For two decades, it's entertained, educated and — on more than a few occasions — polarized electronic music fans. As Barcelona's Sonar festival enters its 20th year, it's still as contrary as when it first started: an event with equal space in its heart for abstract electronica and the Pet Shop Boys, and whose lofty subtitle — International Festival of Advanced Music and New Media Art — doesn't preclude it from booking Deadmau5 and Skrillex.