While most radio stations are looking for ways to expand their online presence, CBS Interactive Music Group is doing the opposite with social music site Last.fm. by giving the brand an all new station to be broadcast on the company’s HD multicast stations. Last.fm will effectively take over CBS (NYSE: CBS) HD broadcast stations starting in four markets: New York, Los Angeles, Chicago and San Francisco. The broadcasts kick off on Monday, October 5. The broadcast—it will be the same in all markets—will feature a mix of music aggregated and influenced by the service’s user-generated weekly charts. It’ll also broadcast live performances and interviews from the Last.fm studios in New York, David Goodman, president of the recently formed CBSi Music Group said during a a press conference Thursday evening. Goodman also told paidContent that CBS will devote more resources toward promoting HD radio in general, something that could give Microsoft’s upcoming Zune HD devices a boost as well. (And vice versa, since it could make HD radio more affordable and more portable for some who have yet to take the leap.)

Listeners will be able to access the station through the Last.fm site as well as CBS Radio and Yahoo Music radio sites. Naturally, it will also be available on the related mobile apps for the iPhone, iPod Touch and certain BlackBerrys. You can’t get HD radio on a regular player; only HD radio receivers in New York, Los Angeles, Chicago and San Francisco will also be able to access the station, which will use the spectrum space occupied by CBS RADIO’s WWFS-FM (102.7 HD2), KCBS-FM (93.1 HD2), WXRT-FM (93.1 HD3) and KITS-FM (105.3 HD3), respectively. “It is the flip side of what usually happens—in most cases, a media property becomes a website last—but in this case, it’s a site that becomes an over-the-air property,” Goodman told paidContent. “This will give us greater ability to generate more awareness of Last.fm in the U.S. and we’re going to be promoting HD radio heavily.”

I also caught up with CBS Interactive CEO Quincy Smith, who conceded that Last.fm has not been as serious about branding in the U.S. as it should be. Smith: “Its peers have been more aggressive and HD radio is a good platform to help get that started. It plays really well to something that would otherwise be a linear feed, ironically. But radio is the first media platform that’s all about interactivity. It makes sense for Last.fm to reverse the circle.”

• Last.fm Founders Quit, CBS’ Goodman GM For Now; But Why?