Rdio announced Thursday it has added the catalogs of distributors CD Baby and TuneCore. Rdio CEO Drew Larner tells Billboard.biz the deal increases Rdio's catalog by 3 million tracks to over 18 million.

Rdio, currently working on a program to directly compensate artists that brings it new subscribers, wants to be seen as being artist-friendly. These deals will help. "Obvious the self-service aspect of what these guys do is going to bring a lot of indie music to the platform," says Larner. "In the evolution of building the catalog it's a very important thing."

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But why the wait? Rdio already had in its catalog ONErpm, BFM Digital, Zimbalam, AWAL and Ditto. And the service launched in the U.S. - with 7 million tracks - all the way back in August 2010. Two years is a long time to go without two of the larger independent distributors.

"Every deal has its own momentum or lack thereof," Lerner explains. "We had a couple of issues we had to get over in terms of getting these deals done, but I'm pleased we did finally get them done because it's an important step."

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A person might have a difficult time running across holes in a subscription service's catalog. These deals are reminders that subscribers aren't necessarily getting the value they expect. People have been led to believe these services pretty much have all legally available music - whether or not that's the case. In reality a music service launches with a bare minimum of rights holders and adds them over time. Subscription services can reasonably argue they offer a good value today. Their value will increase over time as more catalog is added.

Rdio, which currently operates in 13 markets, has fewer aggregators left to license yet many holes remain (it's inevitable, really). One obvious hole is the Beatles, which no streaming service has in its catalog, and legacy artists such as Led Zepelin and Pink Floyd. And there's a constant workload of artist-owned catalogs and out-of-print catalogs to be added to the catalog.

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Larner believes the superstar holdouts will eventually be lured to Rdio and other subscription services when their royalties reach a certain, unknown threshold. "I think the tipping point is coming. It just hasn't come yet.