The interview is a good insight into what a mastering engineers does, but there is one section of the interview which will make many audiophile ears stand up, when Michael asked: "Do you master things differently for iTunes for example?"

Bobs reply: "Apple has begun a new initiative called "Mastered for iTunes" which greatly improves the sound of iTunes AAC encodes without changing a single piece of hardware on the 250,000,000 players in the field. It can be so dramatic you can easily hear the difference between the new and old technology on your little laptop speakers. Instead of ingesting the music from a CD rip or 16-bit file, the new system uses 24-bit master files for the encode.

The AAC encoder can make use of bits 17-24. An important addition is the realization that the act of AAC encoding can cause clipping where there was none on the original PCM .wav or .aiff file. In classical music this encoder induced clipping can occur at the occasional climaxes or in a typical over-compressed pop/rock recording, many times a second. Apple has created tools to log the number, severity and time of each clip so the mastering engineer can lower the level of the 24-bit master by fractions of a dB and the clips and resulting distortion from them is eliminated.

It is a complicated answer, but a 24-bit AAC encoded file can thus sound better and measure better in certain cases than a normal 16-bit Compact Disc, which unfortunately has been regarded as the gold standard for sound in these comparisons."

The first part of Bob's answer goes along way to explaining the rumours a few months back that Apple would be moving to a 24bit download service. I always doubted that would happen, and I think based on Bob's comments above, that these rumours stemmed from the new requirement of a 24bit source file for encoding.

The last paragraph of Bob's reply is the real interesting part, where he states that one of these newly encoded iTunes AAC files 'can' sound better than the holy grail of the past 25+ years, the CD.

It is a great read and I encourage you to have a read of the full interview on the Positive-Feedback web site, well done Mike!