The tenets of improvisation, instrumental discipline and experimentation are part of the lifeblood of Chicago’s underground music scene. For 26 years, Tortoise has entrenched itself as one of that movement’s staunchest purveyors, and remains defiantly so on its seventh studio LP, The Catastrophist. Coloring outside the lines a bit, the band embraces what could have been a schmaltzy cover of David Essex’s 1973 hit “Rock On,” and instead turns it on its ear, rippling it with heavy, darkly reverbed synths and horns. Elsewhere, “Shake Hands With Danger” is similarly bleak-sounding, with fetching minor-chord mayhem via xylophones and fussy guitar leads supported by rudimentary stadium-rock percussion. It is the sound of a band unafraid to go anywhere and somehow making it sound exciting and unique when they arrive. Contrast that song with the sultry soul of “Yonder Blue,” featuring guest vocals by Yo La Tengo’s Georgia Hubley, and you experience Tortoise’s distinctiveness in just two moves—with nine more songs of innovative otherness still to come.