التعديل السعوي والتعديل الترددي كتاب التلفزيون الملون والعادي

Chapter eighteen

Sound stages in monochrome and color television:

introduction :

The audio signal in all TV sets is transmitted using the FM frequency modulation method, and the choice of this type was due to several important factors. It is known that today there are two systems for radio transmission and reception: AM capacitive modulation and FM frequency modulation.

The FM method has proven to have great reception advantages under various conditions. It is less affected by interference than the AM method and can reject unwanted signals from neighboring stations and channels. More economical transmission within a specific area with higher yield, especially at high frequencies.

1- Capacitive modulation and frequency modulation:

A - Amplitude Modulation (AM): It is the change in the amplitude of the amplified signal or the carrier amplitude according to the change in the amplitude of the carried wave (sound or music - image). The frequency remains constant. The amplitude of the carrier signal is changed by what is called capacitive modulation, Figure (1-18).

B- Frequency Modulation (FM): It is a change in the frequency of the carrier signal according to the change in the frequency of the carrier wave (sound, music, or image), and the amplitude remains almost constant in Figure (2-18). The medium frequency (IF) signal for television sets has been determined In the European system with (55 MHz) and the range of FM radio stations between (88-108) MHz.

The sound wave is defined as having a maximum frequency of (15) kilohertz. The carrier wave is accompanied by several side beams separated by a frequency equal to the modulation frequency. These packages contribute to the sincerity of the sound honesty. Therefore, a number of them are taken to improve the receiver sound. The bandwidth of the frequency modulation on both sides is about (200 kHz), while the bandwidth of the capacitive modulation does not exceed (30 = 215 kHz).

This made the frequency modulation method more suitable in very high frequency waves. (as in television transmission and reception) due to its wide bandwidth
required by each station. This is not possible in long, medium and short wave radios because it unacceptably reduces the number of operating stations. In Figure (3-18) there is a comparison between capacitive and frequency modulation.

Advantages and disadvantages of capacitive modulation and frequency modulation: (FM-AM)

A- The distinguishing advantages of FM frequency modulation 1
1 - Better reception in different conditions.

2 - It is easier to exclude weak stations that transmit on the same frequency range and to receive the required strong station.

3- The cheapest transmission cost because it requires less capacity.

4 - Easier to separate the sound signals from the picture signals.

e - The bandwidth is large, which enables the transmission of all sound frequencies up to (15 kHz).

The disadvantages of the FM modulation are:

1- The receiver is more complex.

2- The method of tuning the receiver is more complicated than that of the AM receiver

B - Characteristic advantages of AM capacitive modulation:

The receiver circuit is simpler.

2- The way to set the receiver is very easy.


The disadvantages of AM are:

1- Exceeding the adjustment causes distortion of the sound.

2- The interference on the signal is greater and appears in the speaker (see the effect of interference on both methods in Figure (4-18).

3 - The difficulty of removing the weak stations from the desired strong stations if they are
frequency convergent.

4- You need more power in the transmitter.

e- The bandwidth is narrow, not exceeding 4.5K (Hz), which limits the sound vibrations.