An analogue television signal is several megahertz wide, so the long, medium and short wavelength ranges are too narrow for it. To transmit such signals, at least ultrashort waves are used. This situation has not changed with the transition to digital television.
Instructions
Step 1
The wavelengths allocated for broadcast television vary from country to country. In Russia, for analog broadcasting on meter waves, the D standard has been adopted, which provides for 12 channels. The first of them corresponds to a frequency of 49.75 MHz for transmitting an image signal and 56.25 MHz for transmitting a sound signal. In the last of them, the image and sound are transmitted, respectively, at frequencies of 223, 25 and 229, 75 MHz. Earlier transmissions on decimeter waves were carried out not in all cities, but today - in almost every one. The frequencies of the channels in this range are set by the K standard. On the first of them, which bears the number 21, frequencies of 471, 25 and 477, 75 MHz are provided for the image and sound signals. The last channel in the range was initially 41 (631, 25 and 637, 75 MHz), then 60 (783, 25 and 789, 75 MHz), and today this is channel number 69 (855, 25 and 861, 75 MHz). Amplitude modulation of the image signal, and frequency modulation of the sound. The attentive reader will consider that in all cases the difference between the frequencies for image and sound transmission is 6.5 MHz. In other countries, this difference may differ, for example, 5.5 MHz (standards B and G).
Step 2
There are large gaps between channels 5 and 6 and 12 and 21. It is impossible to arrange television broadcasting at frequencies falling into these intervals on the air - this can interfere with radio broadcasting and other types of communication. But they can be broadcast by cable, which is often practiced today. In the beginning, TVs were unable to work in these bands - set-top boxes were required. Now, almost all TVs can receive these channels, which received numbers from S1 to S40, on their own. The frequency differences for the transmission of picture and sound signals on these channels also comply with national standards.
Step 3
Digital television broadcasting is conducted at frequencies within the existing decimeter range, so the antennas that already exist can be used. Only between the antenna and the TV is it necessary to place a decoder attachment, or use a TV with a built-in decoder. But thanks to compression in digital broadcasting, it is possible to introduce so-called multiplexes, when several television channels broadcast on one frequency channel. Compression is even more efficient in DVB-T2 than in DVB-T. For cable broadcasting, the DVB-C and DVB-C2 standards are used.
Step 4
In satellite TV broadcasting, frequency ranges corresponding to units and tens of gigahertz are used. Previously, it was also analog, but frequency modulation was also used to transmit image signals. Now satellite broadcasting is carried out in the same bands, but using digital standards, in particular, DVB-S and DVB-S2.
