الموجة الحاملة والأقنية التلفزيونية ١-a .. كتاب التلفزيون الملون والعادي

1-6 Carrier wave and TV channels: The carrier wave of the visual signal is an electromagnetic wave to which a calculation law applies:

Wave speed m/s Wave length in meters = Frequency of the wave Vibration / second or Hertz or Cycle

Since the speed of the electromagnetic wave is equal to the speed of light, which is 300,000 km/s
Capacitive modulation (AM) is used to reduce the visual signal, which means the change in the amplitude of the modulated carrier according to the changes in the amplitude of the composite portable visual signal (Fig. (1) - (18). It is known that there is a method of frequency modulation FM (refer to our simplified book of engineering principles - Part Two Electron) in which the frequency of the modulated carrier wave changes according to the changes of the carried signal. The method of capacitive modulation AM was chosen because its frequency band requires a narrow field, which allows the opportunity for a greater number of channels While the frequency modulation is used to transmit the digital signal (sound) in normal and color television.
Carrier modulation, amplitude modulation, with a compound epitaph signal. The levels are shown as percentages of the maximum amplitude of the carrier wave as follows: 10% of the white level, the black level, 75-100 black level of black.

Figure (1 - 18)

Partial transmission of the side beam: The carrier wave modulated by the composite visual signal is accompanied by a large number of frequencies carrying the image signal, whose frequency ranges from (25) Hz to 5 MHz. And the frequency package is on both sides of the carrier wave. The bandwidth of the original and side frequencies is (10) megahertz (if we consider that the frequency of the modulation wave is 5 megahertz).

The upper side band is the one whose frequency is higher than the carrier frequency, the lower side band is the one whose frequency is less than the carrier frequency.

Both the lower and upper beams contain the entire modulation signal information.

Therefore, it can be limited to transmitting one side beam with the carrier, and thus we lose half of the modulation ability with the canceled side beam. It is called the (single) side beam transmission method. It also requires a higher cost in terms of the price of complex filters
But the benefit in sending one side beam is the possibility of investing the frequency domain in a larger number of channels - the frequency range that would have been occupied by the canceled side beam can be exploited in another transmission channel. Partial transmission of the side beam was used as an appropriate solution to the dilemmas and technical and economic disadvantages facing the full transmission or transmission with one side beam.

This type of transmission is based on partial suppression of one of the two side beams, i.e. leaving the second beam, which is the upper one, complete, and extending to 5 megahertz, and leaving a part of the lower side beam adjacent to the carrier, which is 125 megahertz. This type of transmission creates distortions in the image, because it makes the modulation ratio of the low frequencies of the visible signal greater than the modulation ratio of the high frequencies, that is, the voltage of the modulated signal after detection varies in its value between the high and low frequencies. This distortion is corrected by tuning the receiver in a special way, so the response curve of the mid-frequency amplifier takes the form shown in Figure (1 - 19 ).