Mastered For Itunes: How Audio Engineers Tweak Music For The Ipod Age

By Chris Foresman, Arts Technica, February 23, 2012

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In an age when Apple has become the top music retailer without selling a single physical disc, audio engineers are increasingly creating specially mastered versions of songs and albums designed to counteract the audio degradation caused by compression. Though audiophiles typically scoff at paying for compressed audio, preferring vinyl or high-end digital formats such as DVD-A, mastering engineers are doing their best to create digital masters that can pass through Apple's iTunes algorithms with minimal sonic corruption.

To highlight work done to improve the sound of compressed music files, Apple recently launched a "Mastered for iTunes" section on the iTunes Store. It now also provides a set of recommendations for engineers to follow when preparing master files for submission to the iTunes Store. To qualify for the "Mastered for iTunes" label, Apple says that files should be submitted in the highest resolution format possible, and remastered content should sound significantly better than the original.

How does this work? Ars spoke with Masterdisk Chief Engineer Andy VanDette, who recently completed a project remastering the bulk of Rush's back catalogue. As part of the process, VanDette created special versions of each song specifically for uploading to the iTunes Store. He described the often lengthy, trial-and-error process of trying to make iTunes tracks sound as close as possible to polished CD remasters.

The state of compressed audio

All music purchased from iTunes is compressed using a "lossy" compression algorithm called Advanced Audio Coding (AAC). Lossy compression algorithms toss out some of the information contained in a digital file in exchange for very small file sizes. Formats like AAC (and MP3) try to be intelligent about what information is tossed out in order to maintain fidelity with the original, uncompressed file. They do so by eliminating frequencies and harmonics least likely to be discerned by the average listener.

(The JPEG image format attempts to do the same thing with photos, eliminating details and colors that aren't likely to be noticed by the average viewer. This is why JPEGs can sometimes look blocky if saved at a high compression rate.)

A number of music industry luminaries, including Jimmy Iovine (head of Interscope-Geffen-A&M), Dr. Dre, and most recently Neil Young, have bemoaned the fact most music now plays back from a compressed file, resulting in a "degradation" of the sound an artist originally tried to create.

"We live in the digital age, and unfortunately it's degrading our music, not improving it," Young said in January during the D: Dive Into Media conference.

Young and his cohorts are attempting to make uncompressed, higher-end audio formats a common standard across the industry. Music throughout the last decade is typically recorded using 24-bit samples at 96kHz, and advances in computing power and hard disk space have recently made even higher quality, 24-bit 192kHz digital recording possible.

However, even the standard CD format comes in a much lower resolution-just 16-bit 44.1kHz. Compared to 24-bit 192kHz digital audio, a finished CD only has roughly 15 percent of the information captured during the recording process. Compressing the songs on a CD further into 256kbps AAC "iTunes Plus" format cuts the data down to just one-fifth of the size of CD audio, or as little as three percent of the original 192kHz recordings.

"We're working with [Apple] and other digital services-download services-to change to 24-bit," Iovine said. Young also admitted to working with Apple to make 24-bit audio standard across its mobile devices, though he suggested that no progress has happened since Steve Jobs-known for his love of classic rock-died last October.

As an audio engineer, VanDette is "hopeful" hardware and storage capabilities will one day make uncompressed, 24-bit audio a practical standard. For instance, digital music service HDtracks already offers a catalogue of 24-bit audio files at various sampling rates up to 192kHz. But such audiophile quality is only beneficial to those with expensive stereo equipment capable of reproducing the subtle nuances captured in these higher-quality files.

"I am encouraged to see a growing catalog at HDtracks, but being able to have your entire album collection in your pocket is cool, too," VanDette told Ars. As long as iPhones and iPods are the most common playback equipment, and the iTunes Store the top source for music, compressed audio files are, practically speaking, here to stay for the foreseeable future.

If you can't beat 'em, join 'em

Want an uphill battle? Try pushing the bulk of consumers to embrace niche audiophile formats and upgrade to capable equipment. Instead, audio engineers have taken to mastering versions of songs and albums specifically for the iTunes Store.

A similar mastering process is already done to prepare albums for other physical formats. As previously noted, recording is typically done in a digital 24-bit 96kHz format. However, audio released in CD format is 16-bit 44.1kHz quality, requiring a conversion from the original source. Engineers adjust equalization, levels, compression, noise filters, and other parameters to cram as much of the source material into those limits.

(Returning to our earlier photo analogy, the process is similar to converting a 14-bit RAW file from a DSLR into a standard 8-bit TIFF.)

Recording can also be done at varying bit-depths and sampling rates. Sometimes it's still done using vintage analog gear (see recent Grammy winners, The Foo Fighters). Albums are still released on analog vinyl format, and in some cases are made available in high-end digital formats such as Super Audio CD (SACD) or DVD-Audio (DVD-A). A mastering engineer will take whatever source material is provided-analog or digital-and optimize it for each release format, taking into account each format's unique strengths and limits.

VanDette explained how mastering varies depending on the age of the original recordings as well as the final output format. Many master recordings for Rush albums are from vinyl's heyday, he said. "Back then we would try and hide as much top end as possible, knowing that the end users' styli would be crap."

"Most listeners today swear they love the bottom end on vinyl, but I remember in the heyday of vinyl, it was all about top end," VanDette told Ars. "'If we could only have a clear top end without all those pops and clicks' we thought," he said, noting the tendency of low-end record players to introduce unwanted noise. "Back then, bottom was the enemy. It made the grooves [in the vinyl] too wide, and forced us to turn down the overall level of the disc."

The constraints of vinyl aren't a concern when mastering for a CD, so it's possible to boost overall levels as well as low frequencies without ruining the rest of the mix. "While remastering the classic Rush albums, I added as much LF as I could, always aware not to cloud the classic 'ping' on Neil's snare, muddle Geddy's voice, or bury Alex's guitar," he said.

"These are some finely balanced mixes, even 35 years later," VanDette said. "I wanted to make sure the listener still heard the classic album come through, without it being too loud, boomy, or modern sounding."

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."

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."