مراحل الصوت المنفصلة والمشتركة في التلفزيون .. كتاب التلفزيون الملون والعادي

1-2 Separate and Common Sound Stages in Television:

A - the separate (independent) method:

At the beginning of the manufacture of television sets, the separate (or independent) sound method was used. In which the sound passes through special independent stages. After leaving the mixer in the voter, the first medium amplifier of the composite signal enters, then separates to pass through the medium frequency amplifiers, then the sound detector, then the audio frequency amplifier, and its frequency is according to the European system CIR MHz. This method is no longer used at all due to the large number of its defects and the superiority of the second (common) method over it (Fig. 18-5).

B - the combined method:

It is the common method used in all types of black and white and color monochrome devices. In it, the sound and image signals continue to be accompanied together. The image detector is in monochrome, and there the carrier wave of the frequency of the audio signal, which is (33) MHz, is mixed with the carrier wave of the frequency of the video signal (ideo signal), which is MHz, and we obtain by the method of (super heterodyne) the difference between the two frequencies, which is (5.5 MHz). ). It is called the intermediate frequency IF of the audio signal, and it carries frequency-modulated sound information.

It is worth noting that the audio signal (sound) is suppressed at the beginning of the middle frequency stages of the image for fear of its interference with the image signal. But it is preferred by a special resonance circuit after the detector stage and goes through the following stages:

1 - Intermediate frequency (IF) amplifier stage.

2- The specified stage.

3 - audio signal detector stage.

4- The amplifier of the audio signal, then the speaker.

18-3 The block diagram of the sound stages in monochrome and color TV:

We find in Figure (5-18) that monochrome TV does not require a special detector for sound - as the medium frequency of the sound (IF) and the medium frequency of the image, collide with the super heterodyne conflict method in the image detector, to produce a frequency of 5.5 MHz. It is the sound carrier IF and this frequency is taken either from the output of the image detector or from the image amplifier circuit after it, in order to ensure the gain of the highest level of the audio signal. The mid-frequency signal is fed to the IF amplifier's 5.5 MPa input. As for color TV, it is very similar in sound stages to black and white mono TV. The problem of frequency difference arises between the average frequency of the audio signal (55) MHz and the frequency of the color sub-carrier (4,286 MHz), which is 16,500 MHz, and this conflicting frequency that may occur in the detector The second, it cannot be extinguished by a special trap in the visual circuit, because there is important epitaph information at this frequency - and the conflicting frequency of 1214 MHz must not be allowed to appear on the screen, and deleting or avoiding this frequency in the image depends on fine tuning - and the fine tuning switch selects The two frequencies carrying sound and image for the response. Most color TV sets deal with this problem as follows (Fig. 6-18).
At the output of the last stage of the intermediate frequency of the image, the signal is directed to two paths - the first to the second detector of the image, while the other path gives the signal to a separate frequency amplifier for audio. The separate detector for sound mixes the two medium frequencies of sound and image to give the frequency (55) MHz, which is the second medium frequency of sound, which carries all the audio information and passes the output of the output end of the last amplifier of the medium frequency, which feeds the second detector of the image through a group of traps and filters to delete the intermediate frequency for the sound before it reaches the second detector.

The average sound frequency of 5.5 MHz results from the difference between the sound carrier frequency of 0.33 MHz and the image carrier frequency of 38.9 MHz. The middle frequency of the sound is amplified at the amplification stage.