digital radio, audio broadcasting in which an analog audio signal is converted into a digital signal before being transmitted; also known as digital audio broadcasting (DAB) and high-definition radio. Digital radio reception is virtually free of static and fading, pops, and hisses; overall, adjacent stations do not interfere within one another, audio clarity and volume are improved, and weather, noise, and other interference cease to be a factor. Digital radio can be land based (or terrestrial) or transmitted via satellite. In either case, a special receiver is required to decode the multiplexed signal; the receiver may contain a small display that provides information about the audio content (such as the name of the artist or title of the music).
The land-based technology was first deployed in Great Britain in 1995, and has since become established throughout Europe. The first satellite-based digital radio system was WorldSpace, which orbited the first of its three geostationary earth orbit (GEO) satellites, AfriStar, in 1998. Each satellite transmits three overlapping signal beams carrying more than 40 channels of programming; most of world (except mainly North America and Australia) is covered. The first satellite-based system to provide a mobile subscription digital radio service covering the United States was XM Satellite Radio, which orbited two GEO satellites in 2001. XM's ground station transmits digital signals to its satellites, which retransmit them directly to radio receivers on the ground. The receivers unscramble the signal, which contain up to 100 channels of digital audio. In metropolitan areas where tall buildings, overpasses, and other obstacles can interfere with the signals when, for example, the receiver is in a moving vehicle, a network of ground-based repeaters retransmit the signals. The receiver also buffers the signal briefly so that if it loses the satellite signal it can use one from a repeater to maintain a continuous broadcast. Sirius Satellite Radio, which launched national service to the United States in 2002, employs three satellites in inclined elliptical orbits instead of GEO satellites.