The number of podcast users in the United States is expected to increase nearly 15 fold over the next five years, reaching 60 million by 2010, a consumer technology research group said Tuesday.

The increasing use of digital music players and other factors are expected to drive a 101 percent compound annual growth rate in podcast listeners over the next five years, The Diffusion Group said. The number of podcast users this year is expected to reach 4.5 million.

"It's dramatic in terms of percentages, but you need to recognize that we're just in the beginning," Marc Freedman, contributing analyst to the Dallas-based research group, said.

A major contributor to the projected growth is the rise in popularity of digital music players, which are expected to be the favored device for playing podcasts. The number of shipments of digital players in the United States is expected to reach 54.3 million in 2010 from a projected 15.5 million this year, according to The Diffusion Group.

People are also expected to continue using home computers to play podcasts, which are audio files that are often downloaded to a music player for listening on the go.

Other drivers include consumers' acceptance of listening to programs that have already aired, a practice made popular in television programming through the digital video recorder, Freedman said. In addition, RSS, a standard technology used in distributing podcasts to PCs, is expected to help make it easy for people to transfer their favorite programming to digital music players.

The quality of content will be less of a driver, because portable music players are shipping with far more storage capacity than most consumers need for music. Apple Computer Inc.'s popular iPod, for example, has as much as 60GB of storage, enough to store up to 15,000 songs.

"The average consumer only owns 1,000 songs, so from the consumer's perspective, they have all this extra storage that they're willing to fill up with adequate content," Freedman said. "It doesn't have to be compelling."

Podcasts today consist of mostly amateur programming or repurposing of existing content, a practice common among traditional media, Freedman said.

"We're just starting to see some original content, particularly for this type of environment," he said.

RSS, or really simple syndication, is expected to help drive the creation of more content tailored for the medium, because it will enable providers to distribute programming on a regular basis, Freedman said.

Freedman is also chief executive of RazorPop, a Dallas-based entertainment, marketing and technology company specializing in peer-to-peer networks.