Disco began as a gay, black, and brown underground New York City party music scene, which alone was enough to ward off most rockers. The difference between rock and disco was as sociological as it was aesthetic. At its best, disco was galvanizing and affirmative. Its hypnotic power to uplift a broad spectrum of the populace made it the ubiquitous music of the late '70s. Disco was a primal and gaudy fanfare for the apocalypse, a rage for exhibitionism,...