Why Mastered for iTunes Matters

There was a lot of consumer and pro audio press about the introduction of the Mastered for iTunes program, but there are still a lot of music creators and producers who aren’t aware of the program’s details, advantages, or implications.

This article explains the Mastered for iTunes program, and why it’s an important benchmark for digital music in general.

Masters v Premasters

Like nearly every other digital music distribution channel, iTunes doesn’t really want a master; they want a premaster. The so-called ‘mastering’ process is actually two related, but operationally discrete sub-processes:

  1. Premastering, which encompasses all of the aesthetic decisions related to preparing a set of mixes for an audience. Premastering answers any questions related to what the record is going to sound like.
  2. Mastering, which is the process of creating the delivery media that will allow the audience to access the material. In the case of iTunes, this includes encoding an Apple AAC file and adding the appropriate metadata.

It doesn’t matter if you’re a major label dealing directly with iTunes, or a self-released artist using an aggregator; iTunes wants a specific digital audio premaster so that they can do the encoding (i.e. mastering) in-house.

So what do they want, and how do you know that your master is going to turn out the way you would want it to? That’s precisely what the Mastered for iTunes program is all about.

Codecs and Compensated Premasters

The goal of any digital audio codec is to take a high-resolution or CD-quality digital audio file, and generate an encoded file that retains as much of the subjective audio quality as possible, with a reduced file size. Regardless of anyone’s opinion of any particular codec, if your music is going to be sold on iTunes, it’s going to be encoded by Apple using their variable bit rate 256kbps AAC-LC encoding format.

This introduces an important issue. Apple AAC is not lossless. This means the sound of the master may be discernably different than the premaster.

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 and 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:

  1. “An ideal master will have 24-bit 96kHz resolution.”
  2. “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 44.1kHz-192kHz are recommended.

Additionally, Apple recommends that these digital audio files include “a small amount of headroom (roughly 1dBFS).”

When D/A converters reconstruct a continuous waveform using digital audio data, there are analog levels greater than the maximum peak sample level. 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 1dB” 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% of the total US market by dollar value in 2012, after crossing the 50% 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

Rob Schlette

Rob Schlette is chief mastering engineer and owner of Anthem Mastering, in St. Louis, MO. Anthem Mastering provides trusted specialized mastering services to music clients all over the world.
  • Ian

    You make some solid points. i would argue that the whole “Mastered for iTunes” is a bit gimmicky, though. And more importantly, not a true reflection of Apple’s concern for audio quality. I have been doing 24bit masters and protecting against inter-sample peaks for years, so it’s nothing new. I would be inclined to swallow the Apple kool-aid had they even made an attempt to cater to audiophiles. They gave us 256kbps AAC, whoopdie-do. I think for most people this is fine, but still not optimal for listeners like myself. Listeners should have a choice of download format, and with today’s massive data storage capabilities, how expensice would it really be to store a lossless version of the track? It seems Apple is more concerned about maximizing profit than audio integrity (who would have thought?), despite their outward cry for High Quality “premasters”!

    • http://www.anthemmastering.com/ Rob Schlette

      Thanks for reading Ian. MFiT is certainly not a proprietary scheme – it’s really about communication. We might think of these deliverables as common practice, but with the wide spectrum of submissions to aggregators, MFiT is much-needed communication.

      I’m always interested to see discussions of MFiT turn toward the AAC-LC format. MFiT is strictly about the inputs to the iTunes system, and minimizing the signature of the encoding process. Their use of the codec is, at this point, an audience-approved given. Is the argument for ignoring MFiT that there’s no discernible difference between the AAC encode of a track ripped from a CD and a MFiT spec premaster? Is it that you don’t feel that Apple AAC sounds good enough to bother?

    • Ian

      I just think it’s somewhat comical for Apple to come out so heavily in favor of higher quality masters, then refuse to offer ALAC for purchase. If they truly cared about high quality, then they should at least consider offering lossless. Personally, I think 16bit and 44100 sample rate is excellent from both a scientific and pragmatic viewpoint, and it would be nice if they offered that as an alternative to AAC. As a nice bonus, it would encourage some people to think more objectively about the quality of audio blasting their cochlear nerve; which could have some other unforeseen benefits.

  • TN

    so does iTunes dither too?

    • http://www.anthemmastering.com/ Rob Schlette

      Functionally, yes. A basic part of any perceptual coder is noise management.

  • Mo Tyson

    Rob,
    Thank you for the article. I’m still a student in the pro-audio field, so I’m having a little difficulty grasping the MFiT concept and how it affects the rest of the workflow. If Apple prefers a 24-bit 96KHz WAV file as the deliverable, doesn’t that mean the tracking, mixing, and pre-mastering all need to be done at 96K to avoid upsampling?

  • Nichael Mubertz

    MFiT matters for musicbusiness and -industry only, for logistic,
    incasso and digital rights management reasons. Thus matching perfectly
    with wma-files for MS Whimdoze (128k/fixed bitrate MP3′s, by default)!Michael Hubertz, @Hubi1857 on twitter

  • Stefan Peterson

    Interesting stuff, but I would love to have the ALAC option as mentioned by Ian. I just wonder whether this is a hold-back by Apple or by the record-companies. Probably the latter, hopefully it will be an option soon, so we can re-buy the songs we have bought over the years.

    Any good links for ABX-comparisons of MIT vs non-MIT from a CD?

    • http://www.anthemmastering.com/ Rob Schlette

      I’m not aware of any such comparisons currently online. I have some among the ABX results I’m including in a presentation I’m preparing for an upcoming AES student conference. It’ll be the new year before I can dedicate the time to wrapping that up. I’ll do my best to post a link back here.

      Thanks for reading!