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Digital radio standard From Wikipedia, the free encyclopedia
Digital Audio Broadcasting (DAB) is a digital radio standard for broadcasting digital audio radio services in many countries around the world, defined, supported, marketed and promoted by the WorldDAB organisation. The standard is dominant in Europe and is also used in Australia, and in parts of Africa and Asia; as of 2022, 55 countries are actively running DAB broadcasts.[3][4]
DAB was the result of a European research project and first publicly rolled out in 1995, with consumer-grade DAB receivers appearing at the start of this millennium. Initially it was expected in many countries that existing FM services would switch over to DAB, although the take up of DAB has been much slower than expected.[5][6][7][8] As of 2023[update], Norway is the first country to have implemented a national FM radio switch-off,[9][10] with others to follow in the next years.[11][12][13] In recent years, DAB has become the most popular radio listening platform in Norway, Switzerland and the UK,[14] and has become a requirement for all new cars sold in the EU since 2021.[15]
The original version of DAB used the MP2 audio codec; an upgraded version of the system was later developed and released named DAB+ which uses the HE-AAC v2 (AAC+) audio codec and is more robust and efficient. DAB is not forward compatible with DAB+.[16] Today the majority of DAB broadcasts around the world are using the upgraded DAB+ standard, with only the UK, Romania, Brunei, and the Philippines still using a significant number of legacy DAB broadcasts.
DAB is generally more efficient in its use of spectrum than analogue FM radio,[17] and thus can offer more radio services for the same given bandwidth. The broadcaster can select any desired sound quality, from high-fidelity signals for music to low-fidelity signals for talk radio, in which case the sound quality can be noticeably inferior to analog FM. High-fidelity equates to a high bit rate and higher transmission cost. DAB is more robust with regard to noise and multipath fading for mobile listening,[18] although DAB reception quality degrades rapidly when the signal strength falls below a critical threshold (as is normal for digital broadcasts), whereas FM reception quality degrades slowly with the decreasing signal, providing more effective coverage over a larger area.[citation needed] DAB+ is a "green" platform and can bring up to 85 percent energy consumption savings[19] compared to FM broadcasting (but analog tuners are more efficient than digital ones,[20] and DRM+ has been recommended for small scale transmissions).[21]
Similar terrestrial digital radio standards are HD Radio, ISDB-Tb, DRM, and the related DMB.[22]
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The DAB standard was initiated as a European research project called Eureka-147 in the 1980s.[23][24] DAB has been under development since 1981 at the Institut für Rundfunktechnik (IRT). The first DAB demonstrations were held in 1985 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 an 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 1995: the Norwegian Broadcasting Corporation (NRK) launched the first DAB channel in the world on 1 June 1995 (NRK Klassisk),[25] and the BBC and Swedish Radio (SR) launched their first broadcasts later in September[26] while in Germany a pilot broadcast started in Bavaria in October 1995.[27] In the UK, commercial stations started broadcasting in November 1999.[28]
For various reasons such as high receiver costs and limited reception, adoption of DAB had initially been slow, with the exception of the United Kingdom and Denmark. In the UK, DAB radio receivers were high selling and 10% of households owned a DAB radio as of 2005,[29] helped by local manufacturers creating affordable receivers such as the Pure Evoke.[30] In countries where DAB did not take off, efforts were made in later years to "re-launch" it using the newer DAB+ standard:[31] it started gaining traction throughout the 2010s[32] and finally took off in countries like France by 2019.[33] DAB adoption in automobiles became increasingly common during this time, and by 2016 it was standard in most cars sold in the UK, Norway and Switzerland.[34]
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. The AAC+ audio coding standard uses a modified discrete cosine transform (MDCT) audio data compression algorithm.[35][36] This work led to the launch of the DAB+ system.
Trials for DAB-IP were held in London in 2006, as "BT Movio".[37] It competed with DVB-H and MediaFLO which were also under testing.[38]
By 2006, 500 million people worldwide were in the coverage area of DAB broadcasts, although by this time sales of receivers had only taken off in the United Kingdom (UK) and Denmark. In 2006 there were approximately 1,000 DAB stations in operation worldwide.[39] As of 2018, over 68 million devices have been sold worldwide, and over 2,270 DAB services are on air.[4]
In October 2018, the WorldDAB organisation introduced an all-new logo for DAB (specifically DAB+) to replace the previous logo that had been in use since before DAB's initial launch in 1995.[40]
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.
WorldDAB, 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[41] (also known as eAAC+) was adopted. AAC+ uses a modified discrete cosine transform (MDCT) algorithm.[35][36] 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 European Telecommunications Standards Institute (ETSI) TS 102 563.
As DAB is not forward compatible with DAB+, older DAB receivers cannot receive DAB+ broadcasts. However, DAB receivers that were capable of receiving the new DAB+ standard after a firmware upgrade were being sold as early as July 2007. Generally, if a receiver is DAB+ compatible, there will be a sign on the product packaging.
DAB+ broadcasts have launched in several countries like Australia, Czech Republic, Denmark, Germany, Hong Kong (now terminated), Italy, Malta, Norway, Poland, Switzerland, Belgium (October 2017),[42] the United Kingdom and the Netherlands. Malta was the first country to launch DAB+ in Europe in October 2008. South Africa began a DAB+ technical pilot in November 2014 on channel 13F in Band 3. 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+.[43]
In the UK, DAB+ launched in January 2016 following a trial period starting September 2014.[citation needed] Ofcom published a consultation for a new national multiplex containing a mix of DAB and DAB+ services, with the intention of moving all services to DAB+ in the long term.[44] In February 2016, the new national network Sound Digital launched with three DAB+ stations.[45] In August 2021 the BBC started its first regular domestic broadcast of DAB+ in the Channel Islands[46] and followed later with the start of local broadcasts in December 2021 in Cumbria and north Lancashire.[47] Classic FM changed from DAB to DAB+ on 1 January 2024.[48][49]
Digital multimedia broadcasting (DMB) and DAB-IP are both suitable for mobile radio and TV 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 it was designed to be carried on a DAB subchannel. DMB broadcasts in South Korea carry conventional MPEG 1 Layer II DAB audio services alongside their DMB video services.
As of 2017[update], DMB is currently broadcast in Norway, South Korea, and Thailand.
Fifty-five countries provide regular or trial DAB(+) broadcasts.[3] In spectrum management, the bands that are allocated for public DAB services, are abbreviated with T-DAB.
In the European Union, "the European Electronic Communications Code (EECC) entered into force on 20 December 2018, with transposition into national legislation by Member States required by 21 December 2020. The Directive applies to all EU member states regardless of the status of DAB+ in each country. This means that since the end of 2020, across all EU countries, all radios in new cars must be capable of receiving and reproducing digital terrestrial radio."[50]
Following the European Union's obligation in 2020 to include DAB+ receivers in new cars, Belgium has stopped all sales of analogue radio receivers from 1 January 2023. Thus, consumers are no longer able to purchase AM or FM receivers for domestic use. "The obligation to incorporate DAB+ for new cars and domestic radio receivers is a nice step ahead in the digitisation of our radio landscape," commented Benjamin Dalle, the Flemish media minister.[51]
Norway was the first country to announce a complete switch-off of national FM radio stations. The switch-off started on 11 January 2017 and ended on 13 December 2017.[52][53] The 2017 switch-off did not affect some local and regional radio stations. They can continue to transmit on FM until 2027.
The timetable for the closure of FM signals in 2017 were as follows:[54]
SRG SSR, Switzerland's public-service broadcaster, will shut down its FM transmission infrastructure on 31 December 2024. The corporation concluded that maintaining FM broadcasts along with DAB+ and Internet streaming was no longer cost-effective, as due to widespread adoption of DAB+ the share of the public relying exclusively on FM was under ten percent and decreasing.[55] All other FM broadcasters in the country must shut down or convert to DAB+ by 31 December 2026.[56][57][58][59][11][60]
 | This section needs additional citations for verification. (June 2023) |
DAB uses a wide-bandwidth broadcast technology and typically spectra have been allocated for it in Band III (174–240 MHz) and L band (1.452–1.492 GHz), although the scheme allows for operation between 30 and 300 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 to restrict L-Band DAB to terrestrial broadcast to avoid interference.[citation needed]
In January 2017, an updated DAB specification (2.1.1) removed Modes II, III and IV, leaving only Mode I.
From an 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 statistical time-division 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.
DAB initially only used the MPEG-1 Audio Layer II audio codec, which is often referred to as MP2 because of the ubiquitous MP3 (MPEG-1 Audio Layer III).
The newer DAB+ standard adopted the LC-AAC and HE-AAC, including its version 2 audio codecs, commonly known as AAC, AAC+ or aacPlus. AAC+ uses a modified discrete cosine transform (MDCT) algorithm,[35][36] and is approximately three times more efficient than MP2,[89] which means that broadcasters using DAB+ are able to provide far higher audio quality or far more stations than they could with DAB, or a combination of both higher audio quality and more stations.
One of the most important decisions regarding the design of a digital radio broadcasting 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 fixed capacity multiplex at a given level of audio quality.
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 a 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 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 DAB+ standard incorporates 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 original 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.[89]
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 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 1.0 ms, 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 MHz. The OFDM guard interval for TM I is 0.246 ms, which means that the overall OFDM symbol duration is 1.246 ms. 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.
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 block. 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 propagation conditions change, such as when there's high pressure, as signals travel 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.
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 1/2, equating to a maximum bit rate per multiplex of 1,184 kbit/s.
Various different services are embedded into one ensemble (which is also typically called a multiplex). These services can include:
| This section needs additional citations for verification. (June 2023) |
Traditionally, radio programmes were broadcast on different frequencies via AM and FM, and the radio had to be tuned into each frequency as needed. 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.
DAB broadcasts a single multiplex that is approximately 1.5 MHz wide (≈1,000 kilobits per second). That multiplex is then subdivided into multiple digital streams of between 9 and 12 programs. In contrast, FM HD Radio adds its digital carriers to the traditional 270 kilohertz-wide analog channels, with capability of up to 300 kbit/s per station (pure digital mode). The full bandwidth of the hybrid mode approaches 400 kHz.
The first generation DAB uses the MPEG-1 Audio Layer II (MP2) audio codec, which has less efficient compression than newer codecs. The typical bitrate for DAB stereo programs is only 128 kbit/s or less and as a result most radio stations on DAB have a lower sound quality than FM, prompting complaints from listeners.[91] As with DAB+ or T-DMB in Europe, FM HD Radio uses a codec based upon the MPEG-4 HE-AAC standard.
HD Radio is a proprietary system from iBiquity Digital Corporation, a subsidiary of DTS, Inc. since 2015, which is itself owned by Xperi Corporation since 2016. DAB is an open standard deposited at ETSI.
DAB can give substantially higher spectral efficiency, measured in programmes per MHz and per transmitter site, than analogue systems. In many places, this has led to an increase in the number of stations available to listeners, especially outside of the major urban areas. This can be further improved with DAB+ which uses a much more efficient codec, allowing a lower bitrate per channel with little to no loss in quality. If some stations transmit in mono, their bitrate can be reduced compared to stereo broadcasts, further improving the efficiency.
Numerical example: Analog FM requires 0.2 MHz per programme. The frequency reuse factor in most countries is approximately 15 for stereo transmissions (with lesser factors for mono FM networks), meaning (in the case of stereo FM) 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 0.2 MHz over the Band II spectrum of 87.5 to 108.0 MHz, an average 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 / 1,136 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 VHF band II 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 the system becoming noise limited rather than limited by co-channel interference.
The original objectives of converting to digital transmission were to enable higher audio fidelity, more stations and more resistance to noise, co-channel interference and multipath than in analogue FM radio. The improved sound quality is achieved by using CRC and FEC technology, which improves the transmission performance of digital signals.[92] However, many countries in implementing DAB on stereo radio stations use compression to such a degree that it produces lower sound quality than that received from FM broadcasts. This is because of the bit rate levels being too low for the MPEG Layer 2 audio codec to provide high fidelity audio quality.[93]
The BBC Research & Development department states that at least 192 kbit/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[94]
When the BBC reduced the bit-rate of transmission of its classical music station Radio 3 from 192 kbit/s to 160 kbit/s in July 2006, the resulting degradation of audio quality prompted a number of complaints to the corporation.[95] 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. (For comparison, BBC Radio 3 and all other BBC radio stations are streamed online using AAC at 320 kbit/s, described as 'HD', on BBC Radio iPlayer after a period when it was available at two different bit rates.)
Despite the above, a survey in 2007 of DAB listeners (including mobile) has shown most find DAB to have equal or better sound quality than FM.[96]
By 2019, some stations had upgraded to DAB+ but rather than improving sound quality, they instead reduced it to 32 kbit/s or 64 kbit/s, often in mono.[97]
DAB devices perform band-scans over the entire frequency range, presenting all stations from a single list for the user to select from.
DAB is capable of providing metadata alongside the audio stream. Metadata allows visual information, text and graphics - such as the station name and logo, presenter, song title and album artwork - to be displayed while a station is playing. Radio stations can provide the metadata to augment the listening experience, particularly on car receivers which have large display panels.[98]
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, of up to 128 characters in length. This is similar to a feature of FM called RDS, which enables a radiotext of up to 64 characters.
The DAB transmission contains a local time of day and so a device may use this to automatically correct its internal clock when travelling between time zones and when changing to or from Daylight Saving.
DAB is not more bandwidth efficient than analogue measured in programmes per MHz of a specific transmitter (the so-called link spectral efficiency), but it is less susceptible 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 efficient than analogue FM for local radio stations. 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 Southern Norway, radio listeners experienced an increase in available stations from 6 to 21 when DAB was introduced in November 2006.
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 or the audio cutting out altogether.
Due to sensitivity to Doppler shift in combination with multipath propagation, DAB reception range (but not audio quality) is reduced when travelling speeds of more than 120 to 200 km/h, depending on carrier frequency.[18]
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 led to the situation where some stations are being broadcast in mono; see § Audio quality for more details.
DAB transmitters are inevitably more expensive than their FM counterparts. DAB uses higher frequencies than FM and therefore there may be a need to compensate with more transmitters to achieve the same coverage as a single FM transmitter. DAB is commonly transmitted by a different company from the broadcaster who then sells the capacity to a number of radio stations. This shared cost can work out cheaper than operating an individual FM transmitter.
This efficiency originates from the ability a DAB network has in broadcasting more channels per transmitter/network. One network can broadcast 6–10 channels (with MP2 audio codec) or 10–18 channels (with HE AAC codec). Hence, it is thought that the replacement of FM-radios and FM-transmitters with new DAB-radios and DAB-transmitters will not cost any more compared with new FM facilities. It is also argued that the power consumption will be lower for stations transmitted on a single DAB multiplex compared with individual analog transmitters.[99]
Once applied, one operator has claimed that DAB transmission is as low as one-nineteenth of the cost of FM transmission.[100]
The reception quality during the early stage of deployment of DAB was poor even for people who live well within the coverage area. The reason for this is that 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 has been improved upon in the newer DAB+ version that uses stronger error correction coding and as additional transmitters are built.
As with other digital systems, when the signal is weak or suffers severe interference, it will not work at all. DAB reception may also be a problem for receivers when the wanted signal is adjacent to a stronger one. This was a particular issue for early and low cost receivers.
Up to the mid 2010s, a common complaint by listeners is that broadcasters 'squeeze in' more stations per ensemble than recommended[94] by:
The nature of a single-frequency network (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. Delays in the audio encoder and the receiver due to digital processing (e.g. deinterleaving) add to the overall delay perceived by the listener.[18] The signal is delayed, usually by around 1 to 4 seconds and can be considerably longer for DAB+. This has disadvantages:
Time signals, on the contrary, are not a problem in a well-defined network with a fixed delay. The DAB multiplexer adds the proper offset to the distributed time information. The time information is also independent from the (possibly varying) audio decoding delay in receivers since the time is not embedded inside the audio frames. This means that built in clocks in receivers can be precisely correct.
DAB can provide savings for networks of several stations. The original development of DAB was driven by national network operators with a number of channels to transmit from multiple sites. However, for individual stations such as small community or local stations which traditionally operate their own FM transmitter on their own building the cost of DAB transmission will be much higher than analog. Operating a DAB transmitter for a single station is not an efficient use of spectrum or power. With that said, this can be solved to some degree by combining multiple local stations in one DAB/DAB+ mux, similar to what is done on DVB-T/DVB-T2 with local TV stations.
Although FM coverage still exceeds DAB coverage in most countries implementing any kind of DAB services, a number of countries moving to digital switchover have undergone significant DAB network rollouts; as of 2022, the following coverages were given by WorldDAB:[14]
| Country | Coverage (% of population) |
|---|---|
| Kuwait | 100 |
| Malta | 100 |
| Monaco | 100 |
| Denmark | 99.9 |
| Norway | 99.7 |
| Switzerland | 99.5 |
| Germany | 98 |
| United Kingdom | 97.3 |
| Belgium | 97 |
| Czech Republic | 95 |
| Netherlands | 95 |
| Gibraltar | 90 |
| South Korea | 90 |
| Qatar | 90 |
| Croatia | 90 |
| Italy | 86 |
| Slovenia | 85 |
| Austria | 83 |
| Serbia | 78 |
| Tunisia | 75 |
| Poland | 67 |
| Australia | 66 |
| Estonia | 50 |
| Slovakia | 46 |
| Sweden | 43 |
| France | 42 |
| Azerbaijan | 33 |
| Turkey | 30 |
| Montenegro | 29 |
| Spain | 20 |
| Thailand | 17 |
| Algeria | 8 |
| Ukraine | 7 |
| Greece | ? |
| Indonesia | ? |
In 2006 tests began using the much improved HE-AAC codec for DAB+. Hardly any of the receivers made before 2008 support the newer codec, however, making them partially obsolete once DAB+ broadcasts begin and completely obsolete once all MP2 encoded stations are gone. Most new receivers are both DAB and DAB+ compatible; however, the issue is exacerbated by some manufacturers disabling the DAB+ features on otherwise compatible radios to save on licensing fees when sold in countries without current DAB+ broadcasts.
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 higher. 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 have a much shorter battery life, and require higher power (and hence more bulk). This means that they use more energy than analogue Band II VHF receivers. However, thanks to increased integration (radio-on-chip), DAB receiver power usage has been reduced dramatically, making portable receivers far more usable.
Whilst many countries have expected a shift to digital audio broadcasting, a few have moved in the opposite direction following unsuccessful trials.
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