The most controversial genre in the history of pop was launched on a largely unsuspecting public 20 years ago. It was heralded by an album whose cover showed six young black men standing in a circle looking down at the camera. Their expressions were casual, impassive. One pointed a handgun downwards, as if about to blast the helpless viewer to kingdom come. The age of gangsta rap was upon us.

Straight Outta Compton by NWA, aka Niggaz with Attitude, was released in August 1988, a date to be celebrated or deplored according to perspective. Fans venerate it as a landmark album, shifting hip-hop’s centre of gravity from New York to Los Angeles and setting it on the path to becoming the globe-straddling, multi-billion-dollar industry it is today.