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User:Rcooley~enwiki/DAB

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See also: Digital Multimedia Broadcasting
Countries with DAB, DAB+ or DMB broadcasts. (Source:WorldDMBForum country profile)

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, most extensively in Western Europe. As of 2008 there are more than 1,000 stations in operation, broadcasting in DAB format, and 500 million people in the coverage area of DAB broadcasts, worldwide.[1]

An upgraded version of the system has been developed since 2005, called DAB+, which can provide higher audio quality at lower data rates. DVB+ broadcasts are not backwards-compatible with DAB, however, so listeners will be required to purchase new receivers to listen to DAB+ broadcasts.

History[edit]

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 has since become a popular transmission scheme for modern wideband digital communication systems, used in DVB, and 802.11g.

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 held 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 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 HE-AAC v2 audio codec and stronger error correction coding. This work led to the launch of the new DAB+ system.


Channel Allocation[edit]

Traditionally radio program were broadcast on different frequencies via FM and AM, and the radio had to be tuned into each frequency. The amount of bandwidth each channel takes up is determined by the design of the receivers, and cannot be changed.

With DAB, multiple audio streams are multiplexed together into a single, large, stream called a DAB ensemble, which are broadcast together on a single frequency, many times wider than an analog FM signal, from a single transmitter. 

 Within an overall target bit rate for the 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 sound quality of streams. Error correction makes the signal more robust but reduces the total bit rate available for streams.


Efficient Use of Radio Spectrum[edit]

DAB's spectral efficiency on stereo music can range anywhere from just slightly better than analog FM radio, to as much as a factor of 17 as efficient in the case of heavy channel reuse and single-frequency networks (SFNs—see below). For mono speech broadcasts, that can be increased many times more. However, no country has yet decided to cease analog FM transmissions, so most radio channels are transmitted both over FM and DAB, eliminating this potential advantage. 

 FM requires 0.3 MHz per program. 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 programs/transmitter/MHz. 

 DAB with 192 kbit/s codec (for stereo music) requires 1.536 MHz * 192 kbit/s / 1136 kbit/s = 0.26 MHz per audio program. The frequency reuse factor for local programs and multi-frequency broadcasting networks (MFS) is typically 4, resulting in 1 / 4 / (0.26 MHz) = 0.96 programs/transmitter/MHz.  This is 4.3 times as efficient. For single frequency networks (SFN), for example of national programs, the channel re-use factor is 1, resulting in 1/1/0.25 MHz = 3.85 programs/transmitter/MHz, which is 17.3 times as efficient as FM.  For mono audio, bandwidth usage can be as low as 1/8th as much as stereo audio, so as much as 8× more efficient.

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. The number of transmitters becomes dictated by noise, rather than co-channel interference.

Still, in certain areas the introduction of DAB has in fact made possible a greater number 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.


Sound quality[edit]

One of the primary objectives of converting to digital transmission was to enable higher fidelity than analog 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,[2] which offers sound quality that expert listeners find is "usually worse" than FM radio.[3] [4]

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.[5]

The current version of DAB uses the MPEG-1 Audio Layer II (MP2) audio codec, while the newer DAB+ standard has adopted the HE-AAC v2 audio codec. This newer, more efficient audio codec should allow broadcasters using DAB+ to provide broadcasters the opportunity for higher audio quality, more stations than they currently carry, or some combination of the two.

Proponents claim HE-AACv2 is a factor of two to three times more efficient than MP2 (depending on the bitrate used).[6] However, most independent testing has found the advantages of AAC/AAC+ be much less significant, from just barely improved performance on most material (at higher bitrates), to no more than 2× better than MP2 (at very low bitrates).[7] [8]


Reception[edit]

The DAB standard integrates features to reduce the negative consequences of multipath fading and signal noise, which afflict existing analog 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. The newer DAB+ standard uses somewhat stronger error correction coding which should improve fringe reception.

The specialized 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.


Error-correction coding[edit]

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 distorting the playback of the audio.

The new DAB+ standard incorporates 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", meaning that reception will stop entirely, rather than playback with distortion, if the signal isn't strong enough for perfect playback.[9]


Modulation[edit]

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[edit]

DAB's OFDM modulation allows the use of single-frequency networks (SFNs), which means that a network of transmitters can provide coverage to a large area, where all transmitters use the same transmission frequency.

While a receiver will see these multiple signals from several different transmitters that are part of a SFN, to OFDM they will simply appear to be multipath interference from a single signal, and all but the strongest signal will be ignored.

Reception difficulties can arise, however, when the relative delay of multipaths exceeds the OFDM guard interval duration. Transmitters that are part of an SFN need to be very accurately synchronized with other transmitters in the network to avoid problems. There are frequent reports of reception difficulties when tropospheric weather patterns (such as high pressure) result in radio signals traveling farther than usual, and thus arriving at the receiver with a relative delay that is greater than the OFDM guard interval.


Bands and modes[edit]

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


Bit rates[edit]

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[edit]

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


Disorganized, POV Pro/Con List[edit]

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 program 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.)

Digital radio allows relatively easy and inexpensive storage of the radio data stream. Some radios allow radio programs to be losslessly transfered to a computer or other device. Others offer a pause facility on live broadcasts, caching the broadcast stream on local flash memory for resumption of playback moments later.

DAB transmits several channels per multiplex, meaning ownership and maintenance can be outsourced and provided by one organization instead of each radio station, potentially lowering the maintenance cost over time. [10]

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 means time signals are not accurate and that 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.

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.

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).

The complexity of the electronic circuitry required to receive and decode DAB broadcasts is much higher than analog FM. This translates into needing more power, meaning that portable DAB receiver have shorter battery life.[11] DAB+ is even more computationally intensive than DAB, therefore making this power consumption issue even worse.

Poor uptake and the large installed base of FM receivers in homes and cars has caused some broadcasters to close their DAB stations.[12] [13]


DAB+[edit]

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[14] (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.[15] 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+[16], 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.


Regional implementations of DAB[edit]

Main article: Regional implementations of DAB

More 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[edit]


References[edit]

  1. ^ http://www.worlddab.org/about_worlddmb
  2. ^ http://www.wohnort.demon.co.uk/DAB/
  3. ^ http://www.david.robinson.org/commsbill/#1_2_3
  4. ^ http://wo.uio.no/as/WebObjects/theses.woa/wa/these?WORKID=52348
  5. ^ http://www.digitalspy.co.uk/radio/a45367/ofcom-reveals-dab-sound-quality-opinions.html
  6. ^ http://www.ebu.ch/en/technical/trev/trev_305-moser.pdf
  7. ^ Wustenhagen et al, Subjective Listening Test of Multi-channel Audio Codecs, AES 105th Convention Paper 4813, San Francisco 1998
  8. ^ http://www.ebu.ch/CMSimages/en/tec_doc_t3324-2007_tcm6-53801.pdf
  9. ^ http://worlddab.org/pdf/DAB+brochure.pdf
  10. ^ http://www.factum.se/pressroom/pdf/050609%20Switzerland%20has%20chosen%20DAB%20by%20Factum.pdf
  11. ^ http://www.freeplayenergy.com/Devo.html
  12. ^ http://www.ft.com/cms/s/0/7791a438-ea56-11dc-b3c9-0000779fd2ac.html?nclick_check=1
  13. ^ http://www.theregister.co.uk/2008/03/06/dab_fail/
  14. ^ http://www.worlddab.org/upload/uploaddocs/WorldDMBPress%20Release_November.pdf
  15. ^ http://www.digitalradiotech.co.uk/documents/DRDB_UK_DAB+_policy.pdf
  16. ^ release: New High Efficiency Audio Option Added for DAB Digital Radio
  • Stott, J. H.; The How and Why of COFDM, BBC Research Development


External links[edit]

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