وظيفة كاشف الإشارة المرئية .. كتاب التلفزيون الملون والعادي

Chapter VII :

Visual signal detector

1-7 The Function of the Visual Signal Detector The function of the image detector in a monochrome or color television set is to clear the signal - which has passed through the intermediate frequency amplification stages - from its carrier wave and to obtain the visual signal that is used to feed the luminance (Y) and color (Y) sections. C), and the formation of the color image on the screen - as we obtain from it the voltage of the automatic gain control AGC - which controls the amplification of the medium frequency stages and the RF amplification in the channel selector, and this voltage is used as a bias voltage for the transistors of these stages and the visual detector in both mono and color devices is a modulation detector AM capacitive - because the image information is capacitively modified at the transmitting station - a non-linear binary or a synchronous linear binary is used for this, and the detector binary works to pass the positive or negative part of the signal only similar to a rectification circuit, and the polarity of the signal in the detector output is compatible with the following stages of the visual signal and when the output is The detector is negative (as in the American television system). The strongest illumination of the image element is accompanied by the passage of the smallest current intensity in the detector diode - while the highest current passes at the black region of the black image - that is, when the synchronization pulses arrive..and the negative signal cannot be applied to the screen valve network , but applied

on his landing. Conversely, the positive detector signal can be applied to a Siam network the screen .

It is worth noting that the application of the negative polar signal on the screen network shows the image in its negative form (negative), and we will find that the synchronization pulses can also be positive or negative polar (Fig. (7) - (1), (7 - 2) and (7 - 3). ) and the detector circuit can be with a grounded connection as in Figure (5-7)
2-7 Image detector in color TV: In black and white TV, the image and sound signals pass through the intermediate frequency (IF) amplification stages of the visuals, and then enter the image detector. And it contains the frequency of sound and image - by the super heterodyne method) - and produces the average frequency of sound, which is (5.5 MHz in the European system and (4.5 MHz) in the European system

The American

As for color television, it is undesirable for the image and sound signals to pass through the detector, in order to avoid frequency conflict between the sound signal and the carrier signal for color differential, because there is only about 1.25 MHz between them. Sub is not allowed
Suppress this frequency because it carries with it important information about the image. Therefore, the audio signal is separated before the visual detector stage. Because it enters only an independent visual signal that does not carry any audio signal - because we got rid of it by means of a trap tuned at a frequency of (334) MHz - and the signal that was taken before the trap enters a circuit similar to that of a detector. And its only job is to mix the carrier frequency of the image and sound to get The frequency that carries sound information, which is 5.5 megahertz (MHz) - a tuned filter is used at this frequency, which is of the crystal type, as in Figure (6-7).