Watch never-before-seen archival footage of Tom Petty and the Heartbreakers' newly released song

In this Oct. 2, 2007 file photo, singer Tom Petty arrives at the world premiere of the documentary "Runnin' Down a Dream: Tom Petty and the Heartbreakers" in Burbank, Calif. (AP Photo/Chris Pizzello, File)

On Wednesday, "CBS This Morning" debuted never-before-seen archival footage of Tom Petty and the Heartbreakers performing the band's new song, "Keep A Little Soul."

It is the first music to be released since the rock legend's death last October. Petty kept an extensive archive and when his family began pulling tapes from storage earlier this year, they found a trove of unreleased recordings, among them vintage gems like "Keep A Little Soul."

Petty recorded the song in 1982 during the sessions for the "Long After Dark" album. It didn't make the cut then, but the family felt the words resonate today.

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"Keep A Little Soul" will be on a new 60-track boxed set called an "An American Treasure" to be released in September. The career-spanning retrospective is designed to highlight Petty's skills as a songwriter, which he discussed with Anthony Mason in a 2009 interview for CBS Sunday Morning.

"Do you know when you've written a good song?" Mason asked.

"Not always. The B's are the dangerous things, you know….Because they can sometime look like A's," Petty said.

And as for when they reveal themselves, Petty said, "Well, in the recording. You get a little -- if a B is sitting right next to an A, it just can't pull it off."

"An American Treasure" was commissioned by Petty's wife, Dana, and daughter, Adria. "The criteria," Adria told Mason, "was songs that we all felt really said something about dad. It's such a strange thing to lose someone like him. He was much cooler than people could imagine. And I think a lot of people thought he was pretty cool."

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