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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.
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 digitalA digital system is one that uses discrete values rather than a continuous spectrum of values: compare analog. The word comes from the same source as the word digit: the Latin word for finger (counting on the fingers) as these are used for discrete counti spread spectrumSpread-spectrum telecommunications is a technique in which a signal is transmitted in a bandwidth considerably greater than the frequency content of the original information. Note Frequency hopping, direct sequence, PN spreading, time scrambling, chirp, a broadcast technology and operates in the L bandL band (20- cm radar long-band) is a portion of the microwave band of the electromagnetic spectrum ranging roughly from 0. It is used by some communications satellites, and by terrestrial Eureka 147 digital audio broadcasting. In the U. the L band is held, at roughly 1.5 GHz. 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, 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-channelIn-band on-channel IBOC is a method of transmitting digital and analog radio broadcast signals simultaneously on the same frequency. By utilizing a subcarrier, digital information is "piggybacked" on a normal AM or FM analog signal, thus avoiding any comp (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 allocationSince the electromagnetic spectrum is a limited resource, the use of its radio frequency bands is regulated by governments in most countries, in a process known as frequency allocation or spectrum allocation . Since radio propagation and RF technology mar, 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. 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 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.