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'''Digital Audio Broadcasting''' ('''DAB''' |
'''Digital Audio Broadcasting''' ('''DAB''', also known as '''Eureka 147'' or ''E!147'') designed in the 1980s' and early 1990s', is a ] technology for ] ]s. DAB is used in several countries, particularly in Western Europe. As of 2006, nearly 1,000 stations worldwide broadcast in the DAB format.<ref></ref> | ||
An upgraded version of the system has now been developed, called '''DAB+''', which provides higher audio quality at lower data rates. DVB+ is not ] with the original standard, however. | |||
The DAB standard was designed in the 1980s, and ]s have been available in many countries for several years. Proponents claim the standard offers several benefits over existing ] ] radio, such as higher-] audio, more stations in the same broadcast ], and increased resistance to ], ], ], and co-channel ]. However, listening tests carried out by experts have shown that the audio quality on DAB is lower than on FM due to the fact that 98% of stereo stations in the UK, Denmark, Norway and Switzerland (which are the only countries where DAB sales have taken off) are using bit rates levels that are too low.<ref></ref><ref name="robinson"></ref><ref></ref> | |||
⚫ | Due to poor |
||
An upgraded version of the system has now been developed, called '''DAB+'''. The new standard, the full details of which were published in 2007, is not ] with the original standard. The vast majority of existing DAB receivers and those on sale in the shops will not be able to receive DAB+ radio services when they are launched; however, a range of DAB+-upgradeable receivers are now available in the shops. Several countries are expected to launch DAB+ broadcasts over the next few years, including the UK,<ref>http://www.digitalradiotech.co.uk/documents/DRDB_UK_DAB+_policy.pdf</ref> Australia, Italy, Germany, Switzerland and Malta, and podcast services using the DAB+ format will be launched in the UK in 2008.<ref>http://www.ofcom.org.uk/radio/ifi/rbl/dcr/applications/app_national/4digital.pdf</ref> There is also a great deal of interest in using DAB+ in Asian countries, such as China. | |||
DAB+ is approximately three times more efficient than DAB due to the adoption of the ] audio codec.<ref>http://worlddab.org/pdf/DAB+brochure.pdf</ref> This means that it will allow broadcasters to provide far higher audio quality or far more radio stations than with the older version of DAB, or, as is most likely, a combination of both higher audio quality and more radio stations will be provided. Reception quality will also be more robust on DAB+ than on DAB due to the addition of ] error correction coding. | |||
==History== | ==History== | ||
DAB has been under development since 1981 at the ] (IRT). In 1985 the first DAB demonstrations were held at the WARC-ORB in Geneva and in 1988 the first DAB transmissions were made in Germany. Later DAB (or Eureka-147) was developed as |
DAB has been under development since 1981 at the ] (IRT). In 1985 the first DAB demonstrations were held at the WARC-ORB in Geneva and in 1988 the first DAB transmissions were made in Germany. Later, DAB (or Eureka-147) was developed as an inter-governmental, pan-European research initiative, under the Eureka project, which started in 1987. Many of the technologies behind the ] ("MP2") codec were developed as part of the E!147 project. DAB was the first standard based on ] (OFDM) modulation technique, which since then has become one of the most popular transmission schemes for modern wideband digital communication systems. | ||
The choice of audio codec, modulation and error-correction coding schemes was made, and the first trial broadcasts were tested, in 1990. Public demonstrations were made in 1993 in the ]. The protocol specification was finalized in 1993 and adopted by the ] standardization body in 1994, the European community in 1995 and by ] in 1997. Pilot broadcasts were launched in several countries in 1995. | |||
The UK was the first country to receive a wide range of ]s via DAB. Commercial DAB receivers began to be sold in 1999 and over 50 commercial and ] services were available in London by 2001. | The UK was the first country to receive a wide range of ]s via DAB. Commercial DAB receivers began to be sold in 1999 and over 50 commercial and ] services were available in London by 2001. | ||
By 2006, 500 million people worldwide were in the coverage area of DAB broadcasts, although by this time sales had only taken off in the ] and ]. |
By 2006, 500 million people worldwide were in the coverage area of DAB broadcasts, although by this time sales had only taken off in the ] and ]. As of 2006 there are nearly 1,000 DAB stations in operation world wide.<ref></ref> | ||
The standard was coordinated by the European DAB forum, formed in 1995 and reconstituted to the ] in 1997, which represents more than 30 countries. In 2006 the ] became the ] which now presides over both the DAB and DMB standard. | The standard was coordinated by the European DAB forum, formed in 1995 and reconstituted to the ] in 1997, which represents more than 30 countries. In 2006 the ] became the ] which now presides over both the DAB and DMB standard. | ||
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==DAB and FM/AM compared== | ==DAB and FM/AM compared== | ||
Traditionally radio programmes were broadcast on different frequencies via ] and ], and the radio had to be tuned into each frequency. This used up a comparatively large amount of spectrum for a relatively small number of stations, limiting listening choice. DAB is a digital radio broadcasting system that through the application of ] and compression combines multiple audio streams onto a single broadcast frequency called a ]. | Traditionally radio programmes were broadcast on different frequencies via ] and ], and the radio had to be tuned into each frequency. This used up a comparatively large amount of spectrum for a relatively small number of stations, limiting listening choice. DAB is a digital radio broadcasting system that (through the application of ] and compression) combines multiple audio streams onto a single broadcast frequency, called a ]. | ||
Within an overall target bit rate for the ], individual stations can be allocated different bit rates. The number of channels within a ] can be increased by lowering average bit rates, but at the expense of the quality of streams. Error correction under the DAB standard makes the signal more robust but reduces the total bit rate available for streams. | Within an overall target bit rate for the ], individual stations can be allocated different bit rates. The number of channels within a ] can be increased by lowering average bit rates, but at the expense of the quality of streams. Error correction under the DAB standard makes the signal more robust but reduces the total bit rate available for streams. | ||
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==Sound quality== | ==Sound quality== | ||
One of the primary objectives of converting to digital transmission was to enable higher ] than analogue ] radio. | |||
The original objectives of converting to digital transmission were to enable higher ], more stations and more resistance to noise, co-channel interference and ] than in analogue ] radio. However, in the UK, Denmark, Norway and Switzerland, which are the leading countries with regard to implementing DAB, 98% of stereo radio stations on DAB have a lower sound quality than FM due to the bit rate levels they use being too low for the inefficient ] audio codec to provide good audio quality.<ref name="robinson"/> | |||
However, in the UK (where more than 1/3rd of all DAB stations are in operation) more than 98% of all stereo music DAB stations use a bitrate of 128 kbit/s,<ref></ref> which offers sound quality that expert listeners find is "usually worse" than FM radio.<ref name="robinson"></ref> | |||
The following paragraph about bit rate levels to be used on DAB was written by an engineer in the ] Research & Development department and highlights why bit rates as low as 128 kbit/s should not be used on DAB: | |||
{{cquote2|A value of 256 kbit/s has been judged to provide a high quality stereo broadcast signal. However, a small reduction, to 224 kbit/s is often adequate, and in some cases it may be possible to accept a further reduction to 192 kbit/s, especially if redundancy in the stereo signal is exploited by a process of 'joint stereo' encoding (i.e. some sounds appearing at the centre of the stereo image need not be sent twice). At 192 kbit/s, it is relatively easy to hear imperfections in critical audio material.|BBC R&D White Paper WHP 061 June 2003<ref>{{cite web|url=http://www.bbc.co.uk/rd/pubs/whp/whp-pdf-files/WHP061.pdf|title=BBC R&D White Paper WHP 061 June 2003, DAB:An introduction to the EUREKA DAB System and a guide to how it works|accessdate=2007-05-08}}</ref>}} | {{cquote2|A value of 256 kbit/s has been judged to provide a high quality stereo broadcast signal. However, a small reduction, to 224 kbit/s is often adequate, and in some cases it may be possible to accept a further reduction to 192 kbit/s, especially if redundancy in the stereo signal is exploited by a process of 'joint stereo' encoding (i.e. some sounds appearing at the centre of the stereo image need not be sent twice). At 192 kbit/s, it is relatively easy to hear imperfections in critical audio material.|BBC R&D White Paper WHP 061 June 2003<ref>{{cite web|url=http://www.bbc.co.uk/rd/pubs/whp/whp-pdf-files/WHP061.pdf|title=BBC R&D White Paper WHP 061 June 2003, DAB:An introduction to the EUREKA DAB System and a guide to how it works|accessdate=2007-05-08}}</ref>}} | ||
⚫ | Despite these criticism, a recent survey among radio listeners in the UK, revealed that 94% find the sound quality of DAB stations is "much better", "better" or "the same" as FM.<ref></ref> | ||
⚫ | |||
⚫ | However, when the BBC reduced the bit-rate of transmission of ] from 192 kbit/s to 160 kbit/s, (on ] ]) the resulting degradation of audio quality prompted a number of complaints.<ref></ref> The BBC later announced that following this testing of new equipment, it would resume the previous practice of transmitting Radio 3 at 192 kbit/s whenever there were no other demands on bandwidth.<ref></ref> | ||
The UK Government seeks to maximize license-revenue from the available spectrum.{{Fact|date=November 2007}} Therefore it ‘squeezes in’ as many stations as possible. | |||
‘Squeezing in’ techniques include: | |||
*Minimizing the bit-rate, to the lowest level of sound-quality that listeners are willing to tolerate. (This is generally 128 kbit/s for stereo. BBC Radio 3 is exceptional in using 192 kbit/s. There was an outcry when BBC Radio 3 reduced the bit-rate to 160 kbit/s, so 192 kbit/s was restored.) | |||
*Heavy compression - compressing the dynamic range of a signal (reducing sound-quality). | |||
*Having few digital channels broadcasting in stereo. | |||
These factors reduce sound-quality to the point where it is technically inferior to FM. | |||
Maximizing Government license-revenue is not such an issue with TV, so BBC TV audio streams use a bit-rate of 256 kbit/s MP2. | |||
⚫ | Despite |
||
==Benefits of DAB== | ==Benefits of DAB== | ||
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===More stations=== | ===More stations=== | ||
DAB is more bandwidth efficient than analogue for national radio stations due to the use of ]s, enabling more stations to be placed into a smaller section of the spectrum, although it is only marginally more efficient than FM for local radio stations. | DAB is more bandwidth efficient than analogue for national radio stations due to the use of ]s (SFNs), enabling more stations to be placed into a smaller section of the spectrum, although it is only marginally more efficient than FM for local radio stations. | ||
In certain areas — particularly rural areas — the introduction of DAB gives radio listeners a greater choice of radio stations. For instance, in ], radio listeners overnight experienced an increase in available stations from 6 to 21 when DAB was introduced in November 2006. | In certain areas — particularly rural areas — the introduction of DAB gives radio listeners a greater choice of radio stations. For instance, in ], radio listeners overnight experienced an increase in available stations from 6 to 21 when DAB was introduced in November 2006. | ||
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====Music radio stations broadcasting in mono==== | ====Music radio stations broadcasting in mono==== | ||
A large and increasing number of music radio stations and stations that carry drama on DAB in the UK are being broadcast in mono |
A large and increasing number of music radio stations and stations that carry drama on DAB in the UK are being broadcast in mono, while they are virtually all available in stereo on other digital platforms, and FM.<ref>http://www.digitalradiotech.co.uk/articles/Mono-is-the-new-stereo-on-national-DAB.php</ref> | ||
====Reception quality==== | ====Reception quality==== | ||
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====Transmissions cost==== | ====Transmissions cost==== | ||
Transmission on DAB is far more expensive than on FM, and measures taken by broadcasters to limit their costs have resulted in some DAB ensembles having to carry |
Transmission on DAB is far more expensive than on FM, and measures taken by broadcasters to limit their costs have resulted in some DAB ensembles having to carry many channels, forcing bit rates to be reduced to levels that deliver sound quality inferior to traditional FM (''see ]''). | ||
====Compatibility==== | ====Compatibility==== | ||
In 2006 tests |
In 2006 tests began using the much improved ] codec for DAB+. Virtually none of the current receivers in the field support the new codec, however, thus making them partially obsolete once DAB+ broadcasts begin and completely obsolete once the old MPEG-1 Layer 2 stations are switched off. | ||
====Power requirements==== | ====Power requirements==== | ||
As DAB requires digital signal processing techniques to convert from the received digitally encoded signal to the analogue audio content, the complexity of the electronic circuitry required to do this is high. This translates into needing more power to effect this conversion than compared to an analogue FM to audio conversion, meaning that portable receiving equipment will tend to have a shorter battery life, or require higher power (and hence more bulk). | As DAB requires digital signal processing techniques to convert from the received digitally encoded signal to the analogue audio content, the complexity of the electronic circuitry required to do this is high. This translates into needing more power to effect this conversion than compared to an analogue FM to audio conversion, meaning that portable receiving equipment will tend to have a shorter battery life, or require higher power (and hence more bulk).<ref></ref> | ||
As an indicator of this increased power consumption, dual FM/DAB radios quote the length of time they can play on a single charge. For DAB, this is often between one-sixth and one-twelfth of the time they can play when in FM mode. |
As an indicator of this increased power consumption, dual FM/DAB radios quote the length of time they can play on a single charge. For DAB, this is often between one-sixth and one-twelfth of the time they can play when in FM mode.{{fact|date=April 2008}} | ||
====Adoption==== | |||
⚫ | Due to poor uptake and the large ] of FM receivers in homes and particularly in cars, some broadcasters have recently closed their DAB stations. "We do not believe that - with its current cost structure and infrastructure - is an economically viable platform," the commercial broadcaster, GCap Media, said.<ref>http://www.ft.com/cms/s/0/7791a438-ea56-11dc-b3c9-0000779fd2ac.html?nclick_check=1</ref> <ref>http://www.theregister.co.uk/2008/03/06/dab_fail/</ref> | ||
====Other criticism==== | ====Other criticism==== | ||
If the signal reception becomes marginal the audio will first start to burble or cut out rapidly and if the signal continues to degrade the audio will cut out more often. There is also less chance of long distance reception that hobbyists enjoy because each frequency/multiplex is used more often. | If the signal reception becomes marginal the audio will first start to burble or cut out rapidly and if the signal continues to degrade the audio will cut out more often. There is also less chance of long distance reception that hobbyists enjoy because each frequency/multiplex is used more often. | ||
==Technology== | ==Technology== | ||
===Bands and modes=== | ===Bands and modes=== | ||
Eureka 147 DAB uses a wide-bandwidth broadcast technology and typically spectra have been allocated for it in ] (174–240 MHz) and ] (1452–1492 MHz), although the scheme allows for operation almost anywhere above 30 ]. The US military has reserved L-Band in the USA |
Eureka 147 DAB uses a wide-bandwidth broadcast technology and typically spectra have been allocated for it in ] (174–240 MHz) and ] (1452–1492 MHz), although the scheme allows for operation almost anywhere above 30 ]. The US military has reserved L-Band in the USA, blocking its use for other purposes in America. Canada reached an agreement with The United States, saying that they will restrict L-Band DAB to terrestrial broadcast to avoid interference. | ||
DAB has a number of country specific transmission modes (I, II, III and IV). For worldwide operation a receiver must support all 4 modes: | DAB has a number of country specific transmission modes (I, II, III and IV). For worldwide operation a receiver must support all 4 modes: | ||
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The older version of DAB that is being used in the UK, Ireland, Denmark, Norway and Switzerland, uses the ] audio codec, which is also known as ''MP2'' due to computer files using those characters for their ]. | The older version of DAB that is being used in the UK, Ireland, Denmark, Norway and Switzerland, uses the ] audio codec, which is also known as ''MP2'' due to computer files using those characters for their ]. | ||
The new DAB+ standard has adopted the ] version 2 audio codec, commonly known as ''AAC+'' or ''aacPlus''. |
The new DAB+ standard has adopted the ] version 2 audio codec, commonly known as ''AAC+'' or ''aacPlus''. This newer, more efficient audio codec should allow broadcasters using DAB+ to provide higher audio quality, more stations than they currently can, or some combination of the two. | ||
Proponents claim AAC/AAC+ is a factor of two to three times more efficient than MP2 (depending on the bitrate used).<ref>http://www.ebu.ch/en/technical/trev/trev_305-moser.pdf</ref> However, most testing has found the advantages of AAC/AAC+ be much less significant, from just barely improved performance on most material (at higher bitrate), to no more than 2× better than MP2 (at very low bitrates).<ref>Wustenhagen et al, ''Subjective Listening Test of Multi-channel Audio Codecs'', AES 105th Convention Paper 4813, San Francisco 1998</ref> <ref>http://www.ebu.ch/CMSimages/en/tec_doc_t3324-2007_tcm6-53801.pdf</ref> | |||
The choice of audio codec is one of the most important aspects in the design of a digital radio system, because the efficiency of the audio codec determines how many radio stations can be carried on a multiplex at a given level of audio quality. The capacity of a DAB multiplex is fixed, so the more efficient the audio codec is, the more stations can be carried, and vice versa. Similarly, for a fixed bit-rate level, the more efficient the audio codec is the higher the audio quality will be. | |||
====Error-correction coding==== | ====Error-correction coding==== | ||
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==== Single-frequency networks ==== | ==== Single-frequency networks ==== | ||
] allows the use of single-frequency networks (SFN), which means that a network of transmitters can provide coverage to a large area |
] allows the use of single-frequency networks (SFN), which means that a network of transmitters can provide coverage to a large area, where all transmitters use the same transmission frequency. Transmitters that are part of an SFN need to be very accurately synchronized with other transmitters in the network, which requires the transmitters to use very accurate clocks. | ||
When a receiver receives a signal that has been transmitted from the different transmitters that are part of an SFN, the signals from the different transmitters will typically have different delays, but to OFDM they will appear to simply be different multipaths of the same signal. Reception difficulties can arise, however, when the relative delay of multipaths exceeds the OFDM guard interval duration, and there are frequent reports of reception difficulties due to this issue when there is a ''lift'', such as when there's high pressure, due to signals travelling farther than usual, and thus the signals are likely to arrive with a relative delay that is greater than the OFDM guard interval. | When a receiver receives a signal that has been transmitted from the different transmitters that are part of an SFN, the signals from the different transmitters will typically have different delays, but to OFDM they will appear to simply be different multipaths of the same signal. Reception difficulties can arise, however, when the relative delay of multipaths exceeds the OFDM guard interval duration, and there are frequent reports of reception difficulties due to this issue when there is a ''lift'', such as when there's high pressure, due to signals travelling farther than usual, and thus the signals are likely to arrive with a relative delay that is greater than the OFDM guard interval. | ||
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===DAB<sup>+</sup>=== | ===DAB<sup>+</sup>=== | ||
], the organisation in charge of the DAB standards, announced a major non- |
], the organisation in charge of the DAB standards, announced a major non-] upgrade to the Eureka 147 system in 2006 when the ] v2 audio codec<ref>http://www.worlddab.org/upload/uploaddocs/WorldDMBPress%20Release_November.pdf</ref> (also known as ]) was adopted. The new standard, which is called DAB+, has also adopted the ] audio format and stronger ] in the form of ] coding. DAB+ has been standardised as ] TS 102 563. | ||
As DAB+ is not backwards-compatible ordinary DAB receivers cannot receive DAB+ broadcasts |
As DAB+ is not backwards-compatible, ordinary DAB receivers cannot receive DAB+ broadcasts. However, DAB receivers that will be able to receive the new DAB+ standard via a ] went on sale in July 2007. If a receiver is DAB+-upgradeable there will be a sign on the product itself or in the literature for the product, but the vast majority of receivers on sale don't support DAB+ yet. | ||
DAB+ broadcasts have already launched in Italy, and several other countries are also expected to launch DAB+ broadcasts over the next few years, such as Switzerland in 2008, Malta in 2008, Australia on 1st January 2009, Germany in 2009. When DAB+ stations launch in the UK, Norway and Denmark, they will transmit alongside existing DAB stations that use the old ] audio format, and most existing DAB stations are expected to continue broadcasting until the vast majority of receivers support DAB+<ref></ref>, at which point stations using the old DAB format will be switched off. There is also a great deal of interest in using DAB+ in Asian countries, such as China. Read ] for details. | DAB+ broadcasts have already launched in Italy, and several other countries are also expected to launch DAB+ broadcasts over the next few years, such as Switzerland in 2008, Malta in 2008, Australia on 1st January 2009, Germany in 2009, and the UK around 2010-2013.<ref>http://www.digitalradiotech.co.uk/documents/DRDB_UK_DAB+_policy.pdf</ref> When DAB+ stations launch in the UK, Norway and Denmark, they will transmit alongside existing DAB stations that use the old ] audio format, and most existing DAB stations are expected to continue broadcasting until the vast majority of receivers support DAB+<ref></ref>, at which point stations using the old DAB format will be switched off. There is also a great deal of interest in using DAB+ in Asian countries, such as China. Read ] for details. | ||
===DMB=== | ===DMB=== | ||
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More than 20 ], either as a permanent technology or as test transmissions. The UK, along with Denmark, Norway, Belgium, Switzerland and South-Korea maintain a growing base of DAB listeners. | More than 20 ], either as a permanent technology or as test transmissions. The UK, along with Denmark, Norway, Belgium, Switzerland and South-Korea maintain a growing base of DAB listeners. | ||
⚫ | {{Wireless video}} | ||
==See also== | ==See also== | ||
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* | * | ||
* | * | ||
⚫ | {{Wireless video}} | ||
{{Audio broadcasting}} | {{Audio broadcasting}} | ||
Revision as of 19:38, 23 April 2008
Digital Audio Broadcasting ('DAB, also known as Eureka 147 or E!147) designed in the 1980s' and early 1990s', is a digital radio technology for broadcasting radio stations. DAB is used in several countries, particularly in Western Europe. As of 2006, nearly 1,000 stations worldwide broadcast in the DAB format.
An upgraded version of the system has now been developed, called DAB+, which provides higher audio quality at lower data rates. DVB+ is not backwards-compatible with the original standard, however.
History
DAB has been under development since 1981 at the Institut für Rundfunktechnik (IRT). In 1985 the first DAB demonstrations were held at the WARC-ORB in Geneva and in 1988 the first DAB transmissions were made in Germany. Later, DAB (or Eureka-147) was developed as an inter-governmental, pan-European research initiative, under the Eureka project, which started in 1987. Many of the technologies behind the MPEG-1 Audio Layer II ("MP2") codec were developed as part of the E!147 project. DAB was the first standard based on orthogonal frequency division multiplexing (OFDM) modulation technique, which since then has become one of the most popular transmission schemes for modern wideband digital communication systems.
The choice of audio codec, modulation and error-correction coding schemes was made, and the first trial broadcasts were tested, in 1990. Public demonstrations were made in 1993 in the United Kingdom. The protocol specification was finalized in 1993 and adopted by the ITU-R standardization body in 1994, the European community in 1995 and by ETSI in 1997. Pilot broadcasts were launched in several countries in 1995.
The UK was the first country to receive a wide range of radio stations via DAB. Commercial DAB receivers began to be sold in 1999 and over 50 commercial and BBC services were available in London by 2001.
By 2006, 500 million people worldwide were in the coverage area of DAB broadcasts, although by this time sales had only taken off in the UK and Denmark. As of 2006 there are nearly 1,000 DAB stations in operation world wide.
The standard was coordinated by the European DAB forum, formed in 1995 and reconstituted to the World DAB Forum in 1997, which represents more than 30 countries. In 2006 the World DAB Forum became the World DMB Forum which now presides over both the DAB and DMB standard.
In October 2005, the World DMB Forum instructed its Technical Committee to carry out the work needed to adopt the AAC+ audio codec and stronger error correction coding. This work led to the launch of the new DAB+ system.
DAB and FM/AM compared
Traditionally radio programmes were broadcast on different frequencies via FM and AM, and the radio had to be tuned into each frequency. This used up a comparatively large amount of spectrum for a relatively small number of stations, limiting listening choice. DAB is a digital radio broadcasting system that (through the application of multiplexing and compression) combines multiple audio streams onto a single broadcast frequency, called a DAB ensemble.
Within an overall target bit rate for the DAB ensemble, individual stations can be allocated different bit rates. The number of channels within a DAB ensemble can be increased by lowering average bit rates, but at the expense of the quality of streams. Error correction under the DAB standard makes the signal more robust but reduces the total bit rate available for streams.
Use of frequency spectrum and transmitter sites
DAB gives substantially higher spectral efficiency, measured in programmes per MHz and per transmitter site, than analogue communication. However, since there are no plans yet to cease analogue FM transmissions, and most radio channels are transmitted both over FM and digitally, this advantage is not exploited to a high degree.
Numerical example: FM requires 0.3 MHz per programme. The frequency reuse factor is approximately 15, meaning that only one out of 15 transmitters can use the same channel frequency without problems with co-channel interference, i.e. cross-talk. This results in 1 / 15 / (0.3 MHz) = 0.22 programmes/transmitter/MHz. DAB with 192 kbit/s codec requires 1.536 MHz * 192 kbit/s / 1136 kbit/s = 0.26 MHz per audio programme. The frequency reuse factor for local programmes and multi-frequency broadcasting networks (MFS) is typically 4, resulting in 1 / 4 / (0.26 MHz) = 0.96 programmes/transmitter/MHz. This is 4.3 times as efficient. For single frequency networks (SFN), for example of national programmes, the channel re-use factor is 1, resulting in 1/1/0.25 MHz = 3.85 programmes/transmitter/MHz, which is 17.3 times as efficient as FM.
Note the above capacity improvement may not always be achieved at the L-band frequencies, since these are more sensitive to obstacles than the FM band frequencies, and may cause shadow fading for hilly terrain and for indoor communication. The number of transmitter sites or the transmission power required for full coverage of a country may be rather high at these frequencies, to avoid that the system becomes noise limited rather than limited by co-channel interference.
Sound quality
One of the primary objectives of converting to digital transmission was to enable higher fidelity than analogue FM radio.
However, in the UK (where more than 1/3rd of all DAB stations are in operation) more than 98% of all stereo music DAB stations use a bitrate of 128 kbit/s, which offers sound quality that expert listeners find is "usually worse" than FM radio.
A value of 256 kbit/s has been judged to provide a high quality stereo broadcast signal. However, a small reduction, to 224 kbit/s is often adequate, and in some cases it may be possible to accept a further reduction to 192 kbit/s, especially if redundancy in the stereo signal is exploited by a process of 'joint stereo' encoding (i.e. some sounds appearing at the centre of the stereo image need not be sent twice). At 192 kbit/s, it is relatively easy to hear imperfections in critical audio material.
— BBC R&D White Paper WHP 061 June 2003
Despite these criticism, a recent survey among radio listeners in the UK, revealed that 94% find the sound quality of DAB stations is "much better", "better" or "the same" as FM.
However, when the BBC reduced the bit-rate of transmission of Radio 3 from 192 kbit/s to 160 kbit/s, (on 6 July 2006) the resulting degradation of audio quality prompted a number of complaints. The BBC later announced that following this testing of new equipment, it would resume the previous practice of transmitting Radio 3 at 192 kbit/s whenever there were no other demands on bandwidth.
Benefits of DAB
Current AM and FM terrestrial broadcast technology is well established, compatible, and cheap to manufacture. Benefits of DAB over analogue systems are explained below.
Improved end-user features
DAB radios automatically tune to all the available stations, offering a list of all stations.
DAB can carry "radiotext" (in DAB terminology, Dynamic Label Segment, or DLS) from the station giving real-time information such as song titles, music type and news or traffic updates. Advance programme guides can also be transmitted. A similar feature also exists on FM in the form of the RDS. (However, not all FM receivers allow radio stations to be stored by name.)
Some radios offer a pause facility on live broadcasts, caching the broadcast stream on local flash memory, although this function is limited.
More stations
DAB is more bandwidth efficient than analogue for national radio stations due to the use of single-frequency networks (SFNs), enabling more stations to be placed into a smaller section of the spectrum, although it is only marginally more efficient than FM for local radio stations.
In certain areas — particularly rural areas — the introduction of DAB gives radio listeners a greater choice of radio stations. For instance, in South Norway, radio listeners overnight experienced an increase in available stations from 6 to 21 when DAB was introduced in November 2006.
Total cost of ownership
DAB transmits several channels per multiplex, meaning ownership and maintenance can be outsourced and provided by one organisation instead of each radio station, lowering the maintenance cost over time.
Reception quality
The DAB standard integrates features to reduce the negative consequences of multipath fading and signal noise, which afflict existing analogue systems.
Also, as DAB transmits digital audio, there is no hiss with a weak signal, which can happen on FM. However, radios in the fringe of a DAB signal, can experience a "bubbling mud" sound interrupting the audio and/or the audio cutting out altogether.
Less pirate interference
The specialised nature and cost of DAB broadcasting equipment provide barriers to pirate radio stations broadcasting on DAB. In cities such as London with large numbers of pirate radio stations broadcasting on FM, this means that some stations can be reliably received via DAB in areas where they are regularly difficult or impossible to receive on FM due to pirate radio interference.
Variable bandwidth
Mono talk radio, news and weather channels and other non-music programs need significantly less bandwidth than a typical music radio station, which allows DAB to carry these programmes at lower bit rates, leaving more bandwidth to be used for other programs. However, this had led to the situation where numerous music radio stations are being broadcast in mono, see the section on music radio stations broadcasting in mono for more details.
Criticisms of DAB
Music radio stations broadcasting in mono
A large and increasing number of music radio stations and stations that carry drama on DAB in the UK are being broadcast in mono, while they are virtually all available in stereo on other digital platforms, and FM.
Reception quality
The reception quality on DAB can be poor even for people that live well within the coverage area. The reason for this is that the old version of DAB uses weak error correction coding so that when there are a lot of errors with the received data not enough of the errors can be corrected and a "bubbling mud" sound occurs. In some cases a complete loss of signal can happen. This situation will be improved upon in the new DAB standard (DAB+, discussed below) that uses stronger error correction coding and as signal powers are increased.
Poor Marketing
The advantages of DAB have been promoted without mentioning limitations and has led to some consumer disappointment.
Signal Delay
The signal processing required in the receiver (FFT) takes time to perform. This delays the signal to the listener by about 2 seconds (depending on the decoding circuitry used). This has 2 disadvantages: Time signals are not accurate and listeners using a combination of FM and DAB radios (e.g. in different rooms of a house) will not hear an intelligible signal when both receivers are within earshot. This could have been overcome by defining a delay for all DAB receivers and delaying the FM broadcast signal by the same amount.
Coverage
As DAB is at a relatively early stage of deployment, DAB coverage is poor in nearly all countries in comparison to the high population coverage provided by FM.
Transmissions cost
Transmission on DAB is far more expensive than on FM, and measures taken by broadcasters to limit their costs have resulted in some DAB ensembles having to carry many channels, forcing bit rates to be reduced to levels that deliver sound quality inferior to traditional FM (see Criticisms of DAB in the UK).
Compatibility
In 2006 tests began using the much improved HE-AAC codec for DAB+. Virtually none of the current receivers in the field support the new codec, however, thus making them partially obsolete once DAB+ broadcasts begin and completely obsolete once the old MPEG-1 Layer 2 stations are switched off.
Power requirements
As DAB requires digital signal processing techniques to convert from the received digitally encoded signal to the analogue audio content, the complexity of the electronic circuitry required to do this is high. This translates into needing more power to effect this conversion than compared to an analogue FM to audio conversion, meaning that portable receiving equipment will tend to have a shorter battery life, or require higher power (and hence more bulk).
As an indicator of this increased power consumption, dual FM/DAB radios quote the length of time they can play on a single charge. For DAB, this is often between one-sixth and one-twelfth of the time they can play when in FM mode.
Adoption
Due to poor uptake and the large installed base of FM receivers in homes and particularly in cars, some broadcasters have recently closed their DAB stations. "We do not believe that - with its current cost structure and infrastructure - is an economically viable platform," the commercial broadcaster, GCap Media, said.
Other criticism
If the signal reception becomes marginal the audio will first start to burble or cut out rapidly and if the signal continues to degrade the audio will cut out more often. There is also less chance of long distance reception that hobbyists enjoy because each frequency/multiplex is used more often.
Technology
Bands and modes
Eureka 147 DAB uses a wide-bandwidth broadcast technology and typically spectra have been allocated for it in Band III (174–240 MHz) and L band (1452–1492 MHz), although the scheme allows for operation almost anywhere above 30 MHz. The US military has reserved L-Band in the USA, blocking its use for other purposes in America. Canada reached an agreement with The United States, saying that they will restrict L-Band DAB to terrestrial broadcast to avoid interference.
DAB has a number of country specific transmission modes (I, II, III and IV). For worldwide operation a receiver must support all 4 modes:
- Mode I for Band III, Earth
- Mode II for L-Band, Earth and satellite
- Mode III for frequencies below 3 GHz, Earth and satellite
- Mode IV for L-Band, Earth and satellite
Protocol stack
From a protocol stack viewpoint, the technologies used on DAB inhabit the following layers: the audio codec inhabits the application layer. Below that is the physical layer, which contains the error-correction coding and OFDM modulation, dealing with the over-the-air transmission and reception of data. Some aspects of these are described below.
Audio codec
The older version of DAB that is being used in the UK, Ireland, Denmark, Norway and Switzerland, uses the MPEG-1 Audio Layer 2 audio codec, which is also known as MP2 due to computer files using those characters for their file extension.
The new DAB+ standard has adopted the HE-AAC version 2 audio codec, commonly known as AAC+ or aacPlus. This newer, more efficient audio codec should allow broadcasters using DAB+ to provide higher audio quality, more stations than they currently can, or some combination of the two.
Proponents claim AAC/AAC+ is a factor of two to three times more efficient than MP2 (depending on the bitrate used). However, most testing has found the advantages of AAC/AAC+ be much less significant, from just barely improved performance on most material (at higher bitrate), to no more than 2× better than MP2 (at very low bitrates).
Error-correction coding
Error-correction coding (ECC) is an important technology for a digital communication system because it determines how robust the reception will be for a given signal strength - stronger ECC will provide more robust reception than a weaker form.
The old version of DAB uses punctured convolutional coding for its ECC. The coding scheme uses unequal error protection (UEP), which means that parts of the audio bit-stream that are more susceptible to errors causing audible disturbances are provided with more protection (i.e. a lower code rate) and vice versa. However, the UEP scheme used on DAB results in there being a grey area in between the user experiencing good reception quality and no reception at all, as opposed to the situation with most other wireless digital communication systems that have a sharp "digital cliff", where the signal rapidly becomes unusable if the signal strength drops below a certain threshold. When DAB listeners receive a signal in this intermediate strength area they experience a "burbling" sound which interrupts the playback of the audio, and listeners find this to be more unpleasant to listen to than hiss on FM.
The new DAB+ standard has incorporated Reed-Solomon ECC as an "outer layer" of coding that is placed around the "inner layer" of convolutional coding used by the older DAB system, although on DAB+ the convolutional coding uses equal error protection (EEP) rather than UEP. This combination of convolutional coding as the inner layer of coding, followed by a byte interleaver then an outer layer of Reed-Solomon coding - so-called "concatenated coding" - became a popular ECC scheme in the 1990s, and NASA adopted it for its deep-space missions. One slight difference between the concatenated coding used by the DAB+ system and that used on most other systems is that it uses a rectangular byte interleaver rather than Forney interleaving in order to provide a greater interleaver depth, which increases the distance over which error bursts will be spread out in the bit-stream, which in turn will allow the Reed-Solomon error decoder to correct a higher proportion of errors.
The ECC used on DAB+ is far stronger than is used on DAB, which, with all else being equal (i.e. if the transmission powers remained the same), would translate into people who currently experience reception difficulties on DAB receiving a much more robust signal with DAB+ transmissions. It also has a far steeper "digital cliff", and listening tests have shown that people prefer this when the signal strength is low compared to the shallower digital cliff on DAB.
Modulation
Immunity to fading and inter-symbol interference (caused by multipath propagation) is achieved without equalization by means of the OFDM and DQPSK modulation techniques.
Using values for the most commonly used transmission mode on DAB, Transmission Mode I (TM I), the OFDM modulation consists of 1,536 subcarriers that are transmitted in parallel. The useful part of the OFDM symbol period is 1 millisecond, which results in the OFDM subcarriers each having a bandwidth of 1 kHz due to the inverse relationship between these two parameters, and the overall OFDM channel bandwidth is 1,537 kHz. The OFDM guard interval for TM I is 246 microseconds, which means that the overall OFDM symbol duration is 1.246 milliseconds. The guard interval duration also determines the maximum separation between transmitters that are part of the same single-frequency network (SFN), which is approximately 74 km for TM I.
Single-frequency networks
OFDM allows the use of single-frequency networks (SFN), which means that a network of transmitters can provide coverage to a large area, where all transmitters use the same transmission frequency. Transmitters that are part of an SFN need to be very accurately synchronized with other transmitters in the network, which requires the transmitters to use very accurate clocks.
When a receiver receives a signal that has been transmitted from the different transmitters that are part of an SFN, the signals from the different transmitters will typically have different delays, but to OFDM they will appear to simply be different multipaths of the same signal. Reception difficulties can arise, however, when the relative delay of multipaths exceeds the OFDM guard interval duration, and there are frequent reports of reception difficulties due to this issue when there is a lift, such as when there's high pressure, due to signals travelling farther than usual, and thus the signals are likely to arrive with a relative delay that is greater than the OFDM guard interval.
Low power gap-filler transmitters can be added to an SFN as and when desired in order to improve reception quality, although the way SFNs have been implemented in the UK up to now they have tended to consist of higher power transmitters being installed at main transmitter sites in order to keep costs down.
Bit rates
An ensemble has a maximum bit rate that can be carried, but this depends on which error protection level is used. However, all DAB multiplexes can carry a total of 864 "capacity units". The number of capacity units, or CU, that a certain bit-rate level requires depends on the amount of error correction added to the transmission, as described above. In the UK, most services transmit using 'protection level three', which provides an average ECC code rate of approximately ½, equating to a maximum bit rate per multiplex of 1184 kbit/s.
Services and ensembles
Various different services are embedded into one ensemble (which is also typically called a multiplex). These services can include:
- Primary services, like main radio stations
- Secondary services, like additional sports commentaries
- Data services
- Electronic Programme Guide (EPG)
- Collections of HTML pages and digital images (Known as 'Broadcast Web Sites')
- Slideshows, which may be synchronised with audio broadcasts
- Video
- Java Platform Applications
- IP tunneling
- Other raw data
DAB and DMB
Eureka 147 provides the infrastructure for several DAB versions.
DAB
WorldDMB, the organisation in charge of the DAB standards, announced a major non-backwards-compatible upgrade to the Eureka 147 system in 2006 when the HE-AAC v2 audio codec (also known as AAC+) was adopted. The new standard, which is called DAB+, has also adopted the MPEG Surround audio format and stronger error correction coding in the form of Reed-Solomon coding. DAB+ has been standardised as ETSI TS 102 563.
As DAB+ is not backwards-compatible, ordinary DAB receivers cannot receive DAB+ broadcasts. However, DAB receivers that will be able to receive the new DAB+ standard via a firmware upgrade went on sale in July 2007. If a receiver is DAB+-upgradeable there will be a sign on the product itself or in the literature for the product, but the vast majority of receivers on sale don't support DAB+ yet.
DAB+ broadcasts have already launched in Italy, and several other countries are also expected to launch DAB+ broadcasts over the next few years, such as Switzerland in 2008, Malta in 2008, Australia on 1st January 2009, Germany in 2009, and the UK around 2010-2013. When DAB+ stations launch in the UK, Norway and Denmark, they will transmit alongside existing DAB stations that use the old MPEG-1 Audio Layer II audio format, and most existing DAB stations are expected to continue broadcasting until the vast majority of receivers support DAB+, at which point stations using the old DAB format will be switched off. There is also a great deal of interest in using DAB+ in Asian countries, such as China. Read Regional implementations of DAB for details.
DMB
Main article: Digital Multimedia BroadcastingDAB-related standards Digital Multimedia Broadcasting (DMB) and DAB-IP are suitable for mobile radio and TV both because they support MPEG 4 AVC and WMV9 respectively as video codecs. However, a DMB video subchannel can easily be added to any DAB transmission -- as DMB was designed from the outset to be carried on a DAB subchannel. DMB broadcasts in Korea carry conventional MPEG 1 Layer II DAB audio services alongside their DMB video services.
Regional implementations of DAB
Main article: Regional implementations of DABMore than 20 countries provide DAB broadcasts, either as a permanent technology or as test transmissions. The UK, along with Denmark, Norway, Belgium, Switzerland and South-Korea maintain a growing base of DAB listeners.
See also
- ATSC Standards
- DARS
- Digital radio
- DMB (Digital Multimedia Broadcasting)
- DRM (Digital Radio Mondiale)
- Digital television
- ETSI Satellite Digital Radio (SDR)
- European Multimedia Associations Convention (EMMAC)
- FMeXtra
- HD Radio
- ISDB
- OpenCable
- Orthogonal frequency-division multiplexing#Digital radio
- OFDM system comparison table
- Sirius Satellite Radio
- Spectral efficiency comparison table
- T-DMB
- XM Satellite Radio
References
- World DMB forums list of benefits
- World DMB forums list of benefits
- DAB Around the World
- [http://www.david.robinson.org/commsbill/#1_2_3 OFCOM: Regulation in digital broadcasting: DAB digital radio bitrates and audio quality; Dynamic range compression and loudness
- DUO - Digital utgivelse ved Universitetet i Oslo - Lydkvalitetet i DAB digitalradio
- "BBC R&D White Paper WHP 061 June 2003, DAB:An introduction to the EUREKA DAB System and a guide to how it works" (PDF). Retrieved 2007-05-08.
- Ofcom reveals DAB sound quality opinions
- Friends of Radio 3 (FoR3) BBC & R3 News
- Friends of Radio 3 (FoR3) Campaign Update
- BBC has chosen DAB by Factum
- http://www.digitalradiotech.co.uk/articles/Mono-is-the-new-stereo-on-national-DAB.php
- Freeplay Energy Plc
- http://www.ft.com/cms/s/0/7791a438-ea56-11dc-b3c9-0000779fd2ac.html?nclick_check=1
- http://www.theregister.co.uk/2008/03/06/dab_fail/
- http://www.ebu.ch/en/technical/trev/trev_305-moser.pdf
- Wustenhagen et al, Subjective Listening Test of Multi-channel Audio Codecs, AES 105th Convention Paper 4813, San Francisco 1998
- http://www.ebu.ch/CMSimages/en/tec_doc_t3324-2007_tcm6-53801.pdf
- http://worlddab.org/pdf/DAB+brochure.pdf
- http://www.worlddab.org/upload/uploaddocs/WorldDMBPress%20Release_November.pdf
- http://www.digitalradiotech.co.uk/documents/DRDB_UK_DAB+_policy.pdf
- release: New High Efficiency Audio Option Added for DAB Digital Radio
- ETSI Specifications 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)
- Stott, J. H.; The How and Why of COFDM, BBC Research Development
External links
- ETSI EN 300 401 v1.4.1 - Original DAB specification
- ETSI TS 102 563 v1.1.1 - DAB+ enhancement specification
- World DAB Forum
- UK DAB news and information
- Digital One - The UK's Commercial Digital Radio Network
- Digital Online - UK's Digital Portal - (Online Searchable Guide to Stations in the UK)
- DAB Digital Radio News and Information for the UK
- DAB Ensembles Worldwide (also known as "Wohnort", the main part of the site is a list of services currently transmitting)
- DAB and Mobile Java Services
- Audio extracts of UK DAB stations
- Digital broadcasting in Ireland
- DAB in the Netherlands
- E-magazine with information about DAB
- BBC Datasheet on Eureka 147
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