In his 90th year, Elliott Carter is doing something few nonagenarians ever do: he's premiering a striking new string quartet, his fifth. And it's an awe-inspiring piece. The Arditti String Quartet takes up the short phrases that run with and then against one another with sureness, plucking and scraping and making their bows sing. They then delve into each of the five interludes that interrogate the quartet's six sections and play through the disparate splinters of tone and flushes of midrange color as if they were perfectly logical developments. Which they're not. Carter has again brilliantly scripted a chatter of stringed voices--? la the second quartet--that converse quickly, sometimes mournfully, but never straightforwardly. This complexity of conversation is a constant for Carter, coming sharply to light in "90+" and then in Rohan de Saram and Ursula Oppens's heaving read of the 1948 Sonata for Cello and Piano, as well as in virtually all these pieces. This is a monumental recording, extending the documented work of a lamentably underappreciated American composer.