DVB-T

From Free net encyclopedia

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 an MPEG-2 family digital audio/video stream, using OFDM modulation with concatenated channel coding (i.e. COFDM).

1 Technical description of the transmitter
2 Technical description of the receiver
3 References
4 See also
5 External links

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Technical description of the transmitter

Image:Dvbt tx scheme.png With reference to the figure, a short description of the single processing blocks follows.

Source coding and MPEG-2 multiplexing (MUX): video, audio, and data streams are multiplexed into an MPEG-2 PS (MPEG-2 Programme Stream). One or more PSs are joined together into a 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 MPEG-2 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 tecnique 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 univoquely 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.

Available bitrates for a DVB-T system in 8 MHz channels. All (decimal) values in Mbit/s:

Modulation	Coding rate	Guard interval
Modulation	Coding rate	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

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Technical description of the receiver

The receiving STB adopts techniques which are dual to those ones used in the trasmission.

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

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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.

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