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Digital audio broadcasting

Broadcast radio has been in widespread use since the 1920s, and to this time has remained based on the "amplitude modulation" (AM) technologies used at the beginning and the "frequency modulation" (FM) technologies introduced in midcentury.

Broadcast radio has now entered the 21st century, accompanied by a revolution in technology: "Digital Audio Broadcast (DAB), which replaces the traditional analog AM & FM systems with higher fidelity, greater noise immunity, and new services. This article provides an outline of emerging digital radio technology.

Terrestrial digital audio broadcast

Digital audio broadcasting is now moving towards introduction in the US and overseas. While DAB offers many potential benefits, its introduction has been hindered by a lack of standards. Several DAB schemes are being promoted in the US, and none of them are compatible with the "Eureka 147" DAB standard now being implemented in Canada, Europe and parts of Asia.

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 (174~245MHz), and local broadcasts are using L-Band. The US military has reserved L-Band, blocking its use in America, and the Canadians have agreed to 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 merged to form iBiquity.

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. One interesting possibility is that broadcasts may provided digital "tags" to identify themselves, allowing a digital radio receiver to scan for channels by type of music, such as JAZZ or CLASSICAL. Tags would also allow automotive radios to automatically changing 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.

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.

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.

XM 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.

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's free and it's 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, and AmeriStar is expected to join them soon.

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.

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.

greg goebel ( / public domain

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