الدارة الرقمية في المرحلة الراسية .. كتاب التلفزيون الملون والعادي

8-14 The digital circuit in the vertical stage:

Some TV receivers do not have a vertical or horizontal oscillator - and the principle of these circuits depends on digital circuits. It is part of an integrated circuit IC that secures the generation of both vertical and horizontal scanning frequencies. The work of the circuit related to the vertical stage can be summarized. This circuit works on the American system. The number of image lines is 525 lines, the vertical frequency is 60 Hz, and the horizontal frequency is 525 x 60 + 2 = 15750 Hz. But the actual frequency is 59.94 Hz and 15734 Hz ​​- Figure (14-10).

Derivation of the vertical pulse rate:

Vertical pulses with a frequency of 59.94 Hz are obtained by dividing the voltage-controlled oscillator (725) with a frequency of (31468 Hz) by (525). This is done by using a ten-stage counter as shown in the figure - and since the ratio is correct, there is no need for a vertical stability control. And choosing the number (31468) is to reach the required frequency for the vertical, which is 59.94 Hz, and since each field of the image is equal to
(2625 lines), so dividing the number (15734 by 2625) gives the required frequency, and this is not practical as digital dividers do not work on whole numbers. To solve this dilemma, the (VCO) works on the horizontal frequency class, i.e. 31468 Hz, and then it is divided by (525).

VCO stability:

The voltage controlled oscillator (VCO) frequency is stabilized by the DC voltage output .DC of the phase detector is similar to the action of the phase regulator loop, the phase detector compares the frequency and page of the received horizontal synchronization pulses. A wave of pulses of the horizontal output transformer - and any difference that occurs in the output of the DC potentiometer is used to correct the VCO frequency - and it is noted in this case that the VCO frequency. The wave that feeds the phase detector.

Counter work:

The vertical pulses driving the output circuit are now generated in an RC flip flop and fed through a vertical buffer (see Figure 10-14). However, the RS flip flop (vibrator) requires an operating pulse to start and give its output. It also needs a reset pulse to cut the output - these pulses are used for control

Provided with a ten-phase counter as follows:

1- The VOC (voltage controlled oscillator) oscillates at twice the horizontal frequency ie
(31468 Hz), and so the number (525) matches. Line No. (2625 line). which is the beginning of the next field. The count is now set to zero.

. 226.5 lines)

2- The new counting from zero causes the setting pulse to appear in the R-S flip flop oscillator - and it creates the vertical output pulse, and the number of zero always occurs after resetting the ten stages of the counter - and that happens after the number 525 (5, 226 lines).

3- The flip flop remains active until a pulse (Reset) reaches it from line number 16 of the ten-phase counter.

4- At the end of number 16 (8 lines) - the (vibrator) flip-flop is reset and terminates the vertical output pulse - thus the width of this pulse is (8×63.5 microseconds).

e - The count continues to 525, then the cycle is repeated - and this process is continuous as long as the station is in a state of reception by the device.

The vertical output pulses feed the vertical output circuit and vertical deflection coils at a rate of (59.94 Hz), in the system (60 Hz) only