Amplitude modulation (AM) was the dominant form of radio broadcasting during the first 80 years of the 20th century, but channel fading, distortion, and noise led to poor reception quality. These problems were reduced to some extent with the introduction of frequency modulation (FM), which could also provide stereo transmission and CD-quality audio, but analog radio was still not devoid of channel imperfection effects and limited coverage area. During 2003, two commercial start-ups, XM and Sirius (these merged and became SiriusXM™), introduced the huge footprint of subscription-based digital satellite radio in the United States, with a revenue model similar to that of Pay-TV channels. Around the same time, WorldSpace Radio started satellite broadcasts for Asia and Africa.
The Satellite Digital Audio Radio Services (SDARS) enabled mobile car audio listeners to tune into the same radio station anywhere within the satellite’s coverage map, limited only by intermittent blockage of satellite signal due to buildings, foliage, and tunnels. XM satellite radio took the lead in circumventing the blockage problem by installing terrestrial repeaters, which transmit the same satellite audio in dense urban areas and create a hybrid architecture of satellite and terrestrial broadcasts.
Around the same time the traditional terrestrial broadcasters also charted a digital course—for two reasons. First, they perceived that their life span on the analog concourse had to be quite short, as the world migrates to the higher quality digital runway. Second, the frequency spectrum is getting scarce, so additional content within the same bandwidth could be delivered only by digitizing and compressing the old and new content, packaging it, and then broadcasting it. Thus, the world started migrating from analog to digital radio. These techniques for radio broadcast had the advantages of clearer reception, larger coverage area, and ability to pack more content and information within the existing bandwidth of an available analog radio channel—as well as offering users increased control flexibility in accessing and listening to program material.