عمل كاشف الإشارة المرئية .. كتاب التلفزيون الملون والعادي

Chapter

Chrominance and processing of luminance and color signals (C,Y)

20-1 Function of the visual signal detector:

To obtain the signals of the three primary colors (red - green - blue) - the detector processes the visual signal that contains (1) the luminance signal (Y) and is called the luminance signal. This signal carries the necessary information for the image in black and white - (2) - The color difference signals (G-Y), (R-Y), (BY). The luminance and color difference signals drive the component screen diode, to obtain an image identical to what was photographed and broadcast by the imaging camera and the transmitting station, and the detector must achieve a constant pass curve up to (5 MHz). And his work should be in writing so as not to cause any distortions in the signal. Therefore, it requires a signal with a relatively large voltage compared to the visual detector in monochrome television. It is known that the luminance signal covers its frequency range over the entire frequency range of the television channel in the form
(1 - 20) While the color signal gives a frequency range in the upper part of the frequency range of the luminance signal and is located between the frequency
(4-5 MHz) which is the sub-carrier frequency of the coloration signal.

The operations carried out by the detector vary according to the television system that receives it. In the SECAM system, the two sub-carriers of the two color difference signals are frequency modulated. Then they enter the luminance frequency band to modulate the composite visual signal capacitively.

Therefore, the SECAM detector works in the manner of a capacitive modulation detector, and we obtain at its output the luminance signal (Y) whose bandwidth extends from zero to MHz, and we also obtain from the detector’s output the two sub-carriers of the color difference signals at the frequencies (4,406 MHz) and (4 250 MHz) modulated by the two color difference signals by frequency modulation - and each scan line contains only one of them.

The signal comes out from the output of the capacitive detector to an electronic circuit designed to give the two signals of difference or color difference (R-Y) and (B-Y), from the frequency-modulated sub-carriers that are within the frequency band of the luminance signal (Y) and a circuit called the decoder analyzer. By securing the two color difference signals from the two frequency-modulated sub-carriers that do not exist together in any line, the discriminator or the ratio detector performs the process of detecting the frequency modulation, because the sub-carrier is frequency-modulated as we mentioned earlier.

After obtaining the two color difference signals (R-Y) and (B-Y) - special circuits called the Matrix extract the third color difference signal, which is (G-Y), and these four signals (B-Y) - (G-Y) (Y-R) (Y) cooperate in operating Screen valve and color image production.

The extraction of the (G-Y) signal takes place in special circuits that operate according to the equation EY = 10,30ER + 10,59 + 0,11EB, and from which any unknown is extracted by knowing the other known values. The negative signal of luster or luminance (7) is fed to the three main cathodes.

And the color screen LED makes the final matrix to become the stream of the three color signals (RGB) and the luminance signal (Y). As for the NTSC and PAL systems, the modulation is an amplitude modulation of the luminance signal, which contains in its spectrum the two sub-carriers modulated orthogonally by the two color difference signals with the same carrier frequency suppressed. To detect these two signals, a capacitive detection process is first performed to extract the luminance signal (Y) and amplify it in a signal amplifier. The luminance is then fed to the image diode - while part of the decoder output signal is directed to regenerate the sub-carrier that was suppressed when transmitting, and this carrier is generated by a crystal oscillator and its recovery is necessary for the detection process

In the (PAL-D) system, that is, PAL with delay, there is a delay line that delays one of the two color difference signals by a horizontal scan line of 64 microseconds, similar to the SECAM system, and we will explain this in detail. In the simple PAL system (PAL-S), there is no delay line, but it It gives an acceptable image at good reception.