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Information about 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.

DVB-T2

In June 2006, a study group named TM-T2 (Technical Module [=group] 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.[1] 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.

Technical description of the transmitter

Enlarge picture
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 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 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 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 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. 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.
  • 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 width 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 5, 6, 7, or 8 MHz wide channels.
Available bitrates (Mbit/s) for a DVB-T system in 8 MHz channels
Modulation Coding rate Guard interval
1/41/81/161/32
QPSK1/24.9765.5295.8556.032
2/36.6357.3737.8068.043
3/47.4658.2948.7829.048
5/68.2949.2169.75810.053
7/88.7099.67610.24610.556
16-QAM1/29.95311.05911.70912.064
2/313.27114.74515.61216.086
3/414.92916.58817.56418.096
5/616.58818.43119.51620.107
7/817.41819.35320.49121.112
64-QAM1/214.92916.58817.56418.096
2/319.90622.11823.41924.128
3/422.39424.88226.34627.144
5/624.88227.64729.27330.160
7/826.12629.02930.73731.668

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. 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: it uses the Viterbi algorithm.
  • External deinterleaving
  • External decoding
  • MUX adaptation
  • MPEG-2 demultiplexing and source decoding

Countries and territories using DVB-T

America

Chile (experimental), Cuba, Jamaica, Uruguay, Venezuela

Europe

Albania, Andorra, Austria, Belgium, Bulgaria (experimental), Croatia, Czech Republic, Denmark (See DVB-T in Denmark), Estonia, Faroe Islands, Finland, France, Germany, Greece (experimental), Greenland, Hungary (experimental), Ireland (See DVB-T in Ireland), Italy, Lithuania, Luxembourg, Malta, Netherlands, Norway, Romania, Poland (experimental), Portugal, Slovakia (experimental), Serbia, Slovenia, Spain, Sweden (See DVB-T in Sweden), Switzerland, United Kingdom (See DVB-T in United Kingdom).

Asia/Australasia

Australia, India, Malaysia (experimental), Myanmar, New Zealand, Philippines (experimental), Singapore, Taiwan, Vietnam

Africa

Mauritius, Morocco, Namibia, South Africa

See also

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


[ edit ] Video formats
Analog broadcast
525 lines: NTSC | NTSC-J | PAL-M
625 lines: PAL | PAL-N | PALplus | SECAM
Defunct systems: Pre-1940 | 405 lines | 819 lines | Baird-Nipkow | MAC | MUSE
Multichannel audio: BTSC (MTS) | NICAM-728 | Zweiton (A2, IGR)
Hidden signals: Captioning | Teletext | CGMS-A | GCR | PDC | VBI | VEIL | VITC | WSS | XDS
Digital broadcast
Interlaced: SDTV (480i, 576i) | HDTV (1080i)
Progressive: LDTV (240p, 288p, 1seg) | EDTV (480p, 576p) | HDTV (720p, 1080p)
Digital TV standards: MPEG-2: ATSC, DVB, ISDB | MPEG-4: SBTVD
Multichannel audio: AAC (5.1) | Musicam | PCM | LPCM
Hidden signals: Captioning | Teletext | (CPCM/Broadcast flag) | AFD | EPG
Digital cinema: UHDV (2540p, 4320p) | DCI | 22.2 audio
Technical issues: | MPEG transport | Standards conversion | Video processing | VOD
DVB, short for Digital Video Broadcasting, is a suite of internationally accepted open standards for digital television. DVB standards are maintained by the DVB Project, an industry consortium with more than 270 members, and they are published by a
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Digital Terrestrial Television (DTTV or DTT) is an implementation of digital technology to provide a greater number of channels and/or better quality of picture and sound using aerial broadcasts to a conventional antenna (or aerial) instead of a satellite dish or
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In computer science, a channel code is a broadly used term mostly referring to the forward error correction code and bit interleaving in communication and storage where the communication media or storage media is viewed as a channel.
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MPEG-2 is a standard for "the generic coding of moving pictures and associated audio information".[1] It describes a combination of lossy video compression and lossy audio compression (audio data compression) methods which permit storage and transmission of movies using
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H.264 is a standard for video compression. It is also known as MPEG-4 Part 10, or AVC (for Advanced Video Coding). It was written by the ITU-T Video Coding Experts Group (VCEG) together with the ISO/IEC Moving Picture Experts Group (MPEG) as
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data compression or source coding is the process of encoding information using fewer bits (or other information-bearing units) than an un-encoded representation would use through use of specific encoding schemes.
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In electronics, telecommunications and computer networks, multiplexing (short muxing) is a term used to refer to a process where multiple analog message signals or digital data streams are combined into one signal. The aim is to share an expensive resource.
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Program stream (PS or MPEG-PS) is a container format for multiplexing digital audio, video and more. The PS format is specified in MPEG-1 Systems and MPEG-2 Part 1, Systems (ISO/IEC standard 13818-1).
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Transport stream (TS, TP, or MPEG-TS) is a communications protocol for audio, video, and data which is specified in MPEG-2 Part 1, Systems (ISO/IEC standard 13818-1 [1] ).
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A set-top box (STB) or set-top unit (STU) is a device that connects to a television and an external source of signal, turning the signal into content which is then displayed on the television screen.
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bitrate (sometimes written bit rate, data rate or as a variable R or fb) is the number of bits that are conveyed or processed per unit of time. Bit rate is synonymous to data rate and digital bandwidth.
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megabit per second (abbreviated as Mbit/s, Mbps, or mbps) is a unit of data transfer rate equal to 1,000,000 bits per second. Because there are 8 bits in a byte, a transfer speed of 8 megabits per second (8 Mbps) is equivalent to 1,000,000 bytes
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High-definition television (HDTV) is a digital television broadcasting system with a significantly higher resolution than traditional formats (NTSC, SECAM, PAL). While some early analog HDTV formats were broadcast in Europe and Japan, HDTV is usually broadcast digitally,
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In telecommunications, a carrier wave, or carrier is a waveform (usually sinusoidal) that is modulated (modified) with an input signal for the purpose of conveying information, for example voice or data, to be transmitted.
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In telecommunications and recording, a scrambler (often erroneously referred to as a randomizer) is a device that manipulates a data stream before transmitting. The manipulations are reversed by a descrambler at the receiving side.
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correlation, also called correlation coefficient, indicates the strength and direction of a linear relationship between two random variables. In general statistical usage, correlation or co-relation refers to the departure of two variables from independence.
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In computer science, a block code is a type of channel coding. It adds redundancy to a message so that, at the receiver, one can decode with minimal (theoretically zero) errors, provided that the information rate (amount of transported information in bits per sec) would not exceed
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This article or section needs copy editing for grammar, style, cohesion, tone and/or spelling.
You can assist by [ editing it] now. A how-to guide is available, as is general .
This article has been tagged since October 2007.
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In telecommunication, a convolutional code is a type of error-correcting code in which (a) each m-bit information symbol (each m-bit string) to be encoded is transformed into an n-bit symbol, where m/n is the code rate (
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In telecommunication, forward error correction (FEC) is a system of error control for data transmission, whereby the sender adds redundant data to its messages, which allows the receiver to detect and correct errors (within some bound) without the need to ask the sender for
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For the musical use of "modulation" as a change of key, see modulation (music).
In telecommunications, modulation is the process of varying a periodic waveform, i.e.
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While receiving a stream of framed data, frame synchronization is the process by which incoming frame alignment signals, i.e., distinctive bit sequences (a syncword), are identified, i.e.
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Radio is the wireless transmission of signals, by modulation of electromagnetic waves with frequencies below those of visible light. Electromagnetic radiation travels by means of oscillating electromagnetic fields that pass through the air and the vacuum of space.
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Orthogonal Frequency-Division Multiplexing (OFDM) — essentially identical to Coded OFDM (COFDM) — is a digital multi-carrier modulation scheme, which uses a large number of closely-spaced orthogonal sub-carriers.
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In an OFDM symbol the cyclic prefix is a repeat of the end of the symbol at the beginning. The purpose is to allow multipath to settle before the main data arrives at the receiver.
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digital-to-analog converter (DAC or D-to-A) is a device for converting a digital (usually binary) code to an analog signal (current, voltage or electric charge).

An analog-to-digital converter (ADC) performs the reverse operation.
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Very high frequency (VHF) is the radio frequency range from 30 MHz to 300 MHz. It is also known as the meter band or meter wave as the wavelengths range from ten to one meters.
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Cycles per second: 300 MHz to 3 GHz
Wavelength: 1 m to 100 mm Ultra high frequency (UHF) designates a range (band) of electromagnetic waves whose frequency is between 300 MHz and 3 GHz, which is 300 MHz to 3,000 MHz.
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RF may mean:
  • RF, the IATA code for Florida West International Airways
  • RF , NYSE ticker symbol for Regions Financial Corporation
  • Royalty free
Rf or RF may stand for:
  • Radio frequency

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