Digital_audio_broadcasting

Digital Audio Broadcasting

Digital Audio Broadcasting (DAB), also known as Eureka 147, is a digital radio technology for broadcasting radio stations, used in several countries, particularly in Europe. As of 2006, approximately 1,000 stations worldwide broadcast in the DAB format.

The DAB standard was designed in the 1980s, and receivers have been available in many countries for several years. Proponents claim the standard offers several benefits over existing analogue FM radio, such as higher fidelity audio, more stations in the same broadcast spectrum, and increased resistance to noise, multipath, fading, and co-channel interference. However, listening tests carried out by experts in the field of audio have shown that the audio quality on DAB is lower than on FM in the UK, which, with 7m sales, is the country that accounts for the vast majority of global DAB sales-to-date, due to 98% of stereo stations using a bit rate of 128 kbit/s with the MP2 audio codec, which is unable to match the audio quality provided by FM.

An upgraded version of the system was released in February 2007, which is called DAB+. This is not backward-compatible with DAB, which means that receivers that support the new standard will only be able to receive DAB+ broadcasts. DAB+ is apporximately 4 times more efficient than DAB due to the adoption of the AAC+ audio codec, and DAB+ can provide perceived CD-quality audio with as low as 64kbit/s. Reception quality will also be more robust on DAB+ than on DAB due to the addition of Reed-Solomon error correction coding.

Italy has started transmitting DAB+ stations. Malta, Switzerland and Hungary are due to launch DAB+ stations in 2008 and Australia and Germany are planning to launch DAB+ in 2009. The radio industry in the UK is expecting DAB+ stations to launch between 2010 - 2013, and podcast services using the DAB+ format will be launched in the UK in 2009.

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 a research project for the European Union (Eureka project number EU147), which started in 1987 on initiative by a consortium formed in 1986. The MPEG-1 Audio Layer II ("MP2") codec was created as part of the EU147 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.

A choice of audio codec, modulation and error-correction coding schemes and first trial broadcasts were made 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. In 2006 there are approximately 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. This has led to a vast increase in the number of stations available to listeners, especially outside of the major urban conurbations.

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

The original objectives of converting to digital transmission were to enable higher fidelity, more stations and more resistance to noise, co-channel interference and multipath than in analogue FM 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 MPEG Layer 2 audio codec to provide good audio quality.

The following paragraph about bit rate levels to be used on DAB was written by an engineer in the BBC Research & Development department and highlights why bit rates as low as 128 kbit/s should not be used on DAB:

On 6 July 2006 the BBC reduced the bit-rate of transmission of Radio 3 from 192 kbit/s to 160 kbit/s. The resulting degradation of audio quality prompted a number of complaints to the Corporation. 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.

The UK Government seeks to maximize licence-revenue from the available spectrum. 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 and 80 kbit/s or even 64 kbit/s for mono, although with these mono low rates acceptable quality is only achieved with speech only.
  • 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 some criticism of sound quality (see the criticism section), a recent survey among radio listeners in the UK, a territory where the low bit-rates are often criticised, revealed that 94% experience a sound quality that is "much better", "better" or "the same" as FM.

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 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 experienced an increase in available stations from 6 to 21 when DAB was introduced in November 2006.

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 some 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 number of music radio stations and stations that carry drama on DAB in the UK are being broadcast in mono when some argue that they should be in stereo These stations are often available in stereo on other digital platforms, where capacity is not as constrainted, and on FM where applicable.

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 additional transmitters are built.

Signal delay

The nature of a SFN is such that the transmitters in a network must broadcast the same signal at the same time. To achieve synchronization, the broadcaster must counter any differences in propagation time incurred by the different methods and distances involved in carrying the signal from the multiplexer to the different transmitters. This is done by applying a delay to the incoming signal at the transmitter based on a timestamp generated at the multiplexer, created taking into account the maximum likely propagation time, with a generous added margin for safety. Also delays in the receiver due to digital processing (e.g. deinterleaving) add to the overall delay to the listener. This delays the signal to the listener by about 2 seconds (depending on the decoding circuitry used). This has two disadvantages: (i) DAB radios are out of step with live events so time signals are not accurate and the experience of listening to live commentaries on events being watched is impaired, and (ii) 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.

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 too 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 finally 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.

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 only, blocking its use for other purposes in America, and the United States has reached an agreement with Canada that the latter 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. AAC+ is approximately three-times more efficient than MP2, which means that broadcasters using DAB+ will be able to provide far higher audio quality or far more stations than they can on DAB, or, as is most likely, a combination of both higher audio quality and more stations will be provided.

One of the most important decisions regarding the design of a digital radio system is the choice of which audio codec to use, 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 (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 - up to the size of a country - where all transmitters use the same transmission frequency. Transmitters that are part of an SFN need to be very accurately synchronised 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

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-backwardly 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, Malta and Hungary in 2008, Australia on 1 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 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

DAB-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

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

References

  • 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

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