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How is the image formed - How does the TV receiver work .. Electronics

How is the image formed

The TV image consists of a bright spot moving on the cathode ray tube screen and its intensity increases or decreases depending on the brightness or darkness of the different parts that originally make it up. It is worth noting that covering the screen surface with regular lines is called the "scanning" process.

It is noted in Figure (165) that scanning the image begins by moving the spot from point 1 to point B, then it returns to start moving again on the second line at point A and then moves to point D, and so on until the scanning process ends at point H. At that time, the bright spot returns to start the scanning process again starting from point A. It is worth noting that it is possible to use many very close lines in the scanning process. C In addition, controlling the brightness of the spot and scanning the entire image several times per second are sufficient to form the complete image (166).

165 - Scanning the image.
166 - Using the cathode ray to form the image.

167 - Interlaced scanning and using a larger number of lines.

The screen of the television receiver is characterized by being coated from the inside with a phosphorescent material that glows under the influence of the cathode rays that hit the screen at only a small spot on it at any moment, and the continued coating of the tube causes it to glow for a short period after the spot passes, causing the screen to appear illuminated for a long period of time.

The interlaced scanning method is used to prevent some vibration in the image (167).

It is worth noting that the fewer the number of lines, the larger the spot patch and the less clear the details of the image become. The image that contains 405 scanning lines is less clear than the same image that has 625 lines. In addition, some of these lines are used in the process of synchronizing both the transmission and reception processes.

In the interlaced scanning process, the entire image is scanned with half the total number of lines. This scanning is called frame scanning, and it occurs in a period of 1/50 of a second. The remaining lines cover the screen and fill the spaces between the lines of the first frame during another 1/50 second, so the entire process is completed in a period of 1/25 second.

To obtain an image that is exactly the same as the image in the transmitter, the bright spot in the receiver must be moved to coincide with the bright spot in the transmitter. It is known that this process is done by synchronization pulses (128).

In addition, the television receiver circuits are adjusted so that the scanning speed in both the transmitter and receiver is approximately equal.

Therefore, at the end of each scanning line there is a synchronization pulse that terminates the scanning process, i.e., the scanning line circuit is disabled. At the end of scanning the entire image frame there are several successive synchronization pulses that disable the image frame scanning circuit so that the bright spot begins its movement to scan the image frame again.

168 - Scan signal synchronization to control the brightness of the light spot.