DVB-T is an abbreviation for Digital Video Broadcasting — Terrestrial. It means transmitting digital signal (multiplex) and using radio frequency spectrum much more efficiently. Digital broadcasting has many advantages over the old, analog broadcasting that is still in use today.
Terrestrial broadcasting means transmission of TV channels via transmitters which are usually put on high transmitting sites. Until recently there was only analog broadcasting which means one transmitter operating on some frequency and transmitting only one TV channel. Digital video broadcasting (DVB-T) means broadcasting a package of TV channels called multiplex. This kind of broadcasting has many advantages: efficient usage of radio frequency spectrum, better sound and picture quality, possibility of additional services, possibility of high-definition picture (HDTV) and wide screen picture format to mention just the most important properties.
Switching to digital
Because of these advantages we will switch to digital broadcasting (DVB-T). This applies to the whole Europe and later to other countries. There are many standards for digital video broadcasting, DVB-T is one of the widely adopted. It will be used in Europe, Russia, Australia, India and many other countries (see picture below). Each country has its own date for analog switch-off. In Europe most countries will switch off analog Tv transmitters by the year 2012.
Brief history of television broadcasting
Radio frequency spectrum is a natural and limited resource. Usually we are not aware of this fact. Broadcasting (radio and television) is just one of the services which uses this spectrum. One part of radio frequency spectrum is allocated for television services and is divided into channels. Each channel has central frequency and bandwidth (e.g. in UHF frequency band channel bandwidth is 8 MHz). Broadcasting of television picture over radio waves was developed decades ago. Because of physical laws which define propagation of radio waves, transmitters are installed on high sites to have large coverage area. Radio waves, of course, know nothing about country borders. Therefore countries need to reach agreement on frequency usage in order to avoid interference. First major agreement in this regard was reached in 1961 in Stockholm, Sweden, where a conference was held. This conference defined frequency plan which defined technical parameters for every television transmitter allowed to operate. The implication of this agreement was that only a limited part of radio frequency spectrum allocated to television services was assigned to each country. This agreement (also called Stockholm 1961agreement or ST61 for short) also defines precise procedures to be taken before each transmitter is put into operation.
Radio frequency spectrum for DVB-T
Needs for radio frequency spectrum allocated to television services have increased significantly. Every country has come to the point where adding new TV services was not possible because all radio frequency channels (assigned to each country) have already been used. Because of advances in digital technology, efficient compression of audio and video signals and digital broadcasting has become feasible. This altogether has brought to the conclusion that one radio frequency channel can be used to transmit more than one TV program (as is the case of analog broadcasting). Digital video broadcasting (DVB-T) means broadcasting a multiplex, a package of various services. Because of this new technology and a need to redefine radio frequency spectrum allocated to television a conference was held in Geneva, Switzerland in 2006. On this conference countries have agreed on new assignments of radio frequency channels taking into account that broadcasting will be digital (Europe is using DVB-T standard for terrestrial digital video broadcasting). The conference (its agreement is called Geneva 2006 agreement or GE06 for short) decided also that analog television broadcasting should cease by 2015 (each country can decide to switch off analog television transmitters earlier).
DVB-T in detail
Digital Video Broadcasting - Terrestrial (DVB-T) is one of the standards for the broadcast transmission of digital terrestrial television. It is a part of the DVB consortium standards. This system transmits compressed digital audio, video and other data in an MPEG transport stream, using COFDM modulation. DVB-T is used in European countries, Russia, Australia, India, north African countries, middle east, Taiwan and many other countries. There are also other systems for digital terrestrial television: ATSC (USA and Canada), ISDB-T (Japan and Brazil) and DMB-T (China).
OFDM (orthogonal frequency-division multiplexing) is a transmission technique which works by splitting the digital data stream into a large number of slower digital streams, each of which digitally modulate a set of closely spaced adjacent carrier frequencies. In the case of DVB-T, there are two choices for the number of carriers known as 2K-mode (actually 1705 carriers approximately 4 kHz apart) or 8K-mode (actually 6817 carriers approximately 1 kHz apart). DVB-T offers three different modulation schemes (QPSK, 16QAM, 64QAM). DVB-T has been adopted or proposed for digital television broadcasting by many countries, using mainly VHF 7 MHz and UHF 8 MHz channels (Taiwan uses 6 MHz channels).
The DVB-T Standard is published as EN 300 744, Framing structure, channel coding and modulation for digital terrestrial television. This is available from the ETSI website, as is ETSI TS 101 154, Specification for the use of Video and Audio Coding in Broadcasting Applications based on the MPEG-2 Transport Stream (it is not limited to MPEG-2 coding), which gives details of the DVB use of source coding methods for MPEG-2 and, more recently, H.264/MPEG-4 AVC as well as audio encoding systems. Many countries that have adopted DVB-T have published standards for their implementation (the D-Book in the UK, the Italian DGTVi , the ETSI E-Book , Scandivia NorDig). For DVB-T in Slovenia, Post and Electronic Communications Agency of the Republic of Slovenia (APEK) has published Specifications for Slovenian DVB-T receivers that represent a minimum requirements for DVB-T receivers to be used in the Republic of Slovenia.
DVB-T has been further developed into newer standards such as DVB-H (Handheld), now in operation, and DVB-T2, which was recently finalized. BBC R&I was one of the first teams who has developed a modulator and demodulator for DVB-T2 (check their blog How to build a DVB-T2 modulator & demodulator). DVB-T as a digital transmission delivers data in a series of discrete blocks at the symbol rate. DVB-T is a COFDM transmission technique which includes the use of a Guard Interval (interval between symbols). It allows the receiver to cope with strong multipath situations.
Single frequency networks (SFN)
Within a geographical area (allotment), DVB-T also allows single-frequency network (SFN) operation, where two or more transmitters carrying the same data operating on the same frequency. In such cases the signals from each transmitter in the SFN needs to be accurately time-aligned, which is done by a sync information in the stream and timing at each transmitter referenced to GPS. The length of the Guard Interval can be chosen between 1/4, 1/8, 1/16 and 1/32 of the of the symbol length. It is a tradeoff between data rate and SFN capability. The longer the guard interval the larger is the potential SFN area without creating Inter Symbol Interference (ISI). It can be possible to operate SFNs which do not fulfill the guard interval condition if the self interference is properly planned.