Radio broadcasting is the transmission of audio material (called a program) to a geographic area that is intended for general reception by the public, funded by airtime sold between programs.
Amplitude modulation (AM) radio broadcast services have been available for the past 100 years. Most AM radio broadcast systems use relatively low radio frequencies and very narrow radio channel bandwidth to efficiently deliver audio information over large geographic areas. Unfortunately, low frequency used for AM transmission often result in signals that sometimes skip long distances (hundreds of kilometers). This has the potential for interference in distant cities. Amplitude modulation is also easily subject to electrical noise and signal distortion. Recent advancements in AM modulation can allow channel coding for stereo and more reliable (less distorted) radio signals.
To overcome some of the limitations of AM, frequency modulation (FM) was developed. FM transmission is less susceptible to noise and distortion. Unfortunately, most FM broadcast systems use a wider radio channel than AM systems. FM broadcast channels can be up to 20 times the bandwidth of a single AM broadcast channel. The latest advancements in FM broadcasting include conversion from analog to digital and the ability to simultaneously send some additional information (sub-channels) with their audio broadcasts.
The current technology used for FM radio channel broadcast uses less bandwidth than is authorized for transmission. With some modifications to the transmitter, it has been possible for FM broadcast stations to simultaneously send some additional information (sub-channels) with their audio broadcasts. These sub-channels can contain audio or digital information. Sub-channels can be used for data transmission and paging services.
Figure 1 shows a typical radio broadcast system. The radio broadcast system consists of a production studio, a high-power AM or FM transmitter, a communications link between the studio and the transmitter, and network feeds for programming. Radio broadcasting involves the use of various types of information sources called “program sources.” These program sources come from compact discs, tape recordings, soundproof audio studios, remote location sites (such as a van), or other network sources. The production studio controls and mixes the sources of information including audio compact discs, audio studio, audiotape, and other audio sources. A high-power transmitter broadcasts a single radio channel. The studio is connected to the transmitter by a coaxial cable, special leased telephone line (extra high quality), or dedicated radio link. Many radio broadcast stations receive their programming source from a radio broadcast network. This allows a single audio source to be relayed to many radio broadcast transmitters. The diagram also shows how a sub-channel is combined to provide a private audio broadcast service.
Two separate technologies are being tested to bring digital audio and data services to conventional radio broadcasts. The first incorporates digital data into the conventional FM broadcast by adding the digital data signal to the existing audio signal before FM modulation. The second is a fully digital transmission that is transmitted in addition to the conventional FM. This separate signal is added to the conventional FM signal after the FM modulation. Unlike high definition television (HDTV), these systems do not replace the analog service; they provide additional services and are completely compatible with conventional AM or FM broadcasts. The additional services are available only to those users with a receiver capable of accessing the digital data.
The entry of digital transmission into commercial broadcasting represents a revolution in the types of services that will be available to the public in the near future. Compare the possibilities to the many digital satellite features or the digital programming available with CD players. Imagine pressing one button on the car radio to request only news stations, or your preferred music category.
Digital audio broadcasting (DAB) transmits voice and other information using digital radio transmission. The DAB signal is normally shared with additional digital information on a single digital radio channel.
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