Digital Television Broadcasting and Single Frequency Networks

When we talk about digital broadcasting we usually talk about better picture and sound, high definition television, MPEG compression and more choice. However, most people are not aware of technology that lies behind the digital broadcasting and what are the advantages of digital technology other than better picture and sound.

The roots of transition to digital television broadcasting lie in more effective use of radio-frequency spectrum. In analog television world we have radio-frequency channels (frequencies) where each frequency transmits one TV channel (program) and in order to avoid interference the same frequency can be used again only far away. Digital technology enables us to use advanced compression algorithms to compress audio and video signals, consequently we can use one frequency channel to transmit more than one service (usually three to ten and even more TV channels), and we can build a network of transmitters operating on the same frequency thus significantly lower the number of frequencies (channels) needed to cover a territory.

There are a few standards for digital television broadcasting. For terrestrial television countries use systems like ISDB-T, T-DMB, ATSC and DVB-T. DVB-T is maybe the most widely used - it is used in Europe, Russia, Australia, India, and many other countries. All these systems are based on COFDM - Coded Orthogonal Frequency Division Multiplexing. This is a modulation scheme with many thousand closely spaced carries, each carrying digital information.

Frequency plans for DVB-T are based on allotments - areas where all transmitters transmit on the same frequency. How can we transmit on the same frequency without causing interference? Well, there is interference but within some limits it is constructive. It helps in demodulating the signal. The fact is that at any point signals from different transmitters arrive at different times. But since the signal is digital the same signal is received. The length of the symbol - the digital data with which every carrier is modulated - is longer than the difference of arrival times from different transmitters. In addition to this each symbol is prolonged with a guard interval. During the guard interval the same symbols with varying arrival times can be received without any inter-symbol interference. This is the basic principle of Single Frequency Networks (SFN).

Of course, the maximum distance between transmitters operating on the same frequency depends on the length of the guard interval. With proper planning of SFNs the distance of 70 km can be achieved. This means that all transmitters in this area are operating on the same frequency and are broadcasting the same content. This is a huge advantage over analog television where we would need many frequencies to cover the same area.

It is obvious that digital broadcasting has many advantages. Digital networks need to be synchronized to operate properly in SFNs and this can be sometimes tricky. It is also true that the cost of digital network is high but it is divided among many services. And the most important fact is that the cost of the digital network is significantly lower than the cost of the most valuable resource - the radio-frequency spectrum.

More about transition to digital terrestrial television and DVB-T multiplexes in European countries can be found on this interesting website.