the right of it and vice versa.
2- Vertical deflection files: which move the beam with its vertical movement from the top of the screen to the bottom and vice versa.

These coils are fed by a sawtooth current generated and advanced by the oscillator circuit

output circuits for each. And the movement of the beam can be controlled in another way, other than the method of magnetic coils. It is the electrostatic deflection method using deflection plates inside the screen. They are two horizontal plates and two vertical plates, each plate is fed with an appropriate voltage, and an electric field is generated between them that can control the movement of the beam. This is often used in the signal oscilloscope screen.