التحكم الأتوماتيكي بالتباين .. كتاب التلفزيون الملون والعادي

9-6 Automatic Contrast Control:

Some TV sets contain an automatic contrast adjustment circuit so that the contrast of the TV picture is proportional to the lighting of the place. The simplified circuit (Fig. In strong spotlight intensity.

circuit work:

In the circuit shown, we find that the LDR is connected in series with the contrast control resistor and connected to ground. When the light is weak in front of the television set - the resistance of the photocell becomes large - and this increases the DC voltage in the contrast adjustment arm - and this increase in voltage is applied through the contrast transistor 0 to the image amplifier and then the amplifier magnification decreases - and this reduces the contrast to a certain level, and in return it decreases The LDR impedance in strong room light - the DC voltage drop on the contrast control arm increases the gain of the image enlarger - and results in an increase in contrast as required under strong lighting. And there is a switch to cancel the automatic contrast control - so the adjustment process is limited to the manual switch that is within reach of the viewer when it is placed in the first position. Or the photocell is left with the role of automatic contrast control. There is another circuit connected to the LDR photocell, and its work is to adjust the color saturation at the same time with contrast control and it is called (Color Tracking).

9-7 Automatic lighting control:

It also relies on the work of the photocell with light affected resistance (LDR), so it adjusts the lighting as well as the contrast - as in the simplified figure (9-14) and using a single photocell. Any change in the collector DC voltage changes the LED and its illumination
If we assume that the illumination of the place is relatively low, then the resistance (LDR) is high, and since the resistance of the cell and R4 are connected in parallel with the emitter resistance R3, then the total resistance is in the highest value. This resistance will decrease due to the effect of the forward bias (base to emitter) of the transistor Q1, and therefore the collector current Q1 will decrease, causing an increase in the collector voltage, and considering that the collector is directly connected to the cathode of the screen diode, the positivity increases on the cathode of the diode diode, causing a decrease in illumination as well. It is what is required, but if the lighting of the place is relatively high - the resistance (LDR) decreases - and thus the total resistance of the emitter decreases - and the forward bias of Q1 increases - then the collector voltage and the cathode voltage of the screen valve decrease and become less positive and lead to a rise in screen lighting to increase the room lighting contrast control Automatically: In the same previous circuit, the automation is done by contrast by increasing or decreasing the gain of transistor Q1 amplification, according to the emitter resistance affected by the value of the resistance (LDR) - when the external lighting is increased.
Surrounding the device, the resistance of the (LDR) decreases, and with it the resistance of the 01 radiation, so it increases, and thus the amount of image contrast increases, and vice versa, that is, the contrast weakens when the surrounding area lighting is low.

8-9 Control the sharpness or sharpness of the image:

Due to the exposure of the image to noise and the presence of ghosts or errors of the receiving device, it is preferable that the viewer be able to adjust (the accuracy or sharpness of the image) and the circuit shown in Figure (9 - (15) is similar to controlling the tone of the sound, and it contains a variable resistance R1 and a capacitor C1, so the value of the resistance of R1 is reduced As for increasing the value of R1 to its maximum, it has no effect with C1 on the high frequencies of the image.Thus, the viewer can control the sharpness of the image, but the mode of maximum sharpness of the image makes any noise or interference of high frequencies appear clear on the image. By decreasing the sharpness of the image, the effect of these interferences and noises can be mitigated.In some devices, the image sharpness switch is called (Sharpness) or the Peaking Control or (Fidelity Control) switch.