Woodstock: Three Days That Defined a Generation (BBC iPlayer/Amazon Prime/PBS) offers a somewhat unexpected picture of the 1969 rock festival. Its producers wisely chose not to present a bunch of rock acts – the film Woodstock (1970) does that – preferring to go behind the scenes and recount the difficulties in choosing a site and then dealing with crowds as much as 10 times larger than expected. This tale of logistics and improvisation, often fascinating, unfolds in archival footage, stills and voiceovers by the event’s promoters and staff.