Napster Inc. said Sunday it will begin selling music downloads as unprotected MP3 files in the spring, joining other online retailers. The file format change will apply only to single tracks and album purchases, according to a company press release. Tracks downloaded as part of the company's music subscription service will continue to have copyright restrictions. Unlike music files that come with copy protections embedded, MP3 files are compatible with most portable music devices, including Apple Inc.'s market-leading iPod media players, Microsoft Corp.'s Zune and mobile phones that play music. "The ubiquity and cross-platform compatibility of MP3s should create a more level playing field for music services and hardware providers and result in greater ease of use and broader adoption of digital music," Chris Gorog, Napster's chairman and chief executive, said in a statement. The company did not say which record companies had agreed to license music for sale as MP3s via Napster.