التحكم بالمبدل الالكتروني في نظام سيكام .. كتاب التلفزيون الملون والعادي

20-8 Controlling the electronic switch in the SECAM system:

Since this system depends on the simultaneous or sequential exchange between the two colors (R-Y) and (Y), and is symbolized by . Each signal from it comes to one line and the other to the next line, and the two signals do not come together in one line. To ensure color synchronization, the color mapping signal is used. Which is added to the color signal in the transmitter to control the work of the pulse generator that drives and controls the electronic switch in the receiver. In the chapter on television systems, we have shown that we must have three signals at each moment to obtain a color image. Since we have only two signals at each moment, and we can obtain the third signal by using a line delay of (64) microseconds, which is the time required to clear one line. And we get one of the two signals (DR), for example, directly, and the second signal from the delay line, which is (DB), and in the second line, we get directly (DB) and the signal (DR) via . delay line. With us, the color of the image is not affected by using the previous or subsequent font information instead of the specified font, because the font color information is not much different from the previous or subsequent font information. Next or previous line.

​​​​​​The color signal is transmitted through nine lines and during the vertical extinguishing time in the horizontal lines of No. (7-15) in the single frame. And in lines from (320-328) in the even frame, the sign of each color differs from the sign of the other colour. Its impulses are trapezoidal, and each has a special waveform and height. The line mapping signal DR changes linearly over a period of 155 microseconds from zero to the value (125) for the luminance signal. And it stays at the value (1.25) until the end of the line - The color line assignment signal (BR) changes linearly within a period of 10 (20) microseconds from zero to the value - 1.52 and this value remains until the end of the line.

This corresponds to the Burst signal in NTSC and PAL. The electronic commutator is controlled by a pulse generator synchronized with the horizontal scanning pulses in the device. The color mapping signal from the transducer to the channel is (R-Y) when the transducer is on a correct page, and it should have a decreasing frequency in the intervals of lines (7 - 9 - 11 - 13 - 15) as in Figure (14-20) when the carrier is modified The sub with a negative polarity sawtooth pulse - and the color mapping signal arriving at channel (B) (Y) must be of increasing frequency in line intervals (8 - 10 - 12 - 14). When the modulation is carried out with a positive polarity pulse - then the positive polarity sawtooth pulses appear on the input of each discriminator during the direct signal and the delayed signal - and the sawtooth pulses coming out of the (R-Y) and (B-Y) channels are collected in the (G-Y) channel. The resulting sawtooth pulses are integrated, then projected onto the mapping unit and the color killer, in which the integrated mapping pulse is compared. vertical scan pulse. If the pulse is of the correct polarity, the comparator circuit does not give an output. But if the commutator is working with an incorrect page, the polarity of the sawtooth pulses coming out of the discriminator will be negative in polarity. Thus, an integrated pulse of negative polarity reaches the comparator circuit as well - and at that point the comparator circuit gives an output in the form of a pulse that reverses the page of the electronic changer - and at the same time a pulse is applied to the reserved amplifier that makes it close, preventing the passage of the two color signals until the changer works with a page Correct, and when receiving a black and white signal, the pulses for setting the line are absent, so the path of the chrominance signal is closed automatically, and this closure ensures that no noise appears at the output of the coding analyzer. The reason for the closure is due to the receipt of a pulse from the set and color killer unit, so it closes the reserved amplifier permanently.

Electronic switch circuit (SECAM):

Crystal diodes are used to switch the two color channels (Fig. 20-15). The simple switch consists of four crystal diodes, which are received by triggering pulses from a and b, with an inverse page latency. If a is positively polar, then point b is negatively polar. The diodes 1 D and D2 are connected. Then the signal is connected directly to the channel (R) (Y) and the delayed signal is connected to the channel Y -, and if the triggering pulses are of opposite polarity in a and b then 3 and 4 (D) become connected, and the direct signal is connected to the channel (B Y) and the back signal is connected to the (R) channel (Y).

An eight-bit switch is used, so that the signal path is through diodes each time, and this ensures a lower level of distortion, and limits the amplitude of the sub-carrier of the color signal, and the self-capacities become less because the capacity of the two diodes becomes connected in series. This is why it is better than the previous quadruple diode switch