Napster has unveiled the latest version of its music-downloading software, a portable subscription-based service that lets users legally download an unlimited number of songs to their MP3 players for a monthly fee. With Napster To Go, which the company bills as the world's first subscription service for portable devices, users can access any of the tracks in Napster's music library and save them to a portable media player for $US14.95 ($A19.50) per month. The subscription model challenges pay-per-download programs such as Apple Computer's popular iTunes, where customers must buy each individual song or an entire album. The service will offer the service through Microsoft's Windows Media Player software, code named Janus - a digital rights management technology that allows companies to rent out music that customers can listen to on their portable MP3 players. The system works by essentially putting a timer on the tracks loaded on the player. Every time the user connects the player to the PC and the music service, the player automatically checks whether the user's subscription is still in effect. Songs stop playing if the subscription has lapsed. If the user doesn't regularly synchronise the player with the service, the songs go dead as well. "This is potentially the first serious challenge that the iPod is going to face," said Phil Leigh, president of Inside Digital Media. "What these devices are going to be able to do is attack iPod where it's weak.