Variable pricing

There has been friction between Apple and the music industry, with Apple's chief executive Steve Jobs calling the music industry "greedy."

Digital sales - whereby consumers buy songs over the internet - only account for about 5% of the largest record companies revenues, but are growing fast.

Warner, EMI and Sony have expressed interest in a variable price system, where newer songs would cost more than older ones.

However, Universal has been more flexible, arguing that the market needs time to adapt before changes are made to the pricing system.

iTunes is by far the largest player in the digital music segment, with an 80% market share.

The success of iTunes has played an important role in boosting sales of Apple's best-selling iPod portable music player.