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How does the scanning process work - Image signals - How does the TV receiver work .. Electronics

How does the scanning process work

The cathode ray tube emits electrons that are focused focally in a narrow beam as soon as they hit the face of the tube until the latter becomes fluorescent and the tube contains deflecting magnetic coils (169). When the current passes through a pair of coils, it causes the cathode ray to deviate from one side to the other. However, if it passes through the other pair of coils, it moves from bottom to top. There are "frame" scanning coils and "linear" scanning coils according to the need and the "saw" wave that gives a current similar to the teeth of a saw can perform the linear scanning process (170), as the rise of the wave S leads to the deflection of the spot that draws a line across the screen of the device. Then comes the return stage C which returns the spot to a new line at a very high speed. After about half of the lines have passed, the spot reaches the bottom of the screen, then it quickly returns to the top again, also very quickly. Thus, the voltage increases again after each complete return process.

As for the frame scan, it needs another saw wave, but it is slow (not shown in Figure 170). The stage P1 gradually moves the spot to the bottom of the screen, reaching it after about half of the lines have passed, then returns to the top again. As for the stage C1, it is characterized by a sudden change in the voltage, which returns to rise again during the stage C, then reaches the peak during the stage C2.

169 - The pair of deflecting coils used in scanning the image.
170 - Linear scanning and frame scanning waves.

Image signals

The image signals pass after being separated from the audio signals in the mid-frequency amplifier which increases their intensity (171). It is known that each section of this signal is dedicated to performing a specific function, as the linear synchronization pulses that enter into the image signal synchronize the linear scanning oscillator as we explained previously, so that each line starts at the specified moment. As for the frame pulses, their task is to synchronize the frame generator itself.

In addition, the image signals adjust the brightness of the bright spot during its movement, in order to draw the image with all its shades and multiplied details. As for the image signal detector, it is a diode (172) that produces an output that passes to the next amplification stage, then controls the intensity of the cathode beam and the brightness of the bright spot regardless of its position during its movement on the screen.

Figure (173) shows the oscillator that generates the saw current that controls the letter of the bright spot. The frequency of this oscillator can be controlled by adjusting the frame scan controller. If the oscillator frequency is much higher or lower than the frequency of the synchronization pulses, in this case we cannot adjust the scanning process. In this case the image will be cut off and not appear clearly. Therefore, the linear scanning frequency in the receiver must be adjusted so that the scanning process becomes synchronized with the linear synchronization pulses.

171 - Important image circuits in the television receiver.

172 - Detector, image, amplifier and valve.

173 - Frame scan signal generator.

174 - Heating and feeding fuses.