iTunes has offered tracks with the label 'Mastered for iTunes' since February this year, which promises a listening experience closer to CD quality than regular iTunes Plus files. The concept caused some consternation amongst both audiophiles and producers, though, who saw iTunes' promise as an impossible feat. Recording engineer Ian Shepard even went as far as calling the whole idea of being able to more closely emulate a CD's sound through mastering "BS."
However, not everyone was entirely unconvinced. Industry leaders Masterdisk told Ars Technica that they were approached by Rick Rubin, who was dissatisfied with the sound quality of iTunes AACs while working on the Red Hot Chili Peppers' album I'm With You. Because Rubin is such a big name, he caught Apple's attention, leading to it share the details of its encoding process and eventually release tools so that anyone can hear how their master sounds once it's been encoded by Apple.
It's still a divisive topic, with some engineers convinced that Mastered for iTunes files are losing audio when compared to the CD versions. However, others think that they have been able to use the guidelines to tweak their masters until the CD results are comparable. Ultimately, what's more important is that the result sounds good, and it has to be questioned whether the average listener will notice any difference.