Napster began filtering from its directories some files alleged to contain copyrighted songs on Sunday night (3/4), but the system is proving far from foolproof.

The filters apparently block the listings only of titles that more-or-less exactly match those that have been identified as copyrighted by artists, labels and other copyright holders. According to a Webnoize report, Napster was blocking the Metallica [ tickets ] titles "Unforgiven," "Enter Sandman" and "Master of Puppets" on Monday (3/5), but the songs could be found under the names like "Unforgivin," "Enter the Sandman" and "Master orf Puppets."

Napster developed the filtering system in-house in order to comply with a court order to remove links to files containing copyrighted material. The file-swapping company had never before applied a filter to the material that appears in its directories.

Napster first announced plans to implement the filter on Friday (3/2).

Meanwhile, the head of Vivendi Universal, the parent company of major label Universal Music Group, toned down a recent stand he took against Napster's plan to become a subscription-based service, according to the Industry Standard. The company's Jean-Marie Messier reportedly said that content on Duet--a file-swapping service under development by Universal and Sony Music--might license material to Napster.

"If a legitimate service that demonstrates secure technology is developed, we may license Duet content to Napster but at the same time as others such as MSN, Yahoo, Freeserve, Wanadoo and so on," Messier reportedly said during the FT New Media & Broadcasting conference.