مركبة التيار المستمر في الإشارة المرئية .. كتاب التلفزيون الملون والعادي

Chapter Ten:

DC Vehicle Recovery

10-1 DC component in the visual signal:

The composite visual signal contains several compounds, each component serving and performing a specific purpose: the visual signal that carries image information is AC variable currents in proportion to the image elements. As for the other component, it is the DC component that controls the level of the image background - and both components can be separated or changed, one without the other. Also, among the contents of the composite visual signal are the extinguishing pulses, synchronization and equalization. We have studied the processing of the AC component of the visual signal in the image amplifier - and we will discuss in this chapter the continuous component of the DC signal, its processing, its work, and the method of separating and then restoring this component.

Composite visual signal:

Figure (1-10) shows the composite visual signal of several horizontal lines - and between each pulse of synchronization and erasing, the information signal of the television camera is graduated. When these changes are applied to the control network of the screen valve - each change in the signal voltage of a different light point in the intensity of illumination - From the sum of the gradations of illumination, the image appears on the screen in black and white.

DC level offset in the signal:

If we suppose that we took the image signal, but brought its AC changes closer to the level of the visible pulses, as in Figure (10-2), and then moved these changes away from the level of the visible pulses, as in Figure (10) - (2) - (B), then what can we see? Since the level of the vibrating pulses represents the cutting state of the electronic beam in the screen. Bringing the image signal closer towards it makes the background of the image darker - and when the image signal is farther away from the erasing pulses, it makes the background of the image more luminous, and this is similar to lighting a lamp in a room or dimming it - increasing or decreasing the light does not affect the contents of the room

And its objects, but only changes in the level of illumination and clarity. This is what is termed in the image signal by the symbol DC that can be controlled
The level of yolk pulses (extinguishing):

The level of the vibrating pulses in the transmission represents the black level of black, and then the electronic beam is in the cut-off state and the image valve at these moments is dark, and the camera signal and the synchronization pulses come to complement the composite visual signal - and the level can be set between the variable signal For the image and the vibrating pulses to reach the appropriate background illumination, this is possible by the program director or the viewer - but the level of the vibrating pulses and the DC component of the signal must be kept at a constant reference level.

2-10 Loss of a DC vehicle:

The second detector of the image gives a full image signal at its output - and it has erased pulses of one level. the shape
(A - 3 - 10) - But the passage of the signal in the RC linkage circuits between the phases of the image frequency amplifiers makes the extinguishing pulses for the various lines not remain at one level, and this is due to the fact that the connection capacitors make the image signal carry the positive polarity and the negative polarity in a manner corresponding to the zero axis - As in Figure (B). Therefore, the positive peaks of the waves do not have the same level - and in figure (C) three lines appear in the image, one of which is completely white, the second is gray, and the third is dark or black in color - and when the signals of these lines come out of the second image detector and then the link capacitors, the signals become as in Figure (D), that is, the area above the axis is equal to the area below it, and so the DC component was lost in these signals. And we will discuss the effect of that on the image.

The effects of the loss of the DC component on the TV picture: The vibrating pulses must carry the electronic beam to cut off and darken the screen - but the loss of the DC component leads to the appearance of return lines on the screen. mail; This leads to the disappearance of some features of the image with more lighting - in addition to that the manual reset cannot keep up with the great speed at which the lighting of the lines and the image changes. In conclusion, it is necessary to make the erase pulses fall in one level and voltage as it was in the next signal before entering the detector stage, and this is done by inserting a circuit to restore the continuous component to the visual signal so that the lines remain in the image as they were sent, whether they were white, gray, black, or in any degree between them .