As the prime movers of the instrumental post-rock scene, Chicago’s Tortoise have single-handedly injected a level of musical virtuosity not often associated with the indie-rock genre. However, the band has always understood that a flurry of notes alone does not make a group and have centered their explorations on muscular grooves that reflect the autodrones of Kraftwerk. Their sixth studio album and first in five years, Beacons of Ancestorship stakes out some new territory in its Moog-loving keyboard excursions. Their love of primitive, funky synths gives a sinister ‘80s video game appeal to much of what they do. When they turn it up for overdrive on “Yinxianghechengqi,” the sound turns ominous and grinding and makes the album’s previous dark excursions — “High Class Slim Came Floatin’ In,” “Prepare Your Coffin” — sound positively jovial. “The Fall of Seven Diamonds Plus One” is their standard Ennio Morricone’s spaghetti-western homage. “Minors” has a subtle funk under its placid, pleasing tones. Tortoise makes background music that refuses to stay in the background.