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Digital audio broadcasting

From Academic Kids

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.

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 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.

One can use a radio card to hear DAB through a personal computer.

Contents

Terrestrial digital audio broadcast

Digital audio broadcasting is now moving towards introduction in many countries. While DAB offers many potential benefits, its introduction has been hindered by a lack of 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 1980s, although the project started in 1987 and ended in 2000.

Over 285 million people around the world can 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 information such as song titles or traffic updates.

The Eureka 147 scheme is not compatible with the old FM and AM broadcast services. Eureka 147 uses a digital spread spectrum broadcast technology and operates in the L band, at roughly 1.5GHz. In Europe, national and regional broadcasts have been allocated frequencies in Band III (174–245 MHz), and local broadcasts are using L-Band. 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.

US digital radio schemes maintain compatibility with the old analog 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 DAB 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.

Digital radio advocates claim that digital FM sound will be "CD quality", a claim that is certain to be challenged by audiophiles, and that digital AM will be "FM quality".

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.

DAB in the UK

In the United Kingdom, the rollout of digital radio is proceeding. Experimental transmissions by the BBC started in 1995. The digital radio network now reaches about 80% of the population. Most commercial national radio stations also broadcast on DAB, as do many local stations. Receivers are growing in availability, and the 50-pound barrier was broken in 2004.

As of June 2004, the BBC national multiplex contains a number of different services, including primary services like BBC Radio 4, secondary services like BBC Radio Five Live Sports Extra, BBC 6 Music, and BBC 7, and data services like BBC Vision Radio and a test EPG. The multiplex may change configuration, adding extra temporary secondary services, which requires other services on the multiplex to reduce bitrate. Typically, most music services are broadcast at 128 kbit/s stereo within the UK, and speech services at 80 kbit/s mono.

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 is working with Sirius Satellite Radio, previously CD Radio, of New York City, and General Motors is working with XM Satellite Radio of Washington DC 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.

Digital radio for the emerging world

Digital radio is now being provided to the underdeveloped 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 (http://www.drm.org/receivers/globreceivers.htm) 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 in the underdeveloped 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.

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

Services and Ensembles

Different services are embedded in 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 football commentaries
  • Data services
    • Electronic Programme Guide (EPG)
    • Collections of HTML pages
    • Other raw data

An ensemble does not have a maximum bitrate that can be transmitted. Instead, it has a total "capacity unit" level of 864. Capacity Units, or CUs, depend on the amount of error correction added to the transmission: higher error correction enables the signals to be more robust, but allows less to be transmitted. In the UK, most multiplexes offer a protection level of three, which enables a total bitrate per multiplex of 1184 kbit.

See also

External links

es:DAB fr:Digital Audio Broadcasting nl:DAB ja:DAB sv:DAB (Digital Audio Broadcasting)

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