DRM (Digital rights management)
DRM (also known as copy protection) allows copyright owners to control how downloaded music is used. Typically it is used to restrict the number of times a file can be copied onto different computers or music players, or burned to CD. Some subscription services use DRM to prevent users accessing music once their subscription expires.
Most legally purchased music includes DRM protection and the technology is backed by the four major labels, Sony BMG, EMI, Universal and Warner, though there are signs that they are wavering in the face of consumer dissent.
There are several DRM systems. Most protected WMA files use Microsoft's PlaysForSure technology (also know as Janus), although Microsoft has developed a new system for its Zune Marketplace store. Apple uses its own FairPlay technology to protect AAC files sold through its iTunes Store. Of the popular formats, only MP3 remains DRM-free.
DRM is unpopular with listeners. They resent the fact that protected downloads are subject to restrictions that do not affect most music CDs. And they complain of a lack of interoperability – that Apple-encoded files that won't play on Windows devices, and that Windows-encoded files that won't play on Apple devices.
But DRM's days may be numbered. Apple's iTunes Store began selling EMI tracks without protection in May 2007. There are several anti-DRM campaigns and blogs, listed in Find out more.