Concert films are, inevitably, pretty unrewarding spectacles. Iffy sound, endless crowd shots, migraine-inducing overuse of the zoom lens, and a sense of “well, you had to be there, of course” permeating every grainy long-shot of a “legendary” ten-minute Credence jam, all add up to an experience akin to someone telling you about a dream they had last night. That this Maysles brothers’ Altamont doc was rescued from such Woodstock-the-Moviedom by the tragic events caught therein is certainly nothing to cheer about, but the results leave an indelible mark where your average festival film just washes over. Perhaps working with hindsight, but still believably, the atmosphere of impending disaster is magnificently built up over scenes of preparation for the speedway gig – from thePartridgean attempts at crowd control (“You’re rendering that scaffolding unsafe!”) to the massive communication gap between the Hell’s Angels security detachment and the strung-out organisers. Meanwhile The Stones hang about in local civic offices as the finer points of car park regulation are ironed out. By the time the famous on-screen audience stabbing is being played repeatedly, in slow motion to the shell-shocked band by the directors in a squalid edit suite, you’re as sucked into the nightmare as they are. Chilling, compelling stuff.