دارة تثبيت مستوى السواد (BlackLevel Clamping) .. كتاب التلفزيون الملون والعادي

11-20 BlackLevel Clamping circuit:

This circuit is found only in the color device, and its work is based on fixing the bias of the cathode of the screen valve on the bias of the cut-off, which makes the screen in a state of darkness (black) when there is no color signal - it reflects the black and white TV in which the screen appears bright white when there is no signal Sent from and uses the horizontal reverberation pulse to timing the process of stabilizing the black level, as it is applied to the modulation and delay circuit composed of the RC network, and we get the appropriate modulation and delay, and in Figure (19-20) the two transistors maintain the continuous bias of the Q base at the level determined by the position of the switch Lighting - and that circuit whistles automatically during the horizontal regression period to prevent conflict with the program during the time of the back threshold of the horizontal extinguishing pulse as follows:

​​​​​​The horizontal reverberation pulse, after passing through the modulation and delay circuit, reaches the Q base in the form of a positive pulse, and its phase reverses to apply to the Q2 base and lead it to the saturation state. The large current in 02 leads to charging the capacitor C to a potential equal to the collector voltage of the transistor Q2, and this positive charge is applied to a base as a bias voltage during the next scan line. It has the charge of the capacitor C.

After the end of the pulse, 2 and 02 return to the cut-off state, and the capacitor C regulates the Q bias - that is, the level of screen illumination - and the circuit remains on. The pulse of extinguishing the next horizontal line, and the horizontal and vertical extinguishing must be secured, to make the screen valve Cut OFF during the periods of horizontal and vertical rebound - and stopping the extinguishing circuits (erasing) makes the white return lines appear on the screen - and for this reason the The colored devices are provided with a special extinguishing circuit that is fed with horizontal and vertical pulses, so that these pulses operate the extinguishing circuit during the regression periods, to bring the bias of the screen valve radiator to the cut-off state, thus dimming the screen, and this process takes place either by affecting the cathodes or the networks controlling the screen valve. Or by influencing circuits to amplify the luminance signal and deliver it to the pieces.

12-20 Circuit of the Notch Filter:

This circuit is for color TV only, and it suppresses frequencies close to the color carrier frequency (4.43) MHz, it filters the components of the color signal C and distances them from the lighting amplifier Y, and this helps mitigate the effect of the interference of the color signal on the lighting information Y - and this circuit only works Through the color transmission, its effect is eliminated in the black and white transmission automatically by using a color killer voltage, because the effect of the rejection filter distorts the clarity of the small details of the image as it suppresses some of the light signal compounds (luminance) with the color signal. Therefore, it is automatically separated when receiving black and white broadcasts.

The rejection filter consists of a special resonant circuit with a resonant frequency (4.43 MHz) Fig
​​​​​​(20 - 20) The presence of the filter leads to the occurrence of a loophole at this frequency, at which time all the components of the color signal are removed, and the luminance information (Y) remains only during the reception of the color broadcast, and the voltage coming from the color killer is positive and its value is ( 11 volts) and due to its application to the diode D: it makes it in a state of forward connection, a short state, and then the coil L works with the capacitor as a serial resonant circuit with zero impedance at the frequency (4.43 MHz). That frequency is canceled from the composite visual signal, so only the signal L passes through. As for when receiving the black and white broadcast, the positive voltage given by the color killer circuit is cut off, so the binary D: becomes cut off. This will disable the discard filter .

20-13 ABC automatic lighting control circuit:

This circuit is used in some colored devices to control the screen lighting automatically, and proportional to the external lighting of the location of the device - and it is done by means of a photocell of cadmium alloys (CDS), which strongly scatters the external light by converting the light into an electrical resistance that is inversely proportional to the intensity of the lighting and the change in the value of the resistance It leads through the transistor to change the voltage of one of the poles of the integrated circuit, which is the contrast pole - the color that needs a voltage between
(5 - 9 volts), because changing the resistance of the photocell connected to the base of the transistor changes the collector current of the transistor, which in turn changes the voltage of the special pole in the integrated circuit that we mentioned and thus adjusts the color and contrast in proportion to the external lighting, and the work of the automatic lighting control circuit can be canceled By a special key to return tuning
Lighting and contrast manual keys.

Bell filter: Bell filter:

​​​​​​The PELL filter extracts the chrominance signal and separates it from the luminance signal (Y). This filter is tuned to the sub-carrier of the color whose central frequency is (286 MHz) - and its characteristic curve resembles a bell Fig.