When we talk about digital broadcasting, we often talk about a better quality picture and sound, HDTV, MPEG compression and more choice. However, most people do not know about the technology that lies at the heart of digital broadcasting and the added benefits of digital technology, not the best picture and sound quality.
The roots of the transition to digital television broadcasts are in a more efficient use of radio spectrum. In the analog world of television, we radio-frequency channels (frequencies), where each frequency transmits a single channel (program) and to avoid interference as the frequency can be used only once away. Digital technology allows the use of advanced compression algorithms to compress audio and video signals, so that we can use one frequency channel to transmit more than one service (usually three to ten or more channels), and we can build a network of transmitters operating on the same frequency significantly lower than the number of frequencies (channels) needed to cover the territory.
There are several standards for digital television broadcasting. For terrestrial television countries, the use of systems such as DVB-T, T-DMB, ATSC and DVB-T. DVB-T, perhaps the most widely used - it is used in Europe, Russia, Australia, India and many other countries. All of these systems based on COFDM - Coded Orthogonal Frequency Division Multiplexing. This modulation scheme with many thousands of closely spaced bears, each of which digital information.
Frequency plans for DVB-T based on the selections - an area where all the transmitters transmit on the same frequency. How can we pass on the same frequency without interference? Well, there are obstacles, but to some extent 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 because the signal is digital signal is received. Character length - digital data with which each carrier is modulated - it's more than the difference in arrival times from different transmitters. In addition, each symbol duration with guard interval. During the guard interval same characters with different arrival times can be obtained without any interference between the characters. This is a basic principle of a single frequency network (SFN).
Of course, the maximum distance between transmitters operating at the same frequency depends on the length of guard interval. With proper planning SFNs distance of 70 km can be achieved. This means that all transmitters in the 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.
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