DVB-T
Encyclopedia : D : DV : DVB : DVB-T
-->DVB-T stands for Digital Video Broadcasting - Terrestrial and it is the DVB European consortium standard for the broadcast transmission of digital terrestrial television. This system transmits a compressed digital audio/video stream, using OFDM modulation with concatenated channel coding (i.e. COFDM). The adopted source coding methods are MPEG-2 and, more recently, H.264.
In January 2006, a study group named TM-T2_SM (Study Mission on Second Generation DVB-T) has been established by DVB Group for investigation of advanced modulation schemes that could be adopted by a second generation digital terrestrial television standard [link].
Contents
Technical description of the transmitter
With reference to the figure, a short description of the single processing blocks follows.
- Source coding and MPEG-2 multiplexing (MUX): compressed video, compressed audio, and data streams are multiplexed into PSs (Programme Streams). One or more PSs are joined together into an MPEG-2 TS (MPEG-2 Transport Stream); this is the basic digital stream which is being transmitted and received by home Set Top Boxes (STB). 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 (see the bottom figure for a complete listing).
- Splitter: two different 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 protected better 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 (in this way, all receivers that are in proximity of the transmission site can lock the HDTV signal, whereas all the other ones, even the farthest, may still be able to receive and decode a SDTV signal).
- MUX adaptation and energy dispersal: the MPEG-2 TS is identified as a sequence of data packets, of fixed length (188 bytes). With a technique called energy dispersal, the byte sequence is decorrelated.
- External encoder: a first level of protection is applied to the transmitted data, using a nonbinary block code, a Reed-Solomon RS (204, 188) 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, such way it becomes more rugged to long sequences of errors.
- Internal encoder: a second level of protection is given by a punctured convolutional code, which is often denoted in STBs menus as FEC (Forward Error Correction). 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 (this is really done by two separate interleaving processes, one operating on bits and another one operating on groups of bits).
- 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 (1512, 3024, or 6048 symbols per block). 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 equalization phase, while TPS signals (Transmission Parameters Signalling) are used to send the parameters of the transmitted signal and to univocally identify the transmission cell.
- OFDM Modulation: the sequence of blocks is modulated according to the OFDM technique, using 2048, 4096, or 8192 carriers (2k, 4k, 8k mode, respectively).
- Guard interval insertion: to decrease receiver complexity, every OFDM block is extended, copying in front of it its own end (cyclic prefix). The length of such guard interval can be 1/32, 1/16, 1/8, or 1/4 that of the original block length.
- DAC and front-end: the digital signal is transformed into an analog signal, with a digital-to-analog converter (DAC), and then modulated to radio frequency (VHF, UHF) by the RF front-end. The occupied bandwidth is designed to accommodate each single DVB-T signal into 6, 7, or 8 MHz wide channels.
| Modulation | Coding rate | Guard interval | |||
|---|---|---|---|---|---|
| 1/4 | 1/8 | 1/16 | 1/32 | ||
| QPSK | 1/2 | 4.976 | 5.529 | 5.855 | 6.032 |
| 2/3 | 6.635 | 7.373 | 7.806 | 8.043 | |
| 3/4 | 7.465 | 8.294 | 8.782 | 9.048 | |
| 5/6 | 8.294 | 9.216 | 9.758 | 10.053 | |
| 7/8 | 8.709 | 9.676 | 10.246 | 10.556 | |
| 16-QAM | 1/2 | 9.953 | 11.059 | 11.709 | 12.064 |
| 2/3 | 13.271 | 14.745 | 15.612 | 16.086 | |
| 3/4 | 14.929 | 16.588 | 17.564 | 18.096 | |
| 5/6 | 16.588 | 18.431 | 19.516 | 20.107 | |
| 7/8 | 17.418 | 19.353 | 20.491 | 21.112 | |
| 64-QAM | 1/2 | 14.929 | 16.588 | 17.564 | 18.096 |
| 2/3 | 19.906 | 22.118 | 23.419 | 24.128 | |
| 3/4 | 22.394 | 24.882 | 26.346 | 27.144 | |
| 5/6 | 24.882 | 27.647 | 29.273 | 30.160 | |
| 7/8 | 26.126 | 29.029 | 30.737 | 31.668 | |
Technical description of the receiver
The receiving STB adopts techniques which are dual to those ones used in the transmission.
- Front-end and ADC: the analog RF signal is converted to base-band and transformed into a digital signal, using an analog-to-digital converter (ADC).
- Time and frequency synchronization: the digital base band signal is searched to identify the beginning of frames and blocks. Eventual problems on the frequency of the components of the signal are corrected, too.
- Guard interval disposal: the cyclic prefix is removed.
- OFDM demodulation
- Frequency equalization: the pilot signals are used to equalize the received signal.
- Demapping
- Internal deinterleaving
- Internal decoding: it uses the Viterbi algorithm.
- External deinterleaving
- External decoding
- MUX adaptation
- MPEG-2 demultiplexing and source decoding
References
- ETSI Standard: EN 300 744 V1.5.1 (2004-11), Digital Video Broadcasting (DVB); Framing structure, channel coding and modulation for digital terrestrial television, download from [ETSI].
See also
- DVB (Digital Video Broadcasting)
- Digital terrestrial television (DTT or DTTV)
- Digital television (DTV)
- ATSC
- ISDB
- DTV channel protection ratios
External links
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