Digital audio broadcasting

(Redirected from Digital Audio Broadcasting)

Digital audio broadcasting or DAB is a developing technology for broadcasting audio programming in digital form. Broadcast radio has been in widespread use since the 1920s, and to this time has remained largely based on the analog "amplitude modulation" (AM) technologies used at the beginning and the "frequency modulation" (FM) technologies introduced in the mid-20th Century. The objective of converting to digital systems is to enable higher fidelity, greater noise immunity, and new services. However, because FM with good reception provides hi-fi sound, digital radio systems around the world rarely match FM's level of audio quality.

The acronym DAB is used both to identify the generic technology of digital audio broadcasting, and specific technical standards, particularly the Eureka 147 standard described below. Standardization of DAB technology is promoted by the World DAB Forum, which represents more than 30 countries, not including the United States.

Some marketing confusion has been engendered by the use of the term "digital"—consumers may associate this with a digital method of tuning, which is commonly found on analogue radios with LCDs, rather than a digital signal.

In addition to regular-style receivers, one can use a radio card to hear DAB through a personal computer and various models are on the market. The Psion Wavefinder was the original DAB radio for the personal computer, with a unique design and colourful screen; however this device is no longer available.

1 Terrestrial digital audio broadcast
2 DAB in the UK
3 Digital radio for the US automotive market
4 DAB in Canada
5 Digital radio for the third world
6 Bands and Modes
7 Services and Ensembles
8 See also
9 External links

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Terrestrial digital audio broadcast

Digital audio broadcasting is now being introduced in many countries. Whilst DAB offers many potential benefits, its introduction has been hindered by a lack of global agreement on standards. Several DAB schemes are being promoted in the United States, none of which are compatible with the "Eureka 147" DAB standard now being implemented in Canada, Europe and parts of Asia. This standard was developed by EUREKA as a research project for the European Union. (Project number EU147.) It is based on orthogonal frequency division modulation for transmitting digital data over a lossy radio channel. DAB broadcasts use the MP2 audio coding technique, a close relative of the popular MP3 format, which was also created as part of the EU147 project. The technology was mainly developed in the late-1980s, with the choice of audio codec, modulation and error correction coding schemes being made in 1990. The project started in 1987 and ended in 2000.

Over 285 million people around the world are able to receive more than 550 different DAB services. The United Kingdom was the first country to receive a wide range of radio stations via DAB, with over 50 commercial and BBC services available in London in 2001.

DAB has the advantage that stations do not have be re-tuned as you move from area to area, such as in a car. Also, you can receive "radiotext" (in DAB terminology, Dynamic Label Segment, or DLS) from the station giving real-time information such as song titles or traffic updates.

Eureka 147 DAB uses a wide-bandwidth broadcast technology and typically operates in Band III or L band, although the scheme allows for operation almost anywhere above 30MHz. The US military has reserved L-Band in the United States, 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.

Unlike Eureka 147, US digital radio schemes maintain compatibility with the old analogue broadcasting schemes using an approach known as in-band on-channel (IBOC). With IBOC, both the analog and digital signals are sent in the same channel, allowing older analog radios to still receive the signal. This simplifies the problem of frequency allocation, since existing radio broadcast channels can be used for digital transmissions.

The three US IBOC schemes are being promoted by USA Digital Radio (USADR), Lucent Technologies, and Digital Radio Express. All three schemes are based on "Coded Orthogonal Frequency Division Multiplexing (COFDM)" modulation, which is also used for European digital TV broadcast (DVB). All three companies have now entered into a joint venture to form iBiquity.

Japan has started terrestrial sound broadcasting using ISDB-Tsb and 2.6GHz Satellite Sound digital broadcasting, with Korea using the latter as well.

The FM digital schemes provide audio at rates from 96 to 128 kilobits per second (kbit/s), with auxiliary "subcarrier" transmissions at up to 64 kbit/s. The AM digital schemes have data rates of about 48 kbit/s, with auxiliary services provided at a much lower data rate. Both the FM and AM schemes use lossy compression techniques to make the best use of the limited bandwidth.

The National Radio Systems Committee (NRSC) and the three IBOC companies began tests in December 1999. Results of these tests remain unclear, which in general describes the status of the terrestrial digital radio broadcasting effort in the US.

The standards issue is one obstacle to the adoption of digital radio. The other problem is a lack of customer demand. Current AM and FM terrestrial broadcast technology is cheap, reliable, and works well, and unless digital systems offer significant new benefits, there will be no strong consumer interest in the new technology.

Although it is possible for digital radio to provide "CD quality", most digital radio implementations around the world provide audio quality with a similar, or lower, quality than FM.

Other digital information may be sent along with the audio as well, such as text indicating artist and title, news headlines, and so on. Broadcasts can provide digital "tags" to identify themselves, allowing a digital radio receiver to scan for channels by type of music, such as JAZZ or CLASSICAL. Tags can also allow automotive radios to automatically change stations as they travel from city to city, to stay with a particular network or music style.

In the United States of America, it costs much more to run a digital radio station than to run a conventional analog station. This is due to the Digital Performance Right in Sound Recordings Act of 1995, which gives sound recording copyright holders the exclusive right to broadcast their works digitally. Thus, analog broadcasters pay royalties only to ASCAP, SESAC, and BMI, who represent the songwriters; digital broadcasters must pay an additional royalty to RIAA, who represents the major record labels.

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DAB in the UK

In the United Kingdom, the rollout of digital radio is proceeding. Experimental transmissions by the BBC started in 1991. The digital radio network now reaches about 80% of the population. Around half of all analogue radio stations are currently transmitting on DAB, but there are a number of digital-only stations. Receivers are growing in availability, and the £50 barrier was broken in 2004.

As of August 2005, the BBC national multiplex contains a number of different services: channels like BBC Radio 4 that are also available on analogue radio, digital-only services such as BBC Radio Five Live Sports Extra, BBC 6 Music and BBC 7, and an EPG. The multiplex may change configuration, sometimes adding extra temporary extra services, which requires other services on the multiplex to reduce bitrate and hence audio quality. Currently, 98% of all stereo radio stations on DAB in the UK use 128 kbit/s joint stereo, which is incapable of providing the same level of audio quality as FM. Speech services usually use 64 kbit/s mono. The only good audio quality service is currently BBC Radio 3, which is usually transmitted at 192 kbit/s in true stereo, but does not sound as good as it does on FM. The audio is encoded as MPEG 1 Layer 2 (MP2), which is approximately 50% less efficient than MP3, meaning that MP2 requires a 50% higher bit rate to achieve a given level of audio quality than MP3. For example, blind listening tests have shown that MP3 now performs very well at a bit rate of 128 kbit/s, and MP2 would require a bit rate of at least 192 kbit/s to provide the same level of audio quality. The way in which DAB has been administered in the UK means, for example, that there are virtually no new local radio stations/formats appearing. Each local multiplex 'bundle' has been awarded to one of the big radio corporate companies and these, naturally, have a vested interest in 'seeing-off' new 'upstart' stations which would compete with their own 'niche' networks. Consequently, leaving aside the BBC, consumers are being asked to buy new radios to receive inferior-audio copies of existing FM stations (and some AM ones) - or 'new' stations, which are simply regionally-networked from London.

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Digital radio for the US automotive market

While traditional radio broadcasters are trying to "go digital", major US automobile manufacturers are exploring DAB satellite radio from orbit on a subscription basis.

Ford and DaimlerChrysler are working with Sirius Satellite Radio, previously CD Radio, of New York City, and General Motors is working with XM Satellite Radio of Washington, D.C. to build and promote satellite DAB radio systems for North America, each offering "CD quality" audio and about a hundred channels. Satellite DAB would allow people on the road to listen to the same stations in any location in the country.

XM Satellite Radio has a constellation of two satellites, both of which were launched in the spring of 2001. The satellites are Boeing (previously Hughes) 702 comsats, and were put into orbit by Boeing Sea Launch boosters. Back-up ground transmitters (repeaters) will be built in cities where satellite signals could be blocked by big buildings.

Sirius Satellite Radio launched a constellation of three Sirius satellites during the course of 2000. The satellites were built by Space Systems/Loral and were launched by Russian Proton boosters.

The services offered by both groups are similar. Half of the channels will be commercial-free and only available by satellite, while the other half will be relays of conventional ground-based broadcast channels. The service will cost about $10 USD a month, not counting the cost of the DAB radio itself.

The FCC has auctioned bandwidth allocations for satellite broadcast in the S band range, around 2.3GHz.

While terrestrial DAB may be a nonstarter (in North America), satellite DAB has some clear advantages. People who lead mobile existences would find it convenient to access familiar stations while on the road, for example. Terrestrial analog broadcast stations are apprehensive about what satellite DAB may do to their business.

The perceived wisdom of the radio industry is that the medium has two great strengths: it is free and it is local. Since satellite radio is neither of these things, it is seen as a niche market at best.

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DAB in Canada

In Canada, digital radio is developing more slowly, although using the same Eureka 147 system as Europe. A number of radio stations in large cities have "transitional digital radio undertakings" which the CRTC requires to broadcast a certain percentage of distinct programming. In addition, one station in Toronto was approved April 17, 2003, (though not yet operational) to broadcast to a niche market: 70 percent of the programs would be in Punjabi, Hindi and Urdu; the owners preferred this to trying to operate an SCMO operation.

To date, there is no strong indication of a move to digital receivers by Canadians. It may be several years yet before digital broadcasting generates sufficient listenership before analog stations begin to shut down.

However, on June 16, 2005, the CRTC approved subscription satellite digital services, two of which are partnerred with the Sirius (Sirius Canada Inc.) and XM services (Canadian Satellite Radio Inc.) in the United States, and a third to be provided by the CHUM radio group using terrestrial transmitters that will only cover mainly urban areas in the south. A coalition of groups including Friends of Canadian Broadcasting and unions have asked the federal cabinet to, with respect to Sirius and Canadian Satellite Radio, strike down the decision or refer it back to the CRTC for a new decision on those two foreign-partnered services.

Pending a decision by the federal cabinet, the satellite services should be operational by December 2005.

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Digital radio for the third world

Digital radio is now being provided to the third (developing) world. A satellite communications company named WorldSpace is setting up a network of three satellites, including "AfriStar", "AsiaStar", and "AmeriStar", to provide digital audio information services to Africa, Asia, and Latin America. AfriStar and AsiaStar are in orbit. AmeriStar cannot be launched as Worldspace transmits on the L-band and would interfere with USA military as mentioned above.

Each satellite provides three transmission beams that can support 50 channels each, carrying news, music, entertainment, and education, and including a computer multimedia service. Local, regional, and international broadcasters are working with WorldStar to provide services.

A consortium of broadcasters and equipment manufacturers are also working to bring the benefits of digital broadcasting to the radio spectrum currently used for terrestrial AM radio broadcasts, including international shortwave transmissions. Over seventy broadcasters are now transmitting programs using the new standard, known as Digital Radio Mondiale (DRM), and commercial DRM receivers are available. DRM's system uses the MPEG-4 based standard aacPlus to code the music and CELP or HVXC for speech programs. At present these are priced too high to be affordable by many in the third world, however.

Low-cost DAB radio receivers are now available from various Japanese manufacturers, and WorldSpace has worked with Thomson Broadcast to introduce a village communications center known as a Telekiosk to bring communications services to rural areas. The Telekiosks are self-contained and are available as fixed or mobile units.

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Bands and Modes

Band III: DAB – frequency band 174–240 MHz
L-Band: DAB – frequency band 1452–1492 MHz
DAB-Mode I, II, III and IV: country specific transmission mode. For worldwide operation a receiver must support all 4 modes:
- Mode I for Band III, Earth
- Mode II for L-Band, Earth and satellite
- Mode III for frequencies below 3 GHz, Earth and satellite
- Mode IV for L-Band, Earth and satellite

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Services and Ensembles

Various different services are embedded into one ensemble (which is also typically called a multiplex). These can be a number of different things, including:

Primary services, like main radio stations
Secondary services, like additional sports commentaries
Data services
- Electronic Programme Guide (EPG)
- Collections of HTML pages and digital images (Known as 'Broadcast Web Sites')
- Slideshows, which may be synchronised with audio broadcasts
- Video
- Java Platform Applications
- Other raw data

An ensemble has a maximum bitrate 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 depend on the amount of error correction added to the transmission: the stronger the error protection is (which requires higher levels of redundant information to be added) this allows the signals to be more robust, but allows less useful data to be transmitted. In the UK, most services transmit using 'protection level three', which equates to a maximum bitrate per multiplex of 1184 kbit/s.

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