Among Kirk McElhearn’s many observations comes this gem from the Pink Floyd song “Wish You Were Here”:
What is interesting, however, is that when looking at waveforms, the Mastered for iTunes file shows a bit of clipping; it’s clearly louder, and there are some peaks that shouldn’t be there.
In their guide for audio engineers (PDF), Apple says:
With digital files, there’s a limit to how loud you can make a track: 0dBFS. Trying to increase a track’s overall loudness beyond this point results in distortion caused by clipping and a loss in dynamic range. The quietest parts of a song increase in volume, yet the louder parts don’t gain loudness due to the upper limits of the digital format.
Although iTunes doesn’t reject files for a specific number of clips, tracks which have audible clipping will not be badged or marketed as Mastered for iTunes.
McElhearn states that he didn’t notice the clipping until he saw the waveform in this particular case, but it seems to me that any clipping should be intolerable by the Mastered for iTunes standard. The people who choose Mastered for iTunes are people who care about details like this.