Digital audio broadcasting
Encyclopedia
Digital Audio Broadcasting (DAB) is a digital
radio
technology for broadcasting
radio station
s, used in several countries, particularly in Europe. As of 2006, approximately 1,000 stations worldwide broadcast in the DAB format.
The DAB standard was initiated as a European research project in the 1980s. The Norwegian Broadcasting Corporation (NRK) launched the very first DAB channel in the world on June 1st 1995 (NRK Klassisk
), and the BBC
and SR
launched their first DAB digital radio broadcasts in September 1995. 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, since DAB reception quality first degrades rapidly when the signal strength falls below a critical threshold, whereas FM reception quality degrades slowly with the decreasing signal.
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. However, "CD sound quality" with MP2 is possible "with 256..192 kbps".
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. 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 64 kbit/s. 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, Switzerland and Germany, have started transmitting DAB+ stations. See Countries using DAB/DMB. However, DAB radio has still not replaced the old FM system in popularity.
(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 station
s 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 United Kingdom
and Denmark
. In 2006 there were 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.
and AM
, 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). DAB broadcasts a single station that is approximately 1500 kilohertz wide (~1000 kilobits per second). That station is then subdivided into multiple digital streams of between 9 and 12 programs. In contrast FM HD Radio shares its digital broadcast with the traditional 200 kilohertz-wide channels, with capability of 300 kbit/s per station (pure digital mode). Thus HD Radio is approximately twice as data-efficient as DAB.
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 programs is only 128 kbit/s and as a result most radio stations on DAB have a lower sound quality than FM, prompting a number of complaints. The newer FM HD Radio uses a codec based upon the MPEG-4
HE-AAC
standard. Psychoacoustic studies/tests have shown HE-AAC codecs achieve the same perceived quality as MP2, but at one-quarter the bit rate (CD quality at 64 kbit/s rather than 256 kbit/s).
A directly related issue with DAB's original inefficient compression is "downgrading" stations from stereophonic to monaural
, in order to include more channels into the limited 1000 kbit/s bandwidth, smaller coverage of markets as compared to analog FM, radios that are overly expensive, poor reception inside vehicles or buildings, and a general lack of interest in DAB in many countries.
, 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.
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.
.
, more stations and more resistance to noise, co-channel interference and multipath than in analogue FM radio. However, the leading countries in implementing DAB on stereo radio stations use compression to such a degree that it produces lower sound quality than that received from non-mobile 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.
The BBC
Research & Development department states that at least 192 kbit/s is necessary for a high fidelity stereo broadcast :
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. 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.
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.)
DAB receivers can display time of day as encoded into transmissions, so is automatically corrected when travelling between time zones and when changing to or from Daylight Saving. This is not implemented on all receivers, and some display time only when in "Standby" mode.
Some radios offer a pause facility on live broadcasts, caching the broadcast stream on local flash memory, although this function is limited.
s. 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.
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.
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.
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.
This efficiency originates from the ability a DAB network has in broadcasting more channels per network. One network can broadcast 6-10 channels (with MPEG audio codec) or 10-16 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 as opposed to newer FM facilities.
Cheaper transmission costs is backed by independent network studies from Teracom (Sweden) and SSR/SRG (Switzerland). Among other things they show that DAB is up to 6 times less expensive than FM.
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 receiver due to digital processing (e.g. deinterleaving) add to the overall delay perceived by the listener. The signal is delayed by 2-4 seconds depending on the decoding circuitry used. This has disadvantages:
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.
As an indicator of this increased power consumption, some radio manaufacturers quote the length of time their receivers can play on a single charge. For a commonly used FM/DAB-receiver from manufacturer PURE, this is stated as: DAB 10 hours, FM 22 hours.
has prompted criticism of the fact that a (large) public system is financially supporting a private company. In general, an open system will permit equipment to be bought from various sources in competition with each other but by selecting a single vendor of codec, with which all equipment must be compatible, this is not possible.
(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
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.
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.
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.
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.
), 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.
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.
). These services can include:
v2 audio codec (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, Malta, Italy, Australia and Germany. Several other countries are also expected to launch DAB+ broadcasts over the next few years, such as Hungary 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+.
(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. So the digital communication is very important for quality sound.
Digital
A digital system is a data technology that uses discrete values. By contrast, non-digital systems use a continuous range of values to represent information...
radio
Radio
Radio is the transmission of signals through free space by modulation of electromagnetic waves with frequencies below those of visible light. Electromagnetic radiation travels by means of oscillating electromagnetic fields that pass through the air and the vacuum of space...
technology for broadcasting
Broadcasting
Broadcasting is the distribution of audio and video content to a dispersed audience via any audio visual medium. Receiving parties may include the general public or a relatively large subset of thereof...
radio station
Radio station
Radio broadcasting is a one-way wireless transmission over radio waves intended to reach a wide audience. Stations can be linked in radio networks to broadcast a common radio format, either in broadcast syndication or simulcast or both...
s, used in several countries, particularly in Europe. As of 2006, approximately 1,000 stations worldwide broadcast in the DAB format.
The DAB standard was initiated as a European research project in the 1980s. The Norwegian Broadcasting Corporation (NRK) launched the very first DAB channel in the world on June 1st 1995 (NRK Klassisk
NRK Klassisk
NRK Classic is a Norwegian radio station of the Norwegian Broadcasting Corporation that broadcasts classical music 24 hours a day.- History :...
), and the BBC
BBC
The British Broadcasting Corporation is a British public service broadcaster. Its headquarters is at Broadcasting House in the City of Westminster, London. It is the largest broadcaster in the world, with about 23,000 staff...
and SR
Sveriges Radio
Sveriges Radio AB – Swedish Radio Ltd – is Sweden's national publicly funded radio broadcaster. The Swedish public-broadcasting system is in many respects modelled after the one used in the United Kingdom, and Sveriges Radio - like Sveriges Television - shares many characteristics with...
launched their first DAB digital radio broadcasts in September 1995. DAB receivers
Receiver (radio)
A radio receiver converts signals from a radio antenna to a usable form. It uses electronic filters to separate a wanted radio frequency signal from all other signals, the electronic amplifier increases the level suitable for further processing, and finally recovers the desired information through...
have been available in many countries since the end of the nineties.
DAB may offer more radio programmes over a specific spectrum than analogue
Analog signal
An analog or analogue signal is any continuous signal for which the time varying feature of the signal is a representation of some other time varying quantity, i.e., analogous to another time varying signal. It differs from a digital signal in terms of small fluctuations in the signal which are...
FM
FM broadcasting
FM broadcasting is a broadcasting technology pioneered by Edwin Howard Armstrong which uses frequency modulation to provide high-fidelity sound over broadcast radio. The term "FM band" describes the "frequency band in which FM is used for broadcasting"...
radio. DAB is more robust with regard to noise
Noise
In common use, the word noise means any unwanted sound. In both analog and digital electronics, noise is random unwanted perturbation to a wanted signal; it is called noise as a generalisation of the acoustic noise heard when listening to a weak radio transmission with significant electrical noise...
and multipath
Multipath interference
Multipath interference is a phenomenon in the physics of waves whereby a wave from a source travels to a detector via two or more paths and, under the right condition, the two components of the wave interfere...
fading
Fading
In wireless communications, fading is deviation of the attenuation that a carrier-modulated telecommunication signal experiences over certain propagation media. The fading may vary with time, geographical position and/or radio frequency, and is often modelled as a random process. A fading channel...
for mobile listening, since DAB reception quality first degrades rapidly when the signal strength falls below a critical threshold, whereas FM reception quality degrades slowly with the decreasing signal.
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
Bit rate
In telecommunications and computing, bit rate is the number of bits that are conveyed or processed per unit of time....
of 128 kbit/s or less, with the MP2
MPEG-1 Audio Layer II
MPEG-1 Audio Layer II or MPEG-2 Audio Layer II is a lossy audio compression format defined by ISO/IEC 11172-3 alongside MPEG-1 Audio Layer I and MPEG-1 Audio Layer III...
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. However, "CD sound quality" with MP2 is possible "with 256..192 kbps".
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. 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 64 kbit/s. 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, Switzerland and Germany, have started transmitting DAB+ stations. See Countries using DAB/DMB. However, DAB radio has still not replaced the old FM system in popularity.
History
DAB has been under development since 1981 at the Institut für RundfunktechnikInstitut für Rundfunktechnik
The Institut für Rundfunktechnik GmbH is the research centre of the German broadcasters , Austria's broadcaster and the Swiss public broadcaster . It is located in Munich and is responsible for the research and standardisation of broadcasting technology...
(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
European Union
The European Union is an economic and political union of 27 independent member states which are located primarily in Europe. The EU traces its origins from the European Coal and Steel Community and the European Economic Community , formed by six countries in 1958...
(EUREKA
EUREKA
EUREKA, often abbreviated as "E!" or "Σ!", is a pan-European research and development funding and coordination organization. EUREKA aims to coordinate efforts of governments, research institutes and commercial companies concerning innovation...
), which started in 1987 on initiative by a consortium formed in 1986. The MPEG-1 Audio Layer II
MPEG-1 Audio Layer II
MPEG-1 Audio Layer II or MPEG-2 Audio Layer II is a lossy audio compression format defined by ISO/IEC 11172-3 alongside MPEG-1 Audio Layer I and MPEG-1 Audio Layer III...
("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
United Kingdom
The United Kingdom of Great Britain and Northern IrelandIn the United Kingdom and Dependencies, other languages have been officially recognised as legitimate autochthonous languages under the European Charter for Regional or Minority Languages...
. The protocol specification was finalized in 1993 and adopted by the ITU-R
ITU-R
The ITU Radiocommunication Sector is one of the three sectors of the International Telecommunication Union and is responsible for radio communication....
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 station
Radio station
Radio broadcasting is a one-way wireless transmission over radio waves intended to reach a wide audience. Stations can be linked in radio networks to broadcast a common radio format, either in broadcast syndication or simulcast or both...
s via DAB. Commercial DAB receivers began to be sold in 1999 and over 50 commercial and BBC
BBC
The British Broadcasting Corporation is a British public service broadcaster. Its headquarters is at Broadcasting House in the City of Westminster, London. It is the largest broadcaster in the world, with about 23,000 staff...
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 United Kingdom
United Kingdom
The United Kingdom of Great Britain and Northern IrelandIn the United Kingdom and Dependencies, other languages have been officially recognised as legitimate autochthonous languages under the European Charter for Regional or Minority Languages...
and Denmark
Denmark
Denmark is a Scandinavian country in Northern Europe. The countries of Denmark and Greenland, as well as the Faroe Islands, constitute the Kingdom of Denmark . It is the southernmost of the Nordic countries, southwest of Sweden and south of Norway, and bordered to the south by Germany. Denmark...
. In 2006 there were 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 FMFM broadcasting
FM broadcasting is a broadcasting technology pioneered by Edwin Howard Armstrong which uses frequency modulation to provide high-fidelity sound over broadcast radio. The term "FM band" describes the "frequency band in which FM is used for broadcasting"...
and AM
AM broadcasting
AM broadcasting is the process of radio broadcasting using amplitude modulation. AM was the first method of impressing sound on a radio signal and is still widely used today. Commercial and public AM broadcasting is carried out in the medium wave band world wide, and on long wave and short wave...
, 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
Multiplexing
The multiplexed signal is transmitted over a communication channel, which may be a physical transmission medium. The multiplexing divides the capacity of the low-level communication channel into several higher-level logical channels, one for each message signal or data stream to be transferred...
and compression combines multiple audio streams onto a relatively narrow band centred on a single broadcast frequency called a DAB ensemble
DAB ensemble
DAB ensembles are groups of Digital audio broadcasting broadcasters transmitting multiple digital radio channels on a single radio transmission....
.
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
DAB ensemble
DAB ensembles are groups of Digital audio broadcasting broadcasters transmitting multiple digital radio channels on a single radio transmission....
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.
FM HD Radio versus DAB
Some countries have implemented Eureka-147 Digital Audio BroadcastingDigital audio broadcasting
Digital Audio Broadcasting 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....
(DAB). DAB broadcasts a single station that is approximately 1500 kilohertz wide (~1000 kilobits per second). That station is then subdivided into multiple digital streams of between 9 and 12 programs. In contrast FM HD Radio shares its digital broadcast with the traditional 200 kilohertz-wide channels, with capability of 300 kbit/s per station (pure digital mode). Thus HD Radio is approximately twice as data-efficient as DAB.
The first generation DAB uses the MPEG-1 Audio Layer II
MPEG-1 Audio Layer II
MPEG-1 Audio Layer II or MPEG-2 Audio Layer II is a lossy audio compression format defined by ISO/IEC 11172-3 alongside MPEG-1 Audio Layer I and MPEG-1 Audio Layer III...
(MP2) audio codec which has less efficient compression than newer codecs. The typical bitrate for DAB programs is only 128 kbit/s and as a result most radio stations on DAB have a lower sound quality than FM, prompting a number of complaints. The newer FM HD Radio uses a codec based upon the MPEG-4
MPEG-4
MPEG-4 is a method of defining compression of audio and visual digital data. It was introduced in late 1998 and designated a standard for a group of audio and video coding formats and related technology agreed upon by the ISO/IEC Moving Picture Experts Group under the formal standard ISO/IEC...
HE-AAC
HE-AAC
High-Efficiency Advanced Audio Coding is a lossy data compression scheme for digital audio defined as a MPEG-4 Audio profile in ISO/IEC 14496-3. It is an extension of Low Complexity AAC optimized for low-bitrate applications such as streaming audio...
standard. Psychoacoustic studies/tests have shown HE-AAC codecs achieve the same perceived quality as MP2, but at one-quarter the bit rate (CD quality at 64 kbit/s rather than 256 kbit/s).
A directly related issue with DAB's original inefficient compression is "downgrading" stations from stereophonic to monaural
Monaural
Monaural or monophonic sound reproduction is single-channel. Typically there is only one microphone, one loudspeaker, or channels are fed from a common signal path...
, in order to include more channels into the limited 1000 kbit/s bandwidth, smaller coverage of markets as compared to analog FM, radios that are overly expensive, poor reception inside vehicles or buildings, and a general lack of interest in DAB in many countries.
Use of frequency spectrum and transmitter sites
DAB gives substantially higher spectral efficiencySpectral efficiency
Spectral efficiency, spectrum efficiency or bandwidth efficiency refers to the information rate that can be transmitted over a given bandwidth in a specific communication system...
, 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 interferenceCo-channel interferenceCo-channel interference or CCI is crosstalk from two different radio transmitters using the same frequency. There can be several causes of co-channel radio interference; four examples are listed here....
, 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 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 / 1136 kbit/s = 0.26 MHz per audio programme. The frequency reuse factor for local programmes and multi-frequency broadcasting networks (MFNMulti-frequency network-Introduction:Data networks, such as wireless communication networks, have to trade off between services customized for a single terminal and services provided to a large number of terminals. For example, the distribution of multimedia content to a large number of resource limited portable devices ...
) 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.
.
Sound quality
The original objectives of converting to digital transmission were to enable higher fidelityFidelity
"Fidelity" is the quality of being faithful or loyal. Its original meaning regarded duty to a lord or a king, in a broader sense than the related concept of fealty. Both derive from the Latin word fidēlis, meaning "faithful or loyal"....
, more stations and more resistance to noise, co-channel interference and multipath than in analogue FM radio. However, the leading countries in implementing DAB on stereo radio stations use compression to such a degree that it produces lower sound quality than that received from non-mobile FM broadcasts. This is because of the bit rate levels being too low for the MPEG Layer 2
MPEG-1 Audio Layer II
MPEG-1 Audio Layer II or MPEG-2 Audio Layer II is a lossy audio compression format defined by ISO/IEC 11172-3 alongside MPEG-1 Audio Layer I and MPEG-1 Audio Layer III...
audio codec to provide high fidelity audio quality.
The BBC
BBC
The British Broadcasting Corporation is a British public service broadcaster. Its headquarters is at Broadcasting House in the City of Westminster, London. It is the largest broadcaster in the world, with about 23,000 staff...
Research & Development department states that at least 192 kbit/s is necessary for a high fidelity stereo broadcast :
When BBC in July 2006 reduced the bit-rate of transmission of Radio 3
BBC Radio 3
BBC Radio 3 is a national radio station operated by the BBC within the United Kingdom. Its output centres on classical music and opera, but jazz, world music, drama, culture and the arts also feature. The station is the world’s most significant commissioner of new music, and its New Generation...
from 192 kbit/s to 160 kbit/s, the resulting degradation of audio quality prompted a number of complaints to the Corporation. 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.Criticism
Broadcasters have been criticized DAB for ‘squeezing in’ more stations per ensemble than recommended, by:- Minimizing the bit-rate, to the lowest level of sound-quality that listeners are willing to tolerate, such as 128 kbit/s for stereo and even 64 kbit/s for mono speech radio.
- Having few digital channels broadcasting in stereo.
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 features for users
DAB radios automatically tune to all the available stations, offering a list for the user to select from.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
Radio Data System
Radio Data System, or RDS, is a communications protocol standard for embedding small amounts of digital information in conventional FM radio broadcasts. RDS standardises several types of information transmitted, including time, station identification and programme information.Radio Broadcast Data...
. (However, not all FM receivers allow radio stations to be stored by name.)
DAB receivers can display time of day as encoded into transmissions, so is automatically corrected when travelling between time zones and when changing to or from Daylight Saving. This is not implemented on all receivers, and some display time only when in "Standby" mode.
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 not more bandwidth efficient than analogue measured in programmes per MHz of a specific transmitter (the so called link spectral efficiency). However, 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 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 SFNSingle-frequency network
A single-frequency network or SFN is a broadcast network where several transmitters simultaneously send the same signal over the same frequency channel.-Overview:...
s. 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
Sørlandet
Southern Norway is the name of the geographical region of the Skagerrak coast of southern Norway consisting of the two counties of Vest-Agder and Aust-Agder...
, 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 analogueAnalog signal
An analog or analogue signal is any continuous signal for which the time varying feature of the signal is a representation of some other time varying quantity, i.e., analogous to another time varying signal. It differs from a digital signal in terms of small fluctuations in the signal which are...
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 radioPirate radio
Pirate radio is illegal or unregulated radio transmission. The term is most commonly used to describe illegal broadcasting for entertainment or political purposes, but is also sometimes used for illegal two-way radio operation...
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.
Transmission costs
It is common belief that DAB is more expensive to transmit than FM. It is true that DAB uses higher frequencies than FM and therefore there is a need to compensate with more transmitters, higher radiated powers, or a combination, to achieve the same coverage. A DAB network is also more expensive than an FM network. However, the last couple of years has seen significant improvement in power efficiency for DAB-transmitters.This efficiency originates from the ability a DAB network has in broadcasting more channels per network. One network can broadcast 6-10 channels (with MPEG audio codec) or 10-16 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 as opposed to newer FM facilities.
Cheaper transmission costs is backed by independent network studies from Teracom (Sweden) and SSR/SRG (Switzerland). Among other things they show that DAB is up to 6 times less expensive than FM.
Reception quality
The reception quality on DAB can be poor even for people that live well within the coverage area. The reason for this is that the old version of DAB uses weak error correction coding, so that when there are a lot of errors with the received data not enough of the errors can be corrected and a "bubbling mud" sound occurs. In some cases a complete loss of signal can happen. This situation will be improved upon in the new DAB standard (DAB+, discussed below) that uses stronger error correction coding and as additional transmitters are built.Signal delay
The nature of a SFNSingle-frequency network
A single-frequency network or SFN is a broadcast network where several transmitters simultaneously send the same signal over the same frequency channel.-Overview:...
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 receiver due to digital processing (e.g. deinterleaving) add to the overall delay perceived by the listener. The signal is delayed by 2-4 seconds depending on the decoding circuitry used. This has disadvantages:
- DAB radios are out of step with live events, so the experience of listening to live commentaries on events being watched is impaired;
- Delayed time signalsTime signalA time signal is a visible, audible, mechanical, or electronic signal used as a reference to determine the time of day.-Audible and visible time signals:...
: Even in a well-defined network with a fixed delay, the listener has to apply an offset when using the broadcast time signal to set a clock. - Listeners using a combination of analog (AM or FM) and DAB radios (e.g. in different rooms of a house) will hear a confusing mixture 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.Compatibility
In 2006 tests began using the much improved HE-AACHE-AAC
High-Efficiency Advanced Audio Coding is a lossy data compression scheme for digital audio defined as a MPEG-4 Audio profile in ISO/IEC 14496-3. It is an extension of Low Complexity AAC optimized for low-bitrate applications such as streaming audio...
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 means that they use more energy than analogue Band II VHF receivers.As an indicator of this increased power consumption, some radio manaufacturers quote the length of time their receivers can play on a single charge. For a commonly used FM/DAB-receiver from manufacturer PURE, this is stated as: DAB 10 hours, FM 22 hours.
Use of Licensed Codecs
The use of MPEG and latterly AACHE-AAC
High-Efficiency Advanced Audio Coding is a lossy data compression scheme for digital audio defined as a MPEG-4 Audio profile in ISO/IEC 14496-3. It is an extension of Low Complexity AAC optimized for low-bitrate applications such as streaming audio...
has prompted criticism of the fact that a (large) public system is financially supporting a private company. In general, an open system will permit equipment to be bought from various sources in competition with each other but by selecting a single vendor of codec, with which all equipment must be compatible, this is not possible.
Bands and modes
DAB uses a wide-bandwidth broadcast technology and typically spectra have been allocated for it in Band IIIBand III
Band III is the name of a radio frequency range within the very high frequency part of the electromagnetic spectrum.Band III ranges from 174 to 230 MHz, and it is primarily used for radio and television broadcasting...
(174–240 MHz) and L band
L band
L band refers to four different bands of the electromagnetic spectrum: 40 to 60 GHz , 1 to 2 GHz , 1565 nm to 1625 nm , and around 3.5 micrometres .-NATO 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 satelliteSatelliteIn the context of spaceflight, a satellite is an object which has been placed into orbit by human endeavour. Such objects are sometimes called artificial satellites to distinguish them from natural satellites such as the Moon....
- Mode III for frequencies below 3 GHz, Earth and satellite
- Mode IV for L-Band, Earth and satellite
Protocol stack
From a OSI modelOSI model
The Open Systems Interconnection model is a product of the Open Systems Interconnection effort at the International Organization for Standardization. It is a prescription of characterizing and standardizing the functions of a communications system in terms of abstraction layers. Similar...
protocol stack
Protocol stack
The protocol stack is an implementation of a computer networking protocol suite. The terms are often used interchangeably. Strictly speaking, the suite is the definition of the protocols, and the stack is the software implementation of them....
viewpoint, the technologies used on DAB inhabit the following layers: the audio codec inhabits the presentation layer
Presentation layer
The presentation layer is layer 6 of the seven-layer OSI model of computer networking and serves as the data translator for the network. It is sometimes called the syntax layer.-Description:...
. Below that is the data link layer
Data link layer
The data link layer is layer 2 of the seven-layer OSI model of computer networking. It corresponds to, or is part of the link layer of the TCP/IP reference model....
, in charge of packet mode statistical multiplexing
Statistical multiplexing
Statistical multiplexing is a type of communication link sharing, very similar to dynamic bandwidth allocation . In statistical multiplexing, a communication channel is divided into an arbitrary number of variable bit-rate digital channels or data streams. The link sharing is adapted to the...
and frame synchronization
Frame synchronization
While receiving a stream of framed data, frame synchronization is the process by which incoming frame alignment signals, i.e., distinctive bit sequences , are identified, i.e., distinguished from data bits, permitting the data bits within the frame to be extracted for decoding or retransmission...
. Finally, the physical layer
Physical layer
The physical layer or layer 1 is the first and lowest layer in the seven-layer OSI model of computer networking. The implementation of this layer is often termed PHY....
contains the error-correction coding, OFDM modulation
Modulation
In electronics and telecommunications, modulation is the process of varying one or more properties of a high-frequency periodic waveform, called the carrier signal, with a modulating signal which typically contains information to be transmitted...
, 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
HE-AAC
High-Efficiency Advanced Audio Coding is a lossy data compression scheme for digital audio defined as a MPEG-4 Audio profile in ISO/IEC 14496-3. It is an extension of Low Complexity AAC optimized for low-bitrate applications such as streaming audio...
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
Code rate
In telecommunication and information theory, the code rate of a forward error correction code is the proportion of the data-stream that is useful...
) 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"
Concatenated error correction codes
In coding theory, concatenated codes form a class of error-correcting codes that are derived by combining an inner code and an outer code...
- became a popular ECC scheme in the 1990s, and NASA
NASA
The National Aeronautics and Space Administration is the agency of the United States government that is responsible for the nation's civilian space program and for aeronautics and aerospace research...
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. 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 (SFNSingle-frequency network
A single-frequency network or SFN is a broadcast network where several transmitters simultaneously send the same signal over the same frequency channel.-Overview:...
), 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 rateBit rate
In telecommunications and computing, bit rate is the number of bits that are conveyed or processed per unit of time....
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
United Kingdom
The United Kingdom of Great Britain and Northern IrelandIn the United Kingdom and Dependencies, other languages have been officially recognised as legitimate autochthonous languages under the European Charter for Regional or Minority Languages...
, 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 multiplexMultiplexing
The multiplexed signal is transmitted over a communication channel, which may be a physical transmission medium. The multiplexing divides the capacity of the low-level communication channel into several higher-level logical channels, one for each message signal or data stream to be transferred...
). These services can include:
- Primary services, like main radio stations
- Secondary services, like additional sports commentaries
- Data services
- Electronic Programme Guide (EPGElectronic program guideElectronic program guides and interactive program guides provide users of television, radio, and other media applications with continuously updated menus displaying broadcast programming or scheduling information for current and upcoming programming...
) - Collections of HTMLHTMLHyperText Markup Language is the predominant markup language for web pages. HTML elements are the basic building-blocks of webpages....
pages and digital imageDigital imageA digital image is a numeric representation of a two-dimensional image. Depending on whether or not the image resolution is fixed, it may be of vector or raster type...
s (Known as 'Broadcast Web Sites') - SlideshowSlideshowA slide show is a display of a series of chosen information or pictures, done for artistic or instructional purposes. Slide shows are conducted by a presenter using an apparatus, such as a carousel slide projector, an overhead projector or in more recent years, a computer running presentation...
s, which may be synchronised with audio broadcasts - VideoVideoVideo is the technology of electronically capturing, recording, processing, storing, transmitting, and reconstructing a sequence of still images representing scenes in motion.- History :...
- Java Platform Applications
- IP tunnelingTunneling protocolComputer networks use a tunneling protocol when one network protocol encapsulates a different payload protocol...
- Other raw data
- Electronic Programme Guide (EPG
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-AACHE-AAC
High-Efficiency Advanced Audio Coding is a lossy data compression scheme for digital audio defined as a MPEG-4 Audio profile in ISO/IEC 14496-3. It is an extension of Low Complexity AAC optimized for low-bitrate applications such as streaming audio...
v2 audio codec (also known as eAAC+) was adopted. The new standard, which is called DAB+, has also adopted the MPEG Surround
MPEG Surround
MPEG Surround , also known as Spatial Audio Coding is a lossy compression format for surround sound that provides a method for extending mono or stereo audio services to multi-channel audio in a backwards compatible fashion...
audio format and stronger error correction coding in the form of Reed-Solomon coding. DAB+ has been standardised as ETSI
European Telecommunications Standards Institute
The European Telecommunications Standards Institute is an independent, non-profit, standardization organization in the telecommunications industry in Europe, with worldwide projection...
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
Firmware
In electronic systems and computing, firmware is a term often used to denote the fixed, usually rather small, programs and/or data structures that internally control various electronic devices...
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, Malta, Italy, Australia and Germany. Several other countries are also expected to launch DAB+ broadcasts over the next few years, such as Hungary 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
MPEG-1 Audio Layer II
MPEG-1 Audio Layer II or MPEG-2 Audio Layer II is a lossy audio compression format defined by ISO/IEC 11172-3 alongside MPEG-1 Audio Layer I and MPEG-1 Audio Layer III...
audio format, and most existing DAB stations are expected to continue broadcasting until the vast majority of receivers support DAB+.
DMB
Digital Multimedia BroadcastingDigital Multimedia Broadcasting
Digital Multimedia Broadcasting is a digital radio transmission technology developed in South Korea as part of the national IT project for sending multimedia such as TV, radio and datacasting to mobile devices such as mobile phones...
(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. So the digital communication is very important for quality sound.
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
- ATSC Standards
- Digital Audio Radio ServiceDigital Audio Radio ServiceDigital Audio Radio Service or DARS refers to any type of digital radio service. In the United States it is the official FCC term for digital radio services....
- Digital Multimedia BroadcastingDigital Multimedia BroadcastingDigital Multimedia Broadcasting is a digital radio transmission technology developed in South Korea as part of the national IT project for sending multimedia such as TV, radio and datacasting to mobile devices such as mobile phones...
- Digital radioDigital radioDigital radio has several meanings:1. Today the most common meaning is digital radio broadcasting technologies, such as the digital audio broadcasting system, also known as Eureka 147. In these systems, the analog audio signal is digitized into zeros and ones, compressed using formats such as...
- Digital televisionDigital televisionDigital television is the transmission of audio and video by digital signals, in contrast to the analog signals used by analog TV...
- Digital Radio MondialeDigital Radio MondialeDigital Radio Mondiale is a set of digital audio broadcasting technologies designed to work over the bands currently used for AM broadcasting, particularly shortwave...
- ETSI Satellite Digital RadioETSI Satellite Digital RadioETSI Satellite Digital Radio describes a standard of satellite digital radio. It is an activity of the european standardisation organisation ETSI....
(SDR) - FMeXtraFMeXtraFMeXtra, now marketed as VuCast and oriented at the broadcast data market like MSN Direct, is an in-band on-channel digital radio broadcasting technology created by Digital Radio Express, now VuCast Media. Unlike iBiquity's HD Radio system, it uses any FM radio station's existing equipment and...
- HD RadioHD RadioHD Radio, which originally stood for "Hybrid Digital", is the trademark for iBiquity's in-band on-channel digital radio technology used by AM and FM radio stations to transmit audio and data via a digital signal in conjunction with their analog signals...
- Internet radio deviceInternet radio deviceAn Internet radio device, also called network music player is a hardware device that is capable of receiving and playing streamed media from either Internet radio stations or a home network.-Background:...
- ISDBISDBIntegrated Services Digital Broadcasting is a Japanese standard for digital television and digital radio used by the country's radio and television stations. ISDB replaced the previously used MUSE "Hi-vision" analogue HDTV system...
- OpenCableOpenCableOpenCable is a set of hardware and software specifications under development in the United States by CableLabs to "define the next-generation digital consumer device" for the cable television industry...
- Orthogonal frequency-division multiplexingOrthogonal frequency-division multiplexingOrthogonal frequency-division multiplexing is a method of encoding digital data on multiple carrier frequencies. OFDM has developed into a popular scheme for wideband digital communication, whether wireless or over copper wires, used in applications such as digital television and audio...
- Satellite RadioSatellite radioSatellite radio is an analogue or digital radio signal that is relayed through one or more satellites and thus can be received in a much wider geographical area than terrestrial FM radio stations...
- Sirius Satellite RadioSirius Satellite RadioSirius Satellite Radio is a satellite radio service operating in North America, owned by Sirius XM Radio.Headquartered in New York City, with smaller studios in Los Angeles and Memphis, Sirius was officially launched on July 1, 2002 and currently provides 69 streams of music and 65 streams of...
- Spectral efficiency comparison table
- T-DMB
- TeletextTeletextTeletext is a television information retrieval service developed in the United Kingdom in the early 1970s. It offers a range of text-based information, typically including national, international and sporting news, weather and TV schedules...
- XM Satellite RadioXM Satellite RadioXM Satellite Radio is one of two satellite radio services in the United States and Canada, operated by Sirius XM Radio. It provides pay-for-service radio, analogous to cable television. Its service includes 73 different music channels, 39 news, sports, talk and entertainment channels, 21 regional...
External links
- ETSI EN 300 401 v1.4.1 - Original DAB specification, etsi.org
- ETSI TS 102 563 v1.1.1 - DAB+ enhancement specification, etsi.org
- World DAB Forum, wordlddab.org
- DAB Ensembles Worldwide (also known as "Wohnort", the main part of the site is a list of services currently transmitting), wohnort.org
- Ofcom – UK broadcasting regulators, ofcom.org.uk
- Digital One - The UK's Commercial Digital Radio Network, ukdigitalradio.com
- DAB Digital Radio News and Information for the UK, uk-dab.info
- UK DAB sceptical news and information, digitalradiotech.co.uk
- Audio extracts of UK DAB stations, dab.blip.tv
- Digital broadcasting in Ireland, rte.i.e.
- DAB in the Netherlands, dabtuners.nl
- E-magazine with information about DAB, digitaleradio.nl
- BBC Datasheet on Eureka 147, bbd.co.uk
- ASA Bans DAB Audio Quality Claims, digitalradiotech.co.uk
- DAB and Mobile Java Services, ebu.ch
- MMBTools, Linux based free/open DAB, DAB+, T-DMB transmission infrastructure (software based), crc.ca
- OpenMokast, free/open software receiver for DAB, DAB+ and T-DMB, openmokast.org
- Opendigitalradio.org, experimentation and techniques for DAB transmission and reception, opendigitalradio.org
- DAB Car Radio, edabcarradio.co.uk