To ensure the overall sound quality of the iTunes catalogue, Apple has developed a set of tools and procedures to be used by sound engineers and mastering studios when preparing a song for their platform.
When mastering for iTunes in our analogue mastering studio, we recapture the analogue audio at a resolution of 24-bit 96 kHz. This format contains more detail from which iTunes’ encoders can create a more accurate encoded file.
To make sure the encoded file is of the highest quality and does not clip anywhere, we use our own set of tools and the software tools provided by Apple. Besides the technical analysis each track is played back on an iPhone and iPad (with and without headphones / earbuds) to hear the track in the way how most iTunes clients consume their music.