Watch over Fourth of July weekend for “Anthem,” a one-hour show on Francis Scott Key and the writing of “The Star-Spangled Banner.” You’ll learn how and why it was written, you’ll hear what it originally sounded like (the first, second, and eleventh notes of the melody have changed), and you’ll see various talking heads, Yank and Brit, opine (I am one of the former). On public TV stations nationwide: Check local listings.