Mastered for iTunes
iTunes Plus tracks available from the iTunes Store use the same 16-bit 44.1kHz quality as CDs, so the same master files created for CD production are typically used to generate the compressed files uploaded to iTunes. However, the compression process can eliminate or distort certain sounds that, while most listeners may not notice consciously, can degrade the listening experience.
"Mastering for iTunes was a different challenge," VanDette told Ars. "You can't get around it—when you throw away 80 percent of the data, the sound changes. It was my quest to make the AAC files sound as close to the CD as possible; I did not want them to be any more loud, hyped, or boomy sounding than the CD."
Because iTunes tracks are typically played back on decidedly average earbuds or computer speakers, there is a tendency for some producers to boost bass frequencies to make up for the tinny sound. However, VanDette said, doing so is not really the answer. For one thing, there's no guarantee that playback will always happen on sub-par equipment. "There are now systems to slip you iPod into that have decent bass response, and computer speakers that have a subwoofer," he said. "I mastered an album for a mega producer who was intent on adding LF for earbuds and laptops. The result was an album that you can't listen to in a car."
Vlado Meller, engineer at Masterdisk, described mastering for iTunes "like polishing your Bentley in total darkness, then turning on the lights to see where you missed."
Jason Ward at Chicago Mastering Service agreed it's a bad idea to try and create masters for specific listening environments. "Most modern hits these days are sounding pretty fatiguing and less than ideal on any system to my ears," Ward told Ars. "Though that probably says much more about what is considered to sound good than the skills of the relevant engineers."
"I just try to make things sound as good as feasible for as wide a range of possible playback environments as possible," Ward said. "The only real tragedy would be to make decisions which would penalize listeners with good playback systems by making decisions to allegedly enhance enjoyment on inferior playback systems."
Creating iTunes-specific masters for Rush's albums required a more nuanced approach than just boosting the bass. "The delicate mix balances of a Rush album dictated that I could only 'nudge' the bottom, not really boost it," VanDette explained. "For iTunes mastering I focused on making up for the losses created by the iTunes AAC algorithm. Generally, I heard changes in level, bottom, top, punch, and imaging."
But not every album, or even every song, could be treated the same way. "On a live album I found the center image was lower, making Geddy's vocal too low in the mix," VanDette said. "It was rare to be able to use one static setting for an entire album."
The problem? The AAC compression algorithm is "quite quirky." Without compressing a song, and carefully listening to it, then comparing to the uncompressed master, there's no way to predict how the sound will change. Vlado Meller, another engineer at Masterdisk, described mastering for iTunes "like polishing your Bentley in total darkness, then turning on the lights to see where you missed."
"There are no accurate real-time tools to help you hear what the algorithm will do," VanDette said. "It was not uncommon to revise tracks three, four, even five times until I got something that compared well with the CD."
How Apple is battling compression
Apple is working to make compressed files available from the iTunes Store sound better in two ways. First, it developed a set of guidelines and tools to help engineers create the best sounding masters. Apple said the conversion process it uses to convert from uncompressed audio to iTunes Plus format is special, downsampling high-resolution audio to 44.1kHz using 32-bit floating point values, which are then converted to AAC directly. This process significantly reduces noise and dithering typically introduced in downsampling, so engineers can submit 24-bit 96kHz files directly.
Apple suggests submitting high-resolution audio files will become more important down the road. "As technology advances and bandwidth, storage, battery life, and processor power increase, keeping the highest quality masters available in our systems allows for full advantage of future improvements to your music," reads their guide to iTunes mastering. "These masters matter—especially given the move into the cloud on post-PC devices."
Apple also provides some basic tools to help engineers audition how the AAC conversion process sounds. These tools convert any WAV or AIF file into 256kbps iTunes Plus AAC files exactly as Apple does. While they don't work in real time as VanDette would prefer, they do automate the conversion process a bit. Apple even offers an Audio Unit plug-in to compare the encoded file to the original in digital audio workstation software.
In addition, Apple created a special landing page in iTunes linking to content specially mastered or remastered for iTunes Plus format. While there's no easily identifiable badge users can look for, some albums specially mastered for iTunes (identified as such when uploaded by the record label) have a small blurb of text at the beginning of the description. "This album is Mastered for iTunes."
The first such album that appeared labelled in the iTunes Store, as far as we can determine, is a live EP by Metallica called Beyond Magnetic, released in December 2011. The description on the iTunes Store clearly states "this EP is Mastered for iTunes." However, VanDette told Ars that his colleague Meller recently worked on the new Red Hot Chili Peppers album I'm With You, released in August 2011. Meller created special masters for the iTunes Store, and it is highlighted on the "Mastered for iTunes" section. That album doesn't have any current indication of special mastering treatment on its description page.
Regardless, the trend of mastering for iTunes is becoming more commonplace. Universal Music Group announced on Tuesday that new albums from Paul McCartney and Madonna have been "Mastered for iTunes," as have catalog titles from U2, Bon Jovi, and John Coltrane, among others. The iTunes Store lists recent albums from Lana Del Ray, The Decemberists, Jane's Addiction, Wilco, and Taylor Swift under the "Mastered for iTunes" label. Remastered albums from Beck, Nirvana, Diana Krall, Herbie Hancock are there, as is the entirety of Pink Floyd's catalog.
While major label artists are starting to have albums mastered just for iTunes, though, indie artists haven't yet taken the plunge, largely due to price concerns. "At our studio, we've never been asked to provide a specific iTunes master," Chicago Mastering partner Bob Weston told Ars. "Although I can see that it has the potential for making the AAC encoded masters sound truer to the CD and LP versions, it would be a time consuming process that the majority of our clients probably wouldn't want to pay for."
The mastering work for Rush's catalogue is finished, and VanDette is waiting on final record label approval before the new remasters will be available on iTunes. (The versions currently available are converted from remasters done in 1997.) Ultimately, he expects to get more requests for specific iTunes mastering as time goes on.
"With the death of the CD being forecast in the near future, I believe that mastering specifically for the most popular consumer format will become more commonplace," VanDette said. "If Apple releases better tools to make it easier—relying on the same algorithm used in iTunes Producer—this could happen much quicker."
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