Not so far ago Apple announced the initiative aimed to form a kind of standard for mastering of audio content intended for distribution through iTunes Store. Mastering guidelines for audio engineers and independent musicians as well as a set of special audio tools are available on Apple site. The guidelines could be summarized as follows:
- If possible master recordings should be submitted to iTunes Store as 96/24 audio files; remastering is advised in cases when it could provide discernible improvements.
- These high resolution files must be free of clipping; it is recommended to have at least 1dB headroom for peak levels.
- Apple highly recommends to preserve full dynamic range of recordings which is usually well defined by creative purposes of the author. Over-compression of dynamic range (“loudness war”) is no longer a way to attract listeners because almost all distribution and broadcasting channels now have technologies to control playback volume regardless of actual loudness of a track. Itunes has them for sure.
- As almost all tracks in iTunes Store are distributed now in iTunes Plus format (44.1/16, stereo, AAC, VBR-constrained, 256 kbit/s) it is recommended before submitting to preview audio material in the above format and to listen it on various consumer devices using both headphones and speakers.
In order to ease following these guidelines Apple provided a set of audio utilities for Mac OS:
afconvert performs sample rate and bit depth conversions as well as AAC encoding exactly the way iTunes Store does. So any author has possibility to preview quality of final AAC file which will be distributed. Conversion from high resolution files into AAC is made through intermediate 32bit .caf file which helps to avoid dithering thus the whole process of encoding preserves the maximum possible quality.
afclip finds all possible clippings in audio file including the ones between samples; it is recommended to test audio material for clipping both before and after encoding as the latter could introduce additional clipping sometimes.
AURoundTripAAC Audio Unit is intended mostly for blind ABX testing of encoded audio material. With this tool an author can test critical parts of the track for encoding artifacts audibility. No doubt this utility will be helpful also for those listeners who would like to test their ability to hear coding artifacts at various bit rates.
Master for iTunes Droplet automates some of the above procedures.
iTunes Store AAC @SoundExpert
First AAC@256 encoded audio tracks appeared in iTunes Store in 2007. Soon after that SoundExpert added AAC@256 encoder from the latest at that moment iTunes to the rating system, though there was no confidence that audio material for iTunes Store prepared with that exact encoder. Now Apple disclosed the whole procedure of encoding. The encoder itself was improved since 2007. Taking all this into account we decided to encode SE test files into iTunes Plus format using Apple mastering guidelines and add them to the 256 kbit/s section.
Encoding was performed on Mac OS X 10.7.3 (11D50b) using two recommended commands:
afconvert test.wav intermediate.caf -d 0 -f caff –soundcheck-generate
afconvert intermediate.caf -d aac -f m4af -u pgcm 2 --soundcheck-read -b 256000 -q 127 -s 2 final.m4a
afconvert final.m4a decode.wav -d LEI24 -f WAVE
Resulting 24bit .wav file was used to prepare SE test files as usually, thus preserving the highest possible audio quality the iTunes Plus format can offer for 44.1/16 source material.
Out of curiosity SE test files were also encoded with the latest iTunes for Windows (10.6.1.7) using iTunes Plus setting. Difference levels for both encodings are in the table below:
Table 1 Difference levels [dB] of SE test files encoded into iTunes Plus format on Mac OS and Windows.
| || J.S.Bach || Bass || Castanets || French male speech || Glockenspiel || Harpsichord || Postscriptum || Mike Oldfield || Quartet |
| Mac OS X || -37.09 || -39.21 || -39.95 || -41.56 || -43.22 || -43.68 || -36.02 || -35.96 || -35.89 |
| Windows || -37.09 || -39.21 || -39.96 || -41.56 || -43.23 || -43.68 || -36.02 || -35.96 || -35.89 |
Similar values with small variance between encoders mean that they use the same psycho-acoustic model but differ in computation procedures. Usually such small variance of Df levels is imperceptible and it can be safely assumed that both encoders produce audibly identical files. So anyone with the latest iTunes installed can encode their music with the exact quality established in iTunes Store.
Mastered For People
Apple's initiative to create some framework standard for digitally distributed music seems actual and praiseworthy. Exactly poor mastered recordings is the main source of “bad sound” problem nowadays. Using of psycho-acoustic encoding on the final stage of distribution just worsens the problem not originates it. Even taking into consideration that iTunes is only a small part of global music distribution, Apple is capable to influence on mastering practices worldwide as the company is well known trend-setter today. Not to mention the sweet marketing goal for the company to give the name – Mastered For iTunes – to this new trend of more careful mastering practices.
On the other hand iTunes Store is only a tiny, almost insignificant part of global music sharing on the internet. P2Pnetworks, cyberlocking and cloud services, social networks and author's personal web sites rapidly become effective channels of music distribution. Distinction between distribution and sharing become less and less obvious. Definitely some mastering guidelines are necessary for authors and musicians who get on in the world this new way. Apple's guidelines with a few remarks could serve the purpose and such “Mastered For People” recommendations may look like follows:
1. If possible retain your audio tracks as multi-track projects for
- further improvement and development
- producing various master records in 2.0, 5.1, 7.1, 3D … formats
- sharing your work as multi-track mixes for “creative listening” - making by listeners their own versions of your tracks
2. Retain at least 96/24 stereo mix with full dynamic range and 1dB headroom for peak levels (-1 dBFS)
3. While making equalization, limiting, dynamic range compression ... of the 96/24 master for final distribution:
- use 32bit mode
- avoid clipping (-1 dBFS)
- lo-fi versions of your track may require special post-mastering
- resulting master record should be 96/24 as well
4. Variants of conversions for final distribution/sharing:
[96/24 master] → downsampling → [48/32] → dithering → [48/16] → FLAC | ALAC | RAR | …
[96/24 master] → downsampling → [48/32] → Vorbis | AAC | MP3 | … @256-320 kbit/s
[96/24 master] → downsampling → [44.1/32] → dithering → [44.1/16] → Audio CD
Sample frequency 44.1kHz seems become more and more obsolete/senseless as it helps only to solve some compatibility issues such as creation of Red Book Audio CDs and using of old audio equipment. Sampling frequency 48kHz serves well both professional and consumer audio today making 44.1kHz unnecessary.
The draft is open for discussion, drop us a line if you are interested.
SoundExpert thanks James Orlov for encoding test files on his perfectly updated MacBook Pro and for technical assistance with Mac OS X.
As usually SoundExpert encourages you to take part in listening tests. All SE ratings, including iTunes Plus 2012 ones, exist only thanks to visitors like you. Testing is short and easy, visit our Testing Room for simple 1-2-3 instruction.
Please, remember, downloading a test file from SoundExpert you will not necessarily get a test file of iTunes Plus 2012. SoundExpert testing is blind testing – you don't know the device you test. This is the only reliable way of getting true audio quality ratings, free from any cheating and various human senses other than perception of sound quality. After you have sent your grade you will see the device you tested.