(AP) - PBS documentary maker Ken Burns is examining the roots of country music and how it has changed through the present day for a multi-episode series on public broadcasting. Country fans have a wait ahead of them, though. PBS said Monday that Burns' country music project isn't set to air until 2018. The noted documentarian has several other projects in the works for PBS, including one on the Gettysburg Address that will air this spring, and films on the Roosevelts, Jackie Robinson and Vietnam. The country series explores the question, "what is country music." It will track the careers of the Carter family, Jimmie Rodgers, Bob Wills, Hank Williams, Johnny Cash, Willie Nelson and others.