DVB-T is an abbreviation for Digital Video Broadcasting – Terrestrial; it is the DVB European-based consortium standard for the broadcast transmission of digital terrestrial television. This system transmits compressed digital audio, video and other data in an MPEG transport stream, using OFDM modulation with concatenated channel coding (i.e. COFDM).
Basics of DVB-T
Rather than carrying the data on a single radio frequency carrier, OFDM 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 or 8K. These are actually 1705 or 6817 carriers that are approximately 4 kHz or 1 kHz apart, depending on whether its a transmission channel of 8, 7, or 6 MHz.
DVB-T also offers many other transmission choices of digital modulation (QPSK, 16QAM, 64QAM) and Code rate --forward error correction (FEC). This allows broadcasters to trade off payload data capacity versus improved reliability of reception in different reception conditions.
DVB-T as a digital transmission delivers data in a series of discrete blocks at the clock or symbol rate. DVB-T includes a "Guard Interval" feature where the receiver ignores the data for a short period around the time when the data changes. Within limits, this allows for the receiver to ignore the effects of multipath reception (e.g. ghosting).
Within a geographical area, use of the guard interval also allows single-frequency network (SFN) operation, where two or more transmitters carrying the same transport stream of services can operate on the same RF channel frequency. In such cases the signals from each transmitter in the SFN needs to be accurately time-aligned, which is usually done by a sync signal in the stream and GPS timing at each transmission point. This allows the overlap region between transmitters, sometimes called the 'mush zone', to be geographically shifted away from population centers.
DVB-T has been adopted or proposed for digital television broadcasting by many countries (see map), using mainly UHF 8 MHz channels, but also 7 MHz (VHF and UHF in Australia), and 6 MHz in Taiwan. Examples include the UK's Freeview.
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, 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 counties that have adopted DVB-T have published standards for their implementation. These include the DTG in the UK, the DGTVi D-Book in Italy,[1] Nordig and Standards Australia AS 4599 (transmission) and AS 4933 (DTV receivers).
DVB-T has been further developed into newer standards such as DVB-H (Handheld), now in operation, and DVB-T2, which is in the final stages of development in 2008.
DVB-T2
In March 2006 DVB decided to study options for an upgraded DVB-T standard. In June 2006, a formal study group named TM-T2 (Technical Module on Next Generation DVB-T) was established by the DVB Group to develop an advanced modulation scheme that could be adopted by a second generation digital terrestrial television standard, to be named DVB-T2.[2]
According to the commercial requirements and call for technologies [3] issued in April 2007, the first phase of DVB-T2 will be devoted to provide optimum reception for stationary (fixed) and portable receivers (i.e., units which can be nomadic, but not fully mobile) using existing aerials, whereas a second and third phase will study methods to deliver higher payloads (with new aerials) and the mobile reception issue. The novel system should provide a minimum 30% increase in payload, under similar channel conditions already used for DVB-T.
The BBC, ITV, Channel 4 and Five have agreed with the regulator Ofcom to convert one UK multiplex (B, or PSB3) to DVB-T2 to increase capacity for HDTV via DTT.[4] They expect the first TV region to use the new standard will be Granada in November 2009 (with existing switched over regions being changed at the same time). It is expected that over time there will be enough DVB-T2 receivers sold to switch all DTT transmissions to DVB-T2, and H.264.
Ofcom recently published its final decision for HDTV using DVB-T2 and MPEG-4 [5]: BBC HD will have one HD slot after DSO at Granada and ITV, C4 and Five will compete for the two remaining HD slots from 2009 until 2012.
The draft standard
From documents published on the Internet by RAI and others, the following characteristics have been devised for the T2 standard:[6][7]
- Modulation will be standard COFDM in one of the modes QPSK, 16-QAM, 64-QAM, or 256-QAM (but not 128-QAM).
- OFDM modes will be 1k, 2k, 4k, 8k, 16k and 32k. The symbol length for 32k mode will be about 4 ms.
- Guard intervals will be 1/128, 1/32, 1/16, 19/256, 1/8, 19/128, and 1/4. (For 32k mode, the maximum is 1/8.)
- FEC will be LDPC and BCH (as in DVB-S2), with rates 1/2, 3/5, 2/3, 3/4, 4/5, and 5/6.
- There will be fewer pilots, in 8 different pilot-patterns, and equalization will be based also on the RAI CD3 system.
- In the 32k mode, a larger part of the standard 8 MHz channel can be used, adding about 2% extra capacity.
- DVB-T2 will be specified for 1.7, 5, 6, 7, 8, and 10 MHz channel bandwidth.
- MISO (Multiple-Inputs Single-Output) may be used (Alamouti scheme), but MIMO will not be used. Diversity receivers can be used (as they are with DVB-T).
- Bundling of more channels into a SuperMUX (called TFS) is not in the standard, but may be added later.
An example for a UK MFN DVB-T profile (QAM64, 2k, CR2/3, GI1/32) and a DVB-T2 equivalent (QAM256, 32k, CR3/5, GI1/128) shows an increase in bit rate from 24.13 Mbit/s to 35.4 Mbit/s (+46.5%).
Another example is an Italian SFN DVB-T profile (QAM64, 8k, CR2/3, GI1/4) and a DVB-T2 equivalent (QAM256, 32k, CR3/5, GI1/16): it shows an increase in bit rate from 19.91 Mbit/s to 33.3 Mbit/s (+67%).
- Status of the DVB-T2 specification
The DVB-T2 draft standard (EN 302 755, ratified by the DVB Steering Board on June 26, 2008, and published on the DVB homepage as 'DVB-T2 standard BlueBook') has been handed over to The European Telecommunications Standards Institute (ETSI) by DVB.ORG on June 20, 2008 (ETSI timetable DVB-T2). The 'ETSI process' will take until April 25, 2009, when the final standard will be published.
- The DVB-T2 physical layer specification is complete and there will be no further technical enhancements.
- The receiver VLSI chip design can be started with confidence in stability of specification.
- Draft PSI/SI (program and system information) specification document agreed with the DVB-TM-GBS group.
Tests
The first test from a real TV-transmitter was performed by the BBC Research & Innovation in the last weeks of June 2008 [1] using channel 53 from the Guildford transmitter, southwest of London. The BBC had developed and built the modulator/demodulator prototype in parallel with the DVB-T2 standard being drafted.
Technical description of a DVB-T transmitter
Scheme of a DVB-T transmission system
With reference to the figure, a short description of the signal 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 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 an 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 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.
- Internal encoder: 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.
- 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 synchronization and equalization phase, while TPS signals (Transmission Parameters Signalling) 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.
Spectrum of a DVB-T signal in 8k mode (note the flat-top characteristics).
- OFDM Modulation: the sequence of blocks is modulated according to the OFDM technique, using 2048, 4096, or 8192 carriers (2k, 4k, 8k mode, respectively). 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 (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. 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 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 base band sample rate provided at the DAC input depends on the channel bandwidth: it is
samples/s, where B is the channel bandwidth expressed in Hz.
| Available bitrates (Mbit/s) for a DVB-T system in 8 MHz 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. Any 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.
- Guard interval disposal: the cyclic prefix is removed.
- OFDM demodulation
- Frequency equalization: the pilot signals equalize the received signal.
- Demapping
- Internal deinterleaving
- Internal decoding: uses the Viterbi algorithm.
- External deinterleaving
- External decoding
- MUX adaptation
- MPEG-2 demultiplexing and source decoding
Countries and territories using DVB-T[8]
DTT broadcasting systems. Countries using DVB-T are shown in blue.
See also
Notes
- ^ "DGTVi - Per la Televisione Digitale Terrestre".
- ^ TM-T2. Second Generation DVB-T, DVB.org
- ^ DVB - Digital Video Broadcasting - DVB-T2
- ^ "3 Freeview HD channels will start 2009 – ukfree.tv – independent digital television and switchover advice, since 2002". Retrieved on 2007-11-25.
- ^ "Ofcom Statement on DTT future, announced on April 3, 2008". Retrieved on 2008-04-09.
- ^ Presentation given by Dr. Morello RAI research centre, Rome, April 3, 2008]
- ^ DVB-T2 Fact Sheet, June 2008
- ^ Official information taken from the DVB website
- ^ El Espectador, Colombia adopta el estándar europeo para la tv digital terrestre, 28 August 2008
- ^ Lankanewspapers.com, Dialog TV launches another South Asia`s first DVBT, 2008-01-24. Retrieved on 2008-01-26.
References
- 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.)
External links
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Wireless video and data distribution methods |
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