دارة الترشيح المشطية (Comb Filter) ١_a .. كتاب التلفزيون الملون والعادي

9-9 Comb filter circuit:

It is a new circuit in some color television sets, especially on the NTSC system. This circuit can widen the response curve of the luminance amplifier and make it simplified to an additional frequency of approximately 1 megahertz - that is, from (32) megahertz to 42 megahertz in the American system. It makes the details of the received image similar to the transmitted image in its details
. It improves the detail in the horizontal lines of the brightness elements by 25%, that is, the fine details.

The name of this circuit comes from its work in combing the frequencies, as it allows some frequencies to pass through and rejects some other frequencies in the same field.

It rejects the horizontal frequency multiples of the luminance signal which is 15734.26 x an integer. For example, x 2 = 31468.52 Hz or x 1 = 94405.56 Hz and so on while passing in the chroma signal the horizontal frequency: 2 x a single whole number such as 173426: 2 x 3 = 23601 Hz or x 7 = 55069.91 Hz or x
35 2753495 Hz and so on.

The chrominance signal frequencies lie between the gaps of the luminance frequencies. Using the comb filter circuit works to comb the chrominance signals from the composite visual signal, and at the same time, the entire frequency range of the luminance signal enters the luminance amplifier section.

The simplified principle of the comb filter circuit: The principle of this circuit depends on Figure (9) - (17) on the entry of the composite visual signal into a delay circuit of 63.5 microseconds, which is the time of one horizontal line. If the signal frequency is 1573426 Hz at the input, then the delay line at the output will be 9360 delay - and the page difference at the input and output is the same. Therefore, any signal that has a frequency of multiples of the horizontal frequency has at the output the same page difference as the input wave - and it is noted in the box diagram that the composite visual signal (7) It also enters the inverter and its signal becomes (7), so when the wave at the entrance has the horizontal frequency, or its multiples, then the two signals in the adder circuit will have zero result - thus, the luminance signal will not have it at the exit of the circuit, i.e. value . But if there is any bias from the horizontal frequency multiplier, the output will not be non-existent, but there will be a signal in the output (colour) of the comb circuit. The output will be maximum when the input signal is a single whole number multiplied by (2) i.e. half of the horizontal frequency i.e. 13 7867 Hz. As we can see in the previous figure, the multiples of ( represent the color information - which is the only information that appears in the output of the primary comb circuit.

As for the amplitude of the color signal, it is doubled at the output of the comb filter circuit, because the first is the delayed signal (63.5) microseconds, and the second signal is direct (75) and inverted (-7) coming from the next line. There is no need to go into the details of the stages of this filter and its work, because it is only included in some devices, especially those with the American NTSC system.