Music radio, particularly top 40, has often acted as both a barometer and an arbiter of musical taste, and radio airplay is one of the defining measures of success in the mainstream musical world. In fact, the rise of rock music to popularity is intimately tied to the history of music radio. Early forms of rock had languished in poor areas of the South. It was enjoyed mostly by rural blacks, with notable exposure in Memphis, Tennessee due to the all African American programming of WDIA. Rock music entered the mainstream during the 1950s because of controversial white DJs such as Dewey Phillips, Alan Freed, Dick Clark and Wolfman Jack with an appreciation for black music.