When we talk about digital broadcasting, we often talk about the best picture and sound quality of high-definition, MPEG compression and a wider choice. However, most people do not know about the technology that lies at the heart of digital broadcasting, and what are the advantages of digital technology, not the best quality picture and sound.
The roots of the transition to digital television broadcasts are in a more efficient use of radio frequency spectrum. In the analog world of television, we radio-frequency channels (frequencies), where each frequency transmit channel 1 (the program), and in order to avoid interfering with the same frequency, can be used again only far. Digital technology allows the use of advanced compression algorithms to compress audio and video signals, and therefore we can use a single frequency channel for the transfer of more than one service (usually 9:57 or more channels), and we can build a network of transmitters operating on the same frequency, which is much lower than the number of frequencies (channels) needed to cover the territory.
There are several standards for digital television. For terrestrial countries, the use of systems, for example, ISDB-T, T-DMB, ATSC and DVB-T. DVB-T, perhaps the most widely used - used in Europe, Russia, Australia, India and many other countries. All of these systems are based on COFDM - Coded orthogonal frequency division multiplexing. This modulation scheme with the many thousands of closely spaced bears, each of which digital information.
Frequency plans DVB-T on the basis of appropriations - areas where all transmitters transmit on the same frequency. How can we pass on the same frequency without interference? Well, there is noise, but within certain limits, it is constructive. It helps to demodulate the signal. The fact is that at any point of signals from different transmitters arrive at different times. But as the digital signal and the same signal is received. The length of the symbol - the figures with which each carrier modulation - is more than the difference in arrival times from different transmitters. In addition, each character is extended with a guard interval. In the guard interval the same characters with different times of arrival can be obtained without any interference between the signs. This is the basic principle of a single frequency network (SFN).
Of course, the maximum distance between transmitters operating on the same frequency depends on the length of the guard interval. With proper planning SFNs distance of 70 km can be achieved. This means that all transmitters in this area, working on the same frequency and broadcast the same content. This is a huge advantage over analog television, where we have a lot of frequencies to cover the same area.
It is obvious that digital broadcasting has many advantages. Digital networks should be synchronized in order to work properly in SFNs, and it can sometimes be difficult. It is also true that the cost of digital network is high, but it is divided between many services. The most important thing is that the cost of digital network is much lower than the cost of the most valuable resource - the radio frequency spectrum.
