دارة التحسس .. كتاب التلفزيون الملون والعادي

10-19 Sensitivity circuit:

Two comparator circuits (IC) are used to determine the presence of a station signal, and if it is picked up, it determines the tuning accuracy. The diagram shows a simplification of the sensor circuits.

Five signals are applied to it, which are vertical synchronization - AFT voltage - image-carrying IF frequency (38.9) MHz - the composite visual signal - audio-carrying IF frequency (33.4) MHz in the European system or
(45.75 MHz) for the IF frequency of the image carrier and (125 MHz) for the IF frequency of the sound carrier in the American system. These five signals are applied to a single comparator. The comparator handles the signals and applies the logical signals to the microcomputer. The output of each comparator is either in the logical high or low mode. Which should be related to the relationship between its entries as follows:

1 - When the positive input (+) is high relative to the negative input (-), the output is in a logical high mode.

2 - When the positive (+) input is low relative to the negative (-) input, the output is in a low logical mode.

For SEEK and AFT mode, the sensitivity inputs to the microcomputer indicate the presence of a station signal or not, and the accuracy of the tuning. In the case of the correct tuning of the existing signal, the five outputs are all in the high logic, and the sensors perform their function regardless of the operating modes, and the microcomputer ignores the inputs of the sensors when they are in the “Seekoff” mode.

The signals from the circuit boards are proportional to the DC voltage applied to the inputs of the respective sensors. For example, a voltage of about 8.5 volts is derived from the pulses of vertical synchronization and is applied to the negative terminal of the comparator 1 and the positive terminal of the comparator 2. If the signal is properly tuned, it forms a correct level voltage that gives a high logic at both ends of the comparator - but if the received signal is poorly tuned, The voltage of the vertical sync pulses drops to a level below the negative terminal of the comparator.
Thus the polarity reverses and the output becomes logic low. In the absence of synchronization pulses, the input voltage to each comparator increases and the output of the comparator 1 opens to logic low. The AFT voltage is applied to the comparators (3-4-5) and the output of the comparator (3-4) is connected to the output of the comparator 5 and determines whether the compensated frequency is too high or too low and the computer behaves with the sensors’ signals, thus ensuring the correction of fine tuning and in the correct direction. Comparators (3 and 4) open to low logic, so the input of the microcomputer is low, and when the outputs of these comparators (1) are uniform, increasing or decreasing the AFT voltage gives a low logic in the microcomputer, and when the AFT voltage increases only, it gives a low logic from the distinguished comparator 5 ( 2) When the AFT voltage increases both discriminators 21 are in logic low - and when the AFT voltage decreases only discriminator 1 is in logic LOW. So the microcomputer acts on these inputs and fine-tunes in the right direction.

When the outputs of the comparators (6-7-8) are common, the absence of one or all of the input signals results in low logic in the microcomputer. The output of these comparators connected with comparator 1 determines whether the signal is present or not.

Channel Indicators: Channel Indicators:

These indicators depend on seven lines (LED) that show the numbers with light-emitting diodes. They consist of two units, one for the ones and the other for the tens number. The integrated circuits do the decoder work, the driver circuits or the incentive (DRIVER) and the Latch circuits. These circuits receive the binary numbers from the inputs A-B-C-D connected in parallel and a pulse from the terminal (EL) to change channels from the direct input. The viewer must press two buttons. When pressing the first number, the binary number is applied to both integrated circuits by means of the microcomputer. But the pulse (EL) is presented to the IC tens, so it activates the (Latch) circuit, so it receives the input of the diode, and if the IC circuit does not receive

dozens pulse from the (Enable) input it ignores the binary inputs, and when the viewer presses the second number the microcomputer presents the binary input signals to both integrated circuits (IC) but since only the single IC receives the (Enable) pulse so it accepts the binary input signals which it ignored IC scores..

​​The number that enters each IC is maintained by the Latch internal circuit until it is canceled or another request is made, and the decoder circuit converts the binary numbers into the equivalent decimal numbers. For example, if we want to display the channel number (21), a wave voltage from the IC impulses is applied to the elements (C, B) of the tens LED display unit and the elements A - - - - - of the single LED display unit, and the diameters of the IC (LT) connected to the branching ensure a quick check The LED lines are since they are grounded. The LED lines will show the number 8 in the event of safety of the circuit and the elements - DC voltage is applied to a pole for both integrated circuits and this pole is usually used for multiplier applications