A federal judge has ordered Apple CEO Steve Jobs to answer questions in an iTunes antitrust case that alleges Apple had a music downloading monopoly.

[ad#Google Adsense 300×250 in story]The questions will be limited to changes Apple made in 2004 that made music files from RealNetworks incompatible with Apple’s iPod, according to Bloomberg.

U.S. Magistrate Judge Howard R. Lloyd wrote that “Jobs has unique, non-repetitive, firsthand knowledge about the issues at the center of the dispute over RealNetworks software.”

The plaintiff asked the court to allow questioning on why Apple refused to license FairPlay, its DRM technology, but the judge refused.

It’s not clear when Jobs will be called to answer the questions. He took a medical leave of absence from Apple in January and is still off at this time.