المخطط الصندوقي لوحدة محلل الترامز (decoder) في نظام سيكام .

20-4 The block diagram of the decoder unit in the SECAM system:

The box diagram (10-20) shows that the signal is taken from the output of the visual detector or from the first amplifier stage of the visual signal to the stage of separation of the discoloration signal. The discoloration signal located in the upper part of the frequency range between (4-5 MHz) is separated. And it is performed by a bell filter (BEL), and at the same time it performs the process of reducing the peak at high frequencies. The signal is then taken to an amplitude limiter to rid it of the capacitive changes in the amplitude during its course, then this signal is amplified and distributed in two separate paths, one direct and the other delayed, and with the help of the electronic switch, after which both the (R-Y) signal and the (BY) signal are obtained each on sharply and sequentially - and the delay line works to impede the arrival of the next signal for a full line time (64 microseconds) - as for the second line, it gives the signal directly to the electronic switch - and the two signals reach the same amplitude. The switch gives one of the two signals, such as (R-Y) to the upper path and the second signal (B-Y) to the lower path, so that each of the two signals travels in an independent path. The detection process takes place after obtaining these two frequency-modulated signals - the detection is frequency (FM) and at its output we get the differential signals (R-Y) or (BY) and their symbols are (ER-EY) and (EB-EY). Both detectors are tuned to a specific frequency - the frequency of the (R-Y) signal detector is tuned to a frequency (4406 Hz and the BY signal detector) is tuned to the frequency
(4,250 MHz) From the exposed signals, the signal (C-Y) and its symbol (EG-EY) can be obtained using a matrix, which performs the necessary operations to extract this signal.

20-5 Lighting and contrast control circuits:

These circuits are controlled by means of the brightness and contrast control keys, the first symbol is for illumination and the second symbol is O for contrast. This key changes the amplitude of the visual signal connected to the screen diode by controlling the gain of one of the luminance amplifiers, by changing the resistance of the emitter circuit, or by changing the voltage of the signal. In the base of the amplifier by means of a voltage divider circuit as in Figure (11-20). In modern devices, an integrated circuit is changed by controlling the voltage applied to one of the terminals of the integrated circuit, so the gain of the amplifier changes.

As for the lighting switch, it adjusts the continuous alignment of the screen valve, and it controls the current of the electronic beam, and thus the level of illumination of the image elements changes. The lighting switch is in a place that controls the transmission of the screen valve and controls its lighting. . This amplifier changes the bias of the cathode of the screen diode, so it changes the level of illumination and the shape (11-20) of the scheme of the lighting and contrast control circuit in the integrated circuit (TBA590).
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