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Chapter Nine

Video Amplifiers

1-9 Visual Amplifier or Luminance Amplifier (Y):

​​​​​​The signal comes out from the image detector or the visual signal detector, and carries the full information about the image, but it is not strong enough to operate the screen valve directly, and this requires the existence of a stage that amplifies this signal, which is (the visual signal amplifier).

The output voltage of the detector is about (2) (V) from peak to peak (P-P). While the screen diode requires a voltage of 80 to 100 (V) from peak to peak for the highest contrast voltage in the picture.

And since the operation of the color screen valve requires three or four visual signals: the basic color signals (Red - Green - Blue) RGB and the luminance signal (Y) or the color difference signal (BY) (Y) (Y) and the luminance signal (Y) and it is called the lighting signal or blink. The signal (Y) is essential in all cases because it represents the visual signal for black and white TV. It is: We get the color difference signals from the frequencies of the sub-carriers, which are integrated into the transmitting station within the frequency spectrum of the luminance signal (Y).
. In the stage of detecting the visual signal, the processes that occur differ according to the television system in force (SECAM-PAL-NTSC), which we explained in a previous chapter. In the SECAM system, for example, the subcarriers of the two color difference signals are frequency modulated (FM). Then they enter the frequency band of the luminance signal to modulate this composite visual signal capacitively (AM). When this signal is detected in the detector stage - which works as a capacitive modulation detector - we get at its output the luminance signal (Y), whose bandwidth ranges between (1-5 MHz), and it carries all the black and white image information. As for the second output of the detector, it gives the sub-carriers of the two color difference signals, and their static frequency is (4.406 MHz) and (4.250) MHz), which are modulated by the two color difference signals by frequency modulation, and there is only one of them in each line. The primary amplifier is received by the detector signal, which contains The luminance signal (Y) or (EY) and the chrominance signal - the presence of the color signal does not affect the operation of the amplifier because the sub-carrier frequency signal is suppressed to prevent this frequency from appearing on the image. The way to adjust it is frequency modulation in the SECAM system or capacitive modulation in the PAL system. Prevents the color signal from appearing at the luminance signal amplifier output. Figure (1-9) A and B shows the block diagram of the signal path after the visual detector in black and white television. and stained
With the box diagram, Figure (2-9) shows the path of the Y-luminance signal in color TV. This path consists of a pre-measerr with a filter that amplifies the luminance signal. The delay line Figure (4-9) delays the luminance signal in order for it to arrive at the same time as the signal of an analyzer Coding - to the color image diode, and the necessity of this delay is due to the fact that the luminance signal is wide-range, as it has a short build-up time and a short passage time, while the coloration signal is narrow-band, as it has a longer rise-up time and a long passage time - that is, if the luminance signal is not delayed, it reaches to the screen before the colors, and there is no compatibility between the image and its colors, and the appropriate delay time is approximately (15) (1) microseconds.As for the final amplifier, it amplifies the luminance signal to the appropriate value to operate the screen valve, and it ranges between (70-100 volts) and has more working conditions. Amplifier of the image signal in monochrome television - since the path of this signal must have a pass curve of appropriate amplitude and page, so that no distortion occurs in transmitting the rapid changes of luminance from white to black), and it must contain a frequency damping circuit. The sub-carrier of the color signal is sufficient - and the continuous component of the signal is restored because it plays an important role in the color image. of terms.