The key to managing this off-site encoding issue is to reference mixes and premasters through a round-trip codec plugin that provides real-time auditioning of the Apple AAC process. Apple has provided the AURoundTripAAC Audio Unit for that very purpose.
In addition, the Sonnox Fraunhofer Pro-Codec provides the same facility. The purpose of these tools is not to create encoded masters, but to audition them using the very same technology that will be used when they are encoded off-site.
If your workstation lacks the technical specs to run these real-time tools, Apple has also provided the afconvert utility. This command-line tool facilitates off-line encoding using the same codec technology as the tools described above.
Best Practices & Deliverables
The Apple AAC codec performs measurably better with high-resolution digital audio input than with CD-quality input. A cursory review of the technology behind variable bit rate encoding should confirm this. Listening tests can do the same.
Apple’s Best Practices for Mastering for iTunes lays out the recommended WAV file deliverables:
“An ideal master will have 24-bit 96 kHz resolution.”
“Don’t upsample files to a higher resolution than their original format.”
To reiterate, a CD premaster is an inferior input to the iTunes delivery system. 24-bit WAV files with sample rates from 44.1 kHz-192 kHz are recommended.
Additionally, Apple recommends that these digital audio files include “a small amount of headroom (roughly 1 dBFS).”
When D/A converters reconstruct a continuous waveform using digital audio data, there are analog levels greater than the maximum peak sample level.