When oversampling is used during playback, this issue can be compounded. It is common practice to leave some amount of headroom to avoid inter-sample clipping, even for un-encoded masters.
Many mixers and hopefully all mastering engineers employ reconstruction metering to manage inter-sample audio levels.
Apple has also provided afclip, which is a command-line tool for checking both sample and 4x oversampled peaks. afclip provides both quantitative and GUI outputs for locating clips.
Bottom line: Apple is requesting 24-bit WAV premasters with native sample rates and “approximately 1 dB” of peak headroom.
Why Mastered For iTunes Matters
CD-DA is not the primary music delivery medium anymore. The 2012 RIAA Music Industry Shipment Stats showed that “digitally distributed formats comprised 59 percent of the total US market by dollar value in 2012, after crossing the 50 percent threshold for the first time in 2011.” This coincides with a slight, but real, increase in revenue. The RIAA’s Latin Music stats show a similar trend.
What is the sense in continuing (intentionally or not) to master strictly for CD-DA? Ignoring a concerted, thoughtful approach to delivering music for digital distribution ignores the statistical bulk of the listening audience.
The Mastered for iTunes Best Practices aren’t the least bit obscure or esoteric. In fact, they’re a good de facto standard for virtually any digital premaster delivery. General awareness of such a standard is certainly overdue, if not dangerously late.
The best practical argument for ignoring something like the Mastered for iTunes program is that streaming music sources are in the midst of replacing digital download outlets. For some specific audiences this is true, however that observation seems to make a better case for coordinated premaster deliver standards than against.
The basic premise behind Mastered for iTunes is that AAC is not CD-DA, so best-quality results require unique deliverables. This has been true (and well-accepted) for vinyl, cassette, broadcast radio, etc. The technology required for listenable streaming music certainly requires a similar (if not more complex) consideration.
Mastered for iTunes is a simple standard for providing predictable high-quality results for one of music’s most popular distribution channels. Beyond that, it is an excellent chance for the pro audio community to get in the good habit of carefully considering and accommodating digital music distribution.
Rob Schlette is chief mastering engineer and owner of Anthem Mastering (anthemmastering.com) in St. Louis, MO, which provides trusted specialized mastering services to music clients across North America.
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