الإشارة المرئية المركبة ونبضات التزامن .. كتاب التلفزيون الملون والعادي

Chapter Thirteen:

Synchronizing Circuits

13-1 Composite visual signal and synchronization pulses:

The composite visual signal includes image information, which is directed to the screen valve to control the electronic beam that illuminates the screen with a signal proportional to the visual signal received to draw the image. We have discussed in the previous chapters the phases and cycles that it passes through. As for the second part of the composite visual signal, it is what includes the horizontal and vertical synchronization pulses, the equalization and erasure pulses, and they cooperate with the horizontal and vertical deflection coils - so they work to direct the beam to scan the screen 625 lines for each image and in two stages. It draws 25 complete images per second and synchronizes precisely with the transmitter, and the effect of the beam does not appear when it bounces. The synchronization pulses are located in the black region of the black, so that no trace of the beam remains on the screen. Vertical and horizontal pulses have the same amplitude. But it differs in waveform, so it synchronizes pulses from the visible signal. Based on the difference in amplitude between them, as the amplitude of the synchronization pulses is always the largest (Fig. (1-13). As for the separation of the horizontal and vertical synchronization pulses, it is done on the basis of the different waveforms for each of them, and this applies to both black-and-white and color devices. Figure (2-13) shows the block diagram of the synchronization and deviation files section.

2-13 Effect of defect or absence of synchronization pulses on the image:

This defect appears in the image in one of the following forms:

1- Vertical synchronization defect.

​​​​​2 - Horizontal synchronization defect.

3- A glitch in the vertical and horizontal synchronization.

1- Vertical synchronization defect:

If the defect or loss is in the vertical synchronization, the image slides up or down at a speed commensurate with the position of the vertical stabilization adjuster. And can prove The image is for a short time if this caliber is adjusted slowly, but soon the image returns to sliding again. As in Figure (13-83).

Horizontal misalignment:

The horizontal scanning frequency is (15625 Hz) in the European system or (1573426 Hz) in the American system. When the horizontal oscillator of the receiver is compatible in frequency and page with the horizontal oscillator of the transmitter - the image is correct in its proper place. But if the horizontal oscillator has a deviation from its correct frequency, then the image appears in the form of wide slashes. We can determine and know if the frequency of the horizontal oscillator is higher or lower than the correct frequency by looking at the direction of the inclination of the broad horizontal lines on the screen. If they slope from right to left, then the frequency is less than the correct frequency. This is because the horizontal frequency of the local oscillator is higher than the correct frequency. In the image (3-13 B) it is noticed that black lines separate the wide oblique lines - and these black lines are formed from the periods of horizontal erasure - and these periods of horizontal erasure do not usually appear in the image - because they are outside the screen at both ends of the image. But if the image is as shown in Figure (13 B3), i.e. the number of its lines changes over time, then this indicates a complete loss of horizontal synchronization, and we may be able to correct the image with the horizontal stabilizer, if the defect is due to misadjustment or a change in the specifications of some elements due to a factor. oblique time.

3- Vertical and horizontal synchronization defect together:

Figure (13-23) shows this defect. The image rotates vertically and the number of slanted lines changes with time - and there are several possibilities that cause this malfunction:

1 - Malfunction in the synchronization interval phase. (pulse rodent)

2- Compression of the synchronization pulses in the IF stage.

3 - A defect in the automatic gain control
(Automatic Gain Governor) AGC.