Digital Audio Broadcasting

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Digital Audio Broadcasting (DAB), 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.[1]

The DAB standard was initiated as a European research project in the 1980s,[2] and the BBC launched the first DAB digital radio in 1995.[3] DAB receivers have been available in many countries since the end of the nineties. DAB may offer more radio programmes over a specific spectrum than analogue FM radio. DAB is more robust with regard to noise and multipath fading for mobile listening, but DAB reception quality degrades rapidly when the signal strength isn't strong, whereas FM reception quality degrades slowly.

An "informal listening test" by Professor Sverre Holm has shown that for stationary listening the audio quality on DAB is lower than FM stereo, due to most stations using a bit rate of 128 kbit/s or less, with the MP2 audio codec, which requires 160 kbit/s to achieve perceived FM quality. 128 kbit/s gives better dynamic range or signal-to-noise ratio than FM radio, but a more smeared stereo image, and an upper cutoff frequency of 14 kHz, corresponding to 15 kHz of FM radio.[4] However, "CD sound quality" with MP2 is possible "with 256..192 kbps".[5]

An upgraded version of the system was released in February 2007, which is called DAB+. DAB is not forward compatible with DAB+, which means that DAB-only receivers will not be able to receive DAB+ broadcasts.[6] DAB+ is approximately twice as efficient as DAB due to the adoption of the AAC+ audio codec, and DAB+ can provide high quality audio with as low as 64kbit/s.[7] Reception quality will also be more robust on DAB+ than on DAB due to the addition of Reed-Solomon error correction coding.

More than 20 countries provide DAB transmissions, and several countries, such as Australia, Italy, Malta and Switzerland, have started transmitting DAB+ stations. See Countries using DAB/DMB. However, DAB radio has still not replaced the old FM system in popularity.

Contents

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 was developed as a research project for the European Union (EUREKA), 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 were approximately 1,000 DAB stations in operation world wide.[8]

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 relatively narrow band centred on 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 an increase in the number of stations available to listeners, especially outside of the major urban areas.

Numerical example: Analog FM requires 0.2 MHz per programme. The frequency reuse factor in most countries is approximately 15, meaning that only one out of 15 transmitter sites can use the same channel frequency without problems with co-channel interference, i.e. cross-talk. Assuming a total availability of 102 FM channels at a bandwidth of 0.2MHz over the Band II spectrum of 87.5 to 108.0 MHz, an avarage of 102/15 = 6.8 radio channels are possible on each transmitter site (plus lower-power local transmitters causing less interference). This results in a system spectral efficiency of 1 / 15 / (0.2 MHz) = 0.30 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 (MFN) is typically 4 or 5, resulting in 1 / 4 / (0.26 MHz) = 0.96 programmes/transmitter/MHz. This is 3.2 times as efficient as analog FM for local stations. For single frequency network (SFN) transmission, 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 12.7 times as efficient as FM for national and regional networks.

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.

Note also that this statement ignores the FM 'capture effect', which allows stations on the same frequency to be located relatively close together: as long as one station is received a couple of dB more strongly than the other at the receiver, the receiver will be 'captured' by the stronger signal and cross-talk does not occur. So in the case of multiple frequency networks, the actual relative efficiency of DAB over FM will not match the figures calculated above.

The statement above also specifies 102 channels being available at 200 kHz width in Band II whereas there are in fact 206 channels available in theory in a band of width 20.5 MHz. The "frequency reuse factor" referred to is a pointless piece of theory because coverage with any given frequency by a single transmitter achieved over a highly-populated area like the UK for instance is heavily limited by terrain and many other physical features.

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, leading countries in implementing DAB, most of stereo radio stations on DAB use compression to such a degree, that it becomes lower than sound quality in non-mobile FM reception. This is because of the bit rate levels being too low for the MPEG Layer 2 audio codec to provide high fidelity audio quality.[9]

The BBC Research & Development department states that at least 192kbit/s is necessary for a high fidelity stereo broadcast :

“ 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[10]

When BBC in July 2006 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.[11] 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.

Satisified listeners

Still, a survey of DAB listeners (including mobile) has shown most find DAB to have equal or better sound quality than FM.[12]

Critisism

Broadcasters have been criticized for ‘squeezing in’ more stations per ensemble than recommended, by:

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.

Lower cost

DAB broadcast many channels over one transmitter (multiplex) which lowers maintenance and transmission costs radically.

More stations

DAB is not more bandwidth efficient than analogue measured in programmes per MHz of a specific transmitter (the so called link spectral efficiency). However, it is more rubust to co-channel interference (cross talk), which makes it possible to reduce the reuse distance, i.e. use the same radio frequency channel more densely. The system spectral efficiency (the average number of radio programmes per MHz and transmitter) is a factor three more more efficient than analog FM for local radio stations, as can be seen in the above numerical example. For national and regional radio networks, the efficiency is improved by more than an order of magnitude due to the use of SFNs. In that case, adjacent transmitters use the same frequency.

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.

Due to sensitivity to doppler shift in combination with multipath propagation, DAB receivers can not operate in travelling speeds of more than 200 to 600 km/h depending on carrier frequency.

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 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.[13] These stations are often available in stereo on other digital platforms, where capacity is not as constrained, 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.[14] 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). Transmission costs effectively put DAB in the hands of large corporate broadcasters and exclude community and pirate stations.

Still, many perceive DAB to be economically viable, as many national broadcasters have FM transmitters nearing end of life, and replacinge them with DAB broadcasting will be equal or less costly than replacing FM transmitters.

DAB is not a power efficiency transmitting system. Typically an 2500 W DAB transmitter requires 14 000 W of electric power. If two or more transmitter stations are required, the power including cooling will pass 40 000 W. The power efficiency on DAB is therefor around 25-35%, while FM can be as high as up to 90%.

Compatibility

In 2006 tests began using the much improved HE-AAC codec for DAB+. Virtually none of the receivers made before 2008 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. However new receivers are both DAB and DAB+ compatible.

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). This effectively means that they're less energy efficient than an analog Band II VHF receiver.

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.[15]

Expensive investment

Small radio channels may not afford expensive DAB transmitters, and so the smaller channels are threatened.

Technology

Bands and modes

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:

Protocol stack

From a OSI model protocol stack viewpoint, the technologies used on DAB inhabit the following layers: the audio codec inhabits the presentation layer. Below that is the data link layer, in charge of packet mode statistical multiplexing and frame synchronization. Finally, the physical layer contains the error-correction coding, OFDM modulation, and 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,[16] 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.

The new DAB+ standard has incorporated Reed-Solomon ECC as an "inner layer" of coding that is placed around the byte interleaved audio frame but inside the "outer layer" of convolutional coding used by the older DAB system, although on DAB+ the convolutional coding uses equal error protection (EEP) rather than UEP since each bit is equally important in DAB+. This combination of Reed-Solomon coding as the inner layer of coding, followed by an outer layer of convolutional 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[16].

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. For details, see the OFDM system comparison table.

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:

DAB+ and DMB

The term DAB most commonly refers both to a specific DAB-standard using the MP2 audio codec, but can sometimes refer to a whole family of DAB related standards, such as DAB+, DMB and DAB-IP.

DAB+

WorldDMB, the organisation in charge of the DAB standards, announced DAB+, a major upgrade to the DAB standard in 2006. When the HE-AAC v2 audio codec[17] (also known as eAAC+) 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 forward compatible with DAB+, older DAB receivers can not 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+, there will be a sign on the product packaging.

DAB+ broadcasts have launched in several countries like Switzerland,[18] Malta, Italy and Australia. Several other countries are also expected to launch DAB+ broadcasts over the next few years, such as Hungary, Germany and Asian countries, such as China and Vietnam. If DAB+ stations launch in established DAB countries, they can transmit alongside existing DAB stations that use the older 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+.[19]

DMB

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.

Norway, South Korea and France are countries currently broadcasting DMB.

List of Countries using DAB

More than 30 countries provide DAB, DAB+ and/or DMB broadcasts, either as a permanent technology or as test transmissions.

See also

References

  1. ↑ World DMB forums list of benefits, worlddab.org
  2. ↑ Sagepub.com
  3. ↑ Mediauk.com
  4. ↑ S. Holm, "Audio quality on the air in DAB digital radio in Norway," in Proc. 31st Audio Engineering Society International Conference, London, UK, June 2007, AES
  5. ↑ harmoniccycle.com, Fraunhofer: MPEG Audio Layer-3
  6. ↑ DAB/DAB+/DMB Receivers, worlddab.org
  7. ↑ EBU.ch
  8. ↑ World DMB forums list of benefits, worlddab.org
  9. ↑ OFCOM: Regulation in digital broadcasting: DAB digital radio bitrates and audio quality; Dynamic range compression and loudness, david.robinson.org
  10. ↑ "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). BBC.co.uk. http://www.bbc.co.uk/rd/pubs/whp/whp-pdf-files/WHP061.pdf. Retrieved 2007-05-08. 
  11. ↑ Friends of Radio 3 (FoR3) BBC & R3 News, for3.org
  12. ↑ Digitalspy.co.uk
  13. ↑ Digitalradiotech.co.uk
  14. ↑ EBU.ch
  15. ↑ Freeplay Energy Plc, Freeplayenergy.com
  16. ↑ 16.0 16.1 Worlddab.org
  17. ↑ Worlddab.org
  18. ↑ Dab-Digitalradio.ch
  19. ↑ Press release: New High Efficiency Audio Option Added for DAB Digital Radio, worlddab.org

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