DVB-T

Basics

Technical description of a DVB-T transmitter

• Source coding and MPEG-2 multiplexing : Compressed video, compressed audio, and data streams are multiplexed into MPEG program streams. One or more MPEG-PS's are joined together into an MPEG transport stream ; this is the basic digital stream which is being transmitted and received by TV sets or home Set Top Boxes. Allowed bitrates for the transported data depend on a number of coding and modulation parameters: it can range from about 5 to about 32 Mbit/s.
• Splitter: Two different MPEG-TSs can be transmitted at the same time, using a technique called Hierarchical Transmission. It may be used to transmit, for example a standard definition SDTV signal and a high definition HDTV signal on the same carrier. Generally, the SDTV signal is more robust than the HDTV one. At the receiver, depending on the quality of the received signal, the STB may be able to decode the HDTV stream or, if signal strength lacks, it can switch to the SDTV one.
• MUX adaptation and energy dispersal: The MPEG-TS is identified as a sequence of data packets, of fixed length. With a technique called energy dispersal, the byte sequence is decorrelated.
• External encoder: A first level of error correction is applied to the transmitted data, using a non-binary block code, a Reed-Solomon RS code, allowing the correction of up to a maximum of 8 wrong bytes for each 188-byte packet.
• External interleaver: Convolutional interleaving is used to rearrange the transmitted data sequence, in such a way that it becomes more rugged to long sequences of errors.
• Internal encoder: A second level of error correction is given by a punctured convolutional code, which is often denoted in STBs menus as FEC. There are five valid coding rates: 1/2, 2/3, 3/4, 5/6, and 7/8.
• Internal interleaver: Data sequence is rearranged again, aiming to reduce the influence of burst errors. This time, a block interleaving technique is adopted, with a pseudo-random assignment scheme.
• Mapper: The digital bit sequence is mapped into a base band modulated sequence of complex symbols. There are three valid modulation schemes: QPSK, 16-QAM, 64-QAM.
• Frame adaptation: the complex symbols are grouped in blocks of constant length. A frame is generated, 68 blocks long, and a superframe is built by 4 frames.
• Pilot and TPS signals: In order to simplify the reception of the signal being transmitted on the terrestrial radio channel, additional signals are inserted in each block. Pilot signals are used during the synchronization and equalization phase, while TPS signals send the parameters of the transmitted signal and to unequivocally identify the transmission cell. The receiver must be able to synchronize, equalize, and decode the signal to gain access to the information held by the TPS pilots. Thus, the receiver must know this information beforehand, and the TPS data is only used in special cases, such as changes in the parameters, resynchronizations, etc.
• OFDM Modulation: The sequence of blocks is modulated according to the OFDM technique, using 1705 or 6817 carriers. Increasing the number of carriers does not modify the payload bit rate, which remains constant.
• Guard interval insertion: to decrease receiver complexity, every OFDM block is extended, copying in front of it its own end. The width of such guard interval can be 1/32, 1/16, 1/8, or 1/4 that of the original block length. Cyclic prefix is required to operate single frequency networks, where there may exist an ineliminable interference coming from several sites transmitting the same program on the same carrier frequency.
• DAC and front-end: The digital signal is transformed into an analogue signal, with a digital-to-analog converter, and then modulated to radio frequency by the RF front end. The occupied bandwidth is designed to accommodate each single DVB-T signal into 5, 6, 7, or 8 MHz wide channels. The base band sample rate provided at the DAC input depends on the channel bandwidth: it is samples/s, where is the channel bandwidth expressed in Hz.

  Technical description of the receiver

• Front-end and ADC: the analogue RF signal is converted to base-band and transformed into a digital signal, using an analogue-to-digital converter.
• Time and frequency synchronization: the digital base band signal is searched to identify the beginning of frames and blocks. Any problems with the frequency of the components of the signal are corrected, too. The property that the guard interval at the end of the symbol is placed also at the beginning is exploited to find the beginning of a new OFDM symbol. On the other hand, continual pilots determine the frequency offset suffered by the signal. This frequency offset might have been caused by Doppler effect, inaccuracies in either the transmitter or receiver clock, and so on. Generally, synchronization is done in two steps, either before or after the FFT, in such way to resolve both coarse and fine frequency/timing errors. Pre-FFT steps involve the use of sliding correlation on the received time signal, whereas Post-FFT steps use correlation between the frequency signal and the pilot carriers sequence.
• Guard interval disposal: the cyclic prefix is removed.
• OFDM demodulation: this is achieved with an FFT.
• Frequency equalization: the pilot signals are used to estimate the Channel Transfer Function every three subcarriers. The CTF is derived in the remaining subcarriers via interpolation. The CTF is then used to equalize the received data in each subcarrier, generally using a Zero-Forcing method. The CTF is also used to weigh the reliability of the demapped data when they are provided to the Viterbi decoder.
• Demapping: since there are Gray-encoded QAM constellations, demapping is done in a "soft" way using nonlinear laws that demap each bit in the received symbol to a more or less reliable fuzzy value between -1 and +1.
• Internal deinterleaving
• Internal decoding: uses the Viterbi algorithm, with a traceback length larger than that generally used for the basic 1/2 rate code, due to the presence of punctured bits.
• External deinterleaving
• External decoding
• MUX adaptation
• MPEG-2 demultiplexing and source decoding

  Countries and territories using DVB-T or DVB-T2

America

• Europe

• .
• . Transition to DVB-T2 scheduled in 2022.
• DVB-T was only used experimentally in two cities, and is being phased out. The official terrestrial broadcasting standard in Romania is DVB-T2, and implementations started in 2015.

• Oceania

• Asia

• Africa

• DTT switch-off

• Switzerland : Swiss public broadcaster SRG terminated DTT network on 3 June 2019. A regional station from the Geneva area has kept broadcasting. A DVB-T2 antenna was later activated in the east of the country to relay Swiss TV to Austrian cable operators. A similar broadcast is planned to cover Grand Geneva.
• Turkey terminated DTT network on 1 June 2017.