In June 2006, a study group named TM-T2 (Technical Module on Next Generation DVB-T [=''DVB-T2'' ) was established by the DVB Group to develop an advanced modulation scheme that could be adopted by a second generation digital terrestrial television standard. TM-T2. Second Generation DVB-T , DVB.org It is expected that work on the DVB-T2 specification will be completed and passed to ETSI for standardization towards the end of 2007. Market deployment is expected for 2009.
With reference to the figure, a short description of the signal processing blocks follows.
- ''' (see the bottom figure for a complete listing).
- 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 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 (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).
- a first level of error correction 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 is used to rearrange the transmitted data sequence, such way it becomes more rugged to long sequences of errors.
- a second level of error correction 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.
- 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).
- the digital bit sequence is mapped into a base band modulated sequence of complex symbols. There are three valid , 16- QAM , 64-QAM.
- 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.
- 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 (Transmission Parameters Signalling) send the parameters of the transmitted signal and to unequivocally identify the transmission cell. It should be noted that 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.
- the sequence of blocks is modulated according to the OFDM technique, using 2048, 4096, or 8192 carriers (2k, 4k, 8k mode, respectively).
- to decrease receiver complexity, every OFDM block is extended, copying in front of it its own end ( Cyclic Prefix ). The width of such guard interval can be 1/32, 1/16, 1/8, or 1/4 that of the original block length.
- 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 5, 6, 7, or 8 MHz wide channels.
The receiving STB adopts techniques which are dual to those ones used in the transmission.
- 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. 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 (whose value and position is determined in the standard and thus known by the receiver) 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.
- the cyclic prefix is removed.
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- the pilot signals equalize the received signal.
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- : it uses the Viterbi Algorithm .
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Cuba , Jamaica , Uruguay , Venezuela
Andorra , Austria , Belgium , Croatia , Czech Republic , Denmark , Estonia , Faroe Islands ,
Finland , France , Germany , Greece (experimental), Hungary (experimental), Ireland , Italy , Luxembourg , Malta , Netherlands , Norway , Portugal , Slovakia (experimental), Serbia , Slovenia , Spain , Sweden , Switzerland , United Kingdom
Australia , India , Malaysia , Myanmar , Singapore , Vietnam
Morocco , Namibia , South Africa
- ETSI Standard: EN 300 744 V1.5.1, ''Digital Video Broadcasting (DVB); Framing structure, channel coding and modulation for digital terrestrial television'', available at ETSI Publications Download Area (this will open ETSI document search engine, to find the latest version of the document enter a search string; free registration is required to download PDF)
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