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," NAPSTER Chairman/CEO CHRIS GOROG said in a statement.

The company did not say which record companies had agreed to license music for sale as mp3s via NAPSTER.

Three of the world's biggest recording companies -- VIVENDI'S UNIVERSAL MUSIC GROUP, WARNER MUSIC GROUP CORP. AND BRITAIN'S EMI GROUP PLC -- cleared the way last year for some online retailers, including Amazon.com, to sell their artists' music as unprotected mp3 files.