المخطط الصندوقي لقسم اللون .. كتاب التلفزيون الملون والعادي

20-3 The box diagram of the color section:

The signal included in this section consists of the luminance signal (Y) and the sub-carrier frequency signal modified by the color information changes of the televised image. Since the signal, after the modification, has undergone a remphase process, the encoding analyzer (DECoder) performs operations opposite to what was done in the transmission in the decoder.
(Coder). The operations that the coding analyzer goes through in its electronic circuits are:

1- The process of lowering the peak (H.F), which is the opposite of the process of raising the peak in the transmitter. And the process of reducing the peak is performed by a filter and a bell amplifier (Bel), which we curved in the form of a bell. Its function is to improve the signal-to-noise ratio it was exposed to during its path from the transmitter to the receiver.

2- Obtaining the two signals that form the consecutive signal (in the SECAM system), so that the two signals appear at the same time.

3- The detection process for colored signals.

Summary of the visual signal path:

If we go back to the stage of the visual signal, we find that the signal enters the second detector of the visual signal after it has left the intermediate frequency (IF) stage of the image. Here, the modification of the signal is detected to give the black and white information or the luminance signal to and the side bands of the color signal (C). Various synchronization pulses and a splash appear. Color (burst) and AGC control signals.

There are several ways to process these signals, but in the end they have one goal. Figure (4-20) illustrates one of these methods. The signal that comes out of the second detector of the visuals is fed to the amplifier of the visuals, where both signals amplify the black and white signal and the color signal. The black-and-white signal is directed to the second amplifier for the visuals, and from this stage it goes to the matrix network or to the cathodes of the screen valve. The color signal (chroma) is taken from the first amplifier for the visuals and connected to the pass-band amplifier in the color section. Color TV may contain an amplifier One or two pass amplifiers - The color signal is also fed to the splash amplifier, using precise gates that separate the splash signal from the color signal.

As for the brightness and contrast, they are controlled in the visual output, as in a monochrome TV

delay line:

The delay line delays the luminance signal Y from the color signal C by no more than 1 microsecond, and this delay is necessary for the two signals to reach the screen diode at the same time. Because the color signal passes through narrow band amplification circuits, which causes a delay in reaching the screen valve, and the lack of a delay line spoils the synchronization of the arrival of the two signals together, and the image appears in different colors. The delay line is located between the output of the visual detector and the first luminance amplification stage (Y).

The delay line in most of the devices is a coil whose number ranges between (1000-20000) a coil winding on a cylinder of a special insulating material, so each coil with the cylinder forms a capacitor, which forms L.C resonant circuits with the number of turns Figure (20). Thus, the time for the X signal to pass is about 106 to 1 microsecond. It matters.

The task in the delay line is to match its input and output impedance so that resonance and reflections do not appear in the image, and this is done by controlling the values ​​of its elements (Fig. As we explained in a previous chapter, as shown in Figure (6-20).
Luminance Amplifiers (Y)

They are broad-field amplifiers that enlarge the visual signal, whose frequency ranges between (20) Hz to 3.5 MHz, to obtain a clear image and colors on the screen, so that the details of low and high frequencies appear well.

In the stages of visual amplification, the method of direct connection or resistance circuits - capacitor RC is used, because the methods of connection to the transformer lead to loss in some high frequencies - and the method of peak files (peak) is used to increase the response to high frequencies. These coils are connected in a parallel, serial or mixed manner, as in Figure (9-20). The peak coils with the stray capacitances of the amplifier circuit form a resonant circuit, the resonant frequency of which is suitable for the high frequencies of the visual signal, and this is what makes the high frequency amplification increase after it decreased in the interconnect circuits. Increasing the magnification of the high frequencies over the low frequencies of the visual signal makes the image appear whiter than the original image.

Therefore, a special circuit is used to control the sharpness of the image (Sharpness), so it passes some high frequencies to the ground to compensate for the effect of the peak coils, if this effect is present. The control circuit for the sharpness of the image is similar to the tone control circuit. Effectively distorts the image.

In most cases, it consists of a transistor that acts as a variable resistance with a resonant circuit.