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Mohammad.etminan (بحث | مشارکت‌ها)
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{{Table Digital video broadcast standards}}
{{بدون منبع}}


'''DVB-T''' مخففی از '''پخش ویدیوی دیجیتالی - زمینی''' است; این [[Digital Video Broadcasting|DVB]] براساس کنسرسیوم استاندارد اروپای برای انتقال پخش از [[تلویزیو دیجیتالی زمینی]] است که اولین بار در سال ۱۹۹۷ در [[بریتانیا]] پخش شد.این سیستم انتقال می‌دهد دیجیتالی کمپرس شد صدا, تصویر و سایر داده‌ها در انتقال سیال [[MPEG]], با استفاده از مدولاسیون [[COFDM]].
'''DVB-T''' مخففی از '''پخش ویدیوی دیجیتالی - زمینی''' است; این [[Digital Video Broadcasting|DVB]] براساس کنسرسیوم استاندارد اروپای برای انتقال پخش از [[تلویزیو دیجیتالی زمینی]] است که اولین بار در سال ۱۹۹۷ در [[بریتانیا]] پخش شد.این سیستم انتقال می‌دهد دیجیتالی کمپرس شد صدا, تصویر و سایر داده‌ها در انتقال سیال [[MPEG]], با استفاده از مدولاسیون [[COFDM]].


== مقدماتی از DVB-T ==
== مقدماتی از DVB-T ==


به جای حمل داده ها بر روی یک فرکانس رادیوی حامل,[[OFDM]] کار می کند که به وسیله جریان داده های دیجیتال را به تعداد زیادی از جریان دیجیتال تقسیم می کند,
به جای حمل داده ها بر روی یک فرکانس رادیوی حامل,[[OFDM]] کار می کند که به وسیله جریان داده های دیجیتال را به تعداد زیادی از جریان دیجیتال تقسیم می کند,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-mode or 8K-mode. These are actually 1705 or 6817 carriers that are approximately 4 kHz or 1 kHz apart.

DVB-T offers three different modulation schemes ([[QPSK]], [[16QAM]], 64QAM).

DVB-T has been adopted or proposed for digital television broadcasting by many countries ([[DVB-T#Countries_and_territories_using_DVB-T.5B3.5D|see map]]), using mainly VHF 7 MHz and UHF 8 MHz channels whereas Taiwan uses 6 MHz channels. Examples include the UK's [[Freeview (United Kingdom)|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 countries that have adopted DVB-T have published standards for their implementation. These include the D-Book in the UK, the Italian DGTVi <ref>{{cite web | url = http://www.dgtvi.it | title = DGTVi - Per la Televisione Digitale Terrestre}}</ref>, the ETSI E-Book and Scandivia NorDig.

DVB-T has been further developed into newer standards such as [[DVB-H]] (Handheld), now in operation, and [[DVB-T2]], which was recently finalised.

DVB-T as a digital transmission delivers data in a series of discrete blocks at the symbol rate. DVB-T is a COFDM transmission technique which includes the use of a Guard Interval. It allows the receiver to cope with strong multipath situations. Within a geographical area, DVB-T also allows [[single-frequency network]] (SFN) operation, where two or more transmitters carrying the '''same''' data operate on the same frequency. In such cases the signals from each transmitter in the SFN needs to be accurately time-aligned, which is done by sync information in the stream and timing at each transmitter referenced to [[GPS]].

The length of the Guard Interval can be chosen. It is a trade off between data rate and SFN capability. The longer the guard interval the larger is the potential SFN area without creating Inter Symbol Interference (ISI).
It can be possible to operate SFNs which do not fulfil the guard interval condition if the self-interference is properly planned and monitored.

==شرح فنی سیستم انتقال DVB-T==

[[Image:dvbt tx scheme.svg|thumb|600px|centre|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 ([[Program stream|Programme Streams]]). One or more PSs are joined together into an MPEG-2 TS ([[Transport stream|MPEG-2 Transport Stream]]); this is the basic digital stream which is being transmitted and received by home [[Set top box|Set Top Boxes]] (STB). Allowed [[bitrate]]s 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 wave|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 [[Packet (information technology)|data packets]], of fixed length (188 bytes). With a technique called [[Scrambler (randomizer)|energy dispersal]], the byte sequence is [[correlation|decorrelated]].

* '''External encoder:''' a first level of error correction is applied to the transmitted data, using a non-binary [[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, in such a way that 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 synchronization|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.

[[Image:dvbt spectrum.jpg|thumb|250px|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 [[Orthogonal frequency-division multiplexing|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 analogue signal, with a [[digital-to-analogue converter]] (DAC), and then modulated to radio frequency ([[Very high frequency|VHF]], [[Ultra high frequency|UHF]]) by the [[RF]] [[front-end]]. The occupied bandwidth is designed to accommodate each single DVB-T signal into 5, 6, 7, or 8 [[Megahertz|MHz]] wide channels. The base band sample rate provided at the DAC input depends on the channel bandwidth: it is <math>f_s = \frac{8}{7} B</math> ''samples/s'', where <math>B</math> is the channel bandwidth expressed in Hz.

{| class="wikitable" style="width: 40em;"
|-
! colspan="6" | Available bitrates (Mbit/s) for a DVB-T system in 8&nbsp;MHz channels
|-
! rowspan="2" | Modulation
! rowspan="2" | Coding rate
! colspan="4" | [[Guard interval]]
|-
|''1/4''||''1/8''||''1/16''||''1/32''
|-
! rowspan="5" | [[Phase-shift keying|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
|-
! rowspan="5" | 16-[[Quadrature amplitude modulation|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
|-
! rowspan="5" | 64-[[Quadrature amplitude modulation|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
|}

==شرح فنی گیرنده==

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

* '''Front-end and ADC:''' the analogue RF signal is converted to base-band and transformed into a digital signal, using an [[analogue-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 with 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<ref>Official information taken from the [http://www.dvb.org/about_dvb/dvb_worldwide/index.xml DVB website]</ref>==

[[Image:Digital broadcast standards.svg|thumb|600px|center|DTT broadcasting systems. Countries using DVB-T are shown in blue.]]

{{Multicol|100%}}

===امریکا===

* {{flag|Colombia}} (decided in August 28, 2008) <ref>''[[El Espectador]]'', [http://elespectador.com/node/34794/ Colombia adopta el estándar europeo para la tv digital terrestre], 28 August 2008</ref>
* {{flag|Uruguay}} (decided in September 2007, but not effective yet)
* {{flag|Panama}} (decided in May 12, 2009) <ref>[http://www.dvb.org/news_events/news/panama-adopts-dvb-t/index.xml Panama adopts DVB-T]</ref>

===اروپا===

* {{flag|Albania}}
* {{flag|Andorra}}
* {{flag|Austria}}
* {{flag|Belgium}}
* {{flag|Bosnia and Herzegovina}} (Plan to switch by 2011<ref>http://www.svevijesti.ba/content/view/20671/215/, in Bosnian</ref>)
* {{flag|Bulgaria}} (experimental)
* {{flag|Belarus}} (experimental)
* {{flag|Croatia}} (See [[DVB-T in Croatia]].)
* {{flag|Czech Republic}}
* {{flag|Cyprus}}
* {{flag|Denmark}} (See [[DVB-T in Denmark]].)
:* {{flag|Faroe Islands}}
:* {{flag|Greenland}} ([http://www.nuuktv.gl Nuuk tv])
* {{flag|Estonia}} (uses [[MPEG-4]] video)
* {{flag|Finland}}
* {{flag|France}} (uses [[MPEG-2]] for SD and [[MPEG-4]] for HD transmissions.)
* {{flag|Germany}}
* {{flag|Greece}} (experimental)
* {{flag|Hungary}} (branded [http://www.mindigtv.hu MinDigTV], uses [[MPEG-4]] video exclusively.)
* {{flag|Iceland}}{{Fact|date=November 2008}}

{{Multicol-break}}

* {{flag|Ireland}} (Will use [[MPEG-4]] video, see [[DVB-T in Ireland]].)
* {{flag|Italy}}
* {{flag|Israel}}
* {{flag|Latvia}} (uses H.264/MPEG 4 AVC)
* {{flag|Lithuania}} (uses H.264/MPEG-4 AVC)
* {{flag|Luxembourg}}
* {{flag|Republic of Macedonia}} (experimental, official start on October, 2009)
* {{flag|Malta}}
* {{flag|Montenegro}}
* {{flag|Netherlands}}
* {{flag|Norway}} (uses [[MPEG-4]] video)
* {{flag|Poland}} (uses [[MPEG-4/H.264]] video)
* {{flag|Portugal}} (Uses MPEG-4 video/H.264)
* {{flag|Romania}} (uses [[MPEG-2]] for SD and [[MPEG-4]] for HD transmissions.)
* {{flag|Russia}} (experimental)
* {{flag|Slovakia}}
* {{flag|Serbia}}
* {{flag|Slovenia}} (Use [[MPEG-4]] video from 2007. See [[DVB-T in Slovenia]])
* {{flag|Spain}}
* {{flag|Sweden}} (Uses [[MPEG-2]] for SD and [[MPEG-4]] for HD transmissions. See [[DVB-T in Sweden]].)
* {{flag|Switzerland}}
* {{flag|Turkey}} (experimental)
* {{flag|United Kingdom}} (See [[DVB-T in United Kingdom]].)
* {{flag|Ukraine}} (Uses [[MPEG-4]]/[[AAC]] exclusively. 6 multiplexes to be implemented. MX-4 with 10 channels began broadcasting on April 1, 2009 in Kyiv, Zhytomyr and South Odesa regions of Ukraine. Trial broadcasts started in 2007 and had been conducted using [[MPEG-2]].)

{{Multicol-break}}

===آسیا/استرالیا===

* {{flag|Australia}}
* {{flag|India}} (Uses [[MPEG-4]] and [[MPEG-2]] Video)
* {{flag|Indonesia}} (Experimental)
* {{flag|Iran}} (pilot service)
* {{flag|Israel}}
* {{flag|Kuwait}}
* {{flag|Malaysia}} (experimental)
* {{flag|Myanmar}}
* {{flag|New Zealand}} (see [[Freeview (New Zealand)|Freeview]])
* {{flag|Philippines}} (experimental)
* {{flag|Singapore}}
* {{flag|Saudi Arabia}}
* {{flag|Sri Lanka}} (pilot service <ref>Lankanewspapers.com, [http://www.lankanewspapers.com/news/2008/1/23985.html Dialog TV launches another South Asia's first DVBT], 2008-01-24. Retrieved on 2008-01-26. </ref>)
* {{flag|Taiwan}} (uses MPEG-2 for SD and MPEG-4 for HD transmissions.)
* {{flag|Thailand}}
* {{flag|Vietnam}}
* {{flag|United Arab Emirates}} (Will use MPEG-2 for SD and MPEG-4 for HD transmissions.)

===آفریقا===

* {{flag|Cape Verde}}
* {{flag|Kenya}}(Will use [[MPEG-4]])
* {{flag|Mauritius}}
* {{flag|Morocco}}
* {{flag|Namibia}}
* {{flag|South Africa}} (Uses [[MPEG-4]])
* {{flag|Tunisia}} (experimental)

{{Multicol-end}}

==نیز==

* [[ATSC Standards|ATSC]] (Advanced Television Systems Committee, North American Standard)
* [[Digital audio broadcasting]] (low bitrate video suitable for moving receivers)
* [[DTV channel protection ratios]]
* [[DVB over IP]]
* [[Interactive television]]
* [[Digital terrestrial television]]
* [[DMB-T]] - Digital Multimedia Broadcast-Terrestrial
* [[ISDB]] - Integrated Services Digital Broadcasting
* [[Orthogonal frequency-division multiplexing#OFDM system comparison table|OFDM system comparison table]]
* [[Spectral efficiency#comparison table|Spectral efficiency comparison table]]

==یاداشت ها==

{{reflist}}

==منابع==

* [[ETSI]] Standard: EN 300 744 V1.5.1, ''Digital Video Broadcasting (DVB); Framing structure, channel coding and modulation for digital terrestrial television'', available at [http://pda.etsi.org/pda/queryform.asp 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.)

==پیوندهای خارجی==

* [http://www.dvb.org/ Website of the DVB Project]
* [http://www.dvb-h.org/ DVB-H.org]
* [http://www.digitag.org/ DigiTAG website]
* [http://www.ofcom.org.uk/consult/condocs/dttfuture/ OFCOM DTT future]
* [http://www.bbc.co.uk/digital/index.shtml BBC - Digital - Homepage]
* [http://www.bbc.co.uk/digital/tv/switchover.shtml BBC - Digital - Switchover]
* [http://www.bbc.co.uk/pressoffice/pressreleases/stories/2007/11_november/20/hd.shtml BBC press]

{{تصویر بی سیمی}}
{{قالب های تصویری}}

[[Category:DVB]]

[[ca:DVB-T]]
[[cs:DVB-T]]
[[de:DVB-T]]
[[en:DVB-T]]
[[es:DVB-T]]
[[fr:DVB-T]]
[[hr:DVB-T]]
[[it:DVB-T]]
[[ms:DVB-T]]
[[nl:DVB-T]]
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[[no:DVB-T]]
[[pl:DVB-T]]
[[ru:DVB-T]]
[[sr:DVB-T]]
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[[tr:DVB-T]]
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[[vi:DVB-T]]

نسخهٔ ‏۱۵ اکتبر ۲۰۰۹، ساعت ۱۷:۳۰

الگو:Table Digital video broadcast standards


DVB-T مخففی از پخش ویدیوی دیجیتالی - زمینی است; این DVB براساس کنسرسیوم استاندارد اروپای برای انتقال پخش از تلویزیو دیجیتالی زمینی است که اولین بار در سال ۱۹۹۷ در بریتانیا پخش شد.این سیستم انتقال می‌دهد دیجیتالی کمپرس شد صدا, تصویر و سایر داده‌ها در انتقال سیال MPEG, با استفاده از مدولاسیون COFDM.

مقدماتی از DVB-T

به جای حمل داده ها بر روی یک فرکانس رادیوی حامل,OFDM کار می کند که به وسیله جریان داده های دیجیتال را به تعداد زیادی از جریان دیجیتال تقسیم می کند,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-mode or 8K-mode. These are actually 1705 or 6817 carriers that are approximately 4 kHz or 1 kHz apart.

DVB-T offers three different modulation schemes (QPSK, 16QAM, 64QAM).

DVB-T has been adopted or proposed for digital television broadcasting by many countries (see map), using mainly VHF 7 MHz and UHF 8 MHz channels whereas Taiwan uses 6 MHz channels. 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 countries that have adopted DVB-T have published standards for their implementation. These include the D-Book in the UK, the Italian DGTVi [۱], the ETSI E-Book and Scandivia NorDig.

DVB-T has been further developed into newer standards such as DVB-H (Handheld), now in operation, and DVB-T2, which was recently finalised.

DVB-T as a digital transmission delivers data in a series of discrete blocks at the symbol rate. DVB-T is a COFDM transmission technique which includes the use of a Guard Interval. It allows the receiver to cope with strong multipath situations. Within a geographical area, DVB-T also allows single-frequency network (SFN) operation, where two or more transmitters carrying the same data operate on the same frequency. In such cases the signals from each transmitter in the SFN needs to be accurately time-aligned, which is done by sync information in the stream and timing at each transmitter referenced to GPS.

The length of the Guard Interval can be chosen. It is a trade off between data rate and SFN capability. The longer the guard interval the larger is the potential SFN area without creating Inter Symbol Interference (ISI). It can be possible to operate SFNs which do not fulfil the guard interval condition if the self-interference is properly planned and monitored.

شرح فنی سیستم انتقال DVB-T

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 non-binary 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 analogue signal, with a digital-to-analogue 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 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

شرح فنی گیرنده

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

  • Front-end and ADC: the analogue RF signal is converted to base-band and transformed into a digital signal, using an analogue-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 with 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[۲]

DTT broadcasting systems. Countries using DVB-T are shown in blue.

امریکا

اروپا

آسیا/استرالیا

آفریقا

نیز

یاداشت ها

  1. ↑ "DGTVi - Per la Televisione Digitale Terrestre".
  2. ↑ Official information taken from the DVB website
  3. ↑ El Espectador, Colombia adopta el estándar europeo para la tv digital terrestre, 28 August 2008
  4. ↑ Panama adopts DVB-T
  5. ↑ http://www.svevijesti.ba/content/view/20671/215/, in Bosnian
  6. ↑ Lankanewspapers.com, Dialog TV launches another South Asia's first DVBT, 2008-01-24. Retrieved on 2008-01-26.

منابع

  • 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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