"I spoke again with Bob Ludwig, the mastering engineer quoted in the story, who has submitted "Mastered for iTunes" tracks to Apple. He says the company is simply providing mastering engineers with tools that allow them to see how songs mastered at 24 bits will clip (that is, distort audibly) when they go through the standardized AAC encoding process. That's been difficult to do in the past. Seeing the places where all lossy encoders creates clipping in the music gives engineers a reference for how to adjust the master recordings to avoid that distortion. The uncompressed files are then submitted to iTunes, which creates lossless versions before encoding the songs as 256 kpbs AAC files for sale in the iTunes store. (Through the testing process, Apple has even been submitting its encoded AAC files to mastering engineers to make sure the process hasn't created any unforeseen errors.)

Why is this significant? Because the fact that Apple retains the lossless versions of the high-quality studio masters means that iTunes, at any time it decides to, can begin selling higher-quality encodes, or even lossless files. If this story is, at bottom, about how the demands of consumers shift the balance between convenience and quality, then this new development is Apple's effort to allow humans to correct flaws in a very useful but imperfect technology. It does not mean that the company is locking its users into an inferior format."

"When Apple started pushing its “Mastered for iTunes” section of albums “specially tuned for higher fidelity sound,” it also published a white paper detailing new guidelines asking publishers to submit high-resolution 24-bit/96kHz files instead of the original CD masters for inclusion in the section. Many were under the impression that the 100 or so albums in the new section are sonically closer to the original CD source in comparison to your average AAC encode from iTunes. According to British mastering engineer Ian Shepherd, “null testing” proves that is simply not true. In fact, he proves a “vanilla” iTunes AAC encoding with default settings sounds closer to the original CD than songs that were specifically Mastered for iTunes. "