Ah, but when did the Rock Era begin? That's easier. Everybody knows that Bill Haley and His Comets' rendition of "Rock Around the Clock" was America's first-ever No. 1 rock and roll song, topping the Billboard charts in the summer of 1955 and launching the Rock Era as we know it. Occasionally, musical epochs can be demarcated easily, with a bright temporal line.

So it goes with the era of the music video. The promotional-music-clip format is more than a half-century old, dating to the 1940s and raised to a high-pop-art form by such pre-'80s acts as the Beatles and Queen, among others.

But the music video era, better known as the MTV Era, began unequivocally 30 years ago this weekend—on August 1, 1981, the day Music Television went live on cable TV. The No. 1 song on Billboard's Hot 100 that week was "Jessie's Girl," by a guy so telegenic he was crossing over from a soap opera: General Hospital's Rick Springfield. Appropriately, "Jessie's Girl" came packaged with a fairly slick (for its day) music video.