For audiophiles and music fans alike, Apple just slid something under the door when no one was looking. Head over to the iTunes Store today, and you'll see an entirely new section called Mastered for iTunes. In addition, there's a new PDF buried in Apple's knowledge base that details mastering practices for iTunes Plus, as Ars Technica first reported.

Mastered for iTunes is an attempt to aid mixing and mastering engineers in creating the best-sounding tracks possible when converted to the lossy 256Kbps audio format commonly used in the iTunes Store.

Currently, the section contains roughly 100 albums, spanning a fairly traditional and representative grouping of albums, including Pink Floyd's Dark Side of the Moon, Marvin Gaye's Let's Get It On, numerous classical albums, and (oddly, given Brian Eno's heavy use of distortion and overdriven amps) U2's Achtung Baby. It's not clear whether some of these were already in the store before. For example, some of the albums in the new section, such as Nirvana's Nevermind, have brand-new release dates from February 2012, while others keep their old original dates (1973, for Dark Side of the Moon) or dates of more recent remasters (2002, for Billie Holiday's Lady Sings the Blues), confusing the issue.

Mastering for Compressed Formats

Interestingly, Apple is now providing a freely available set of tools for mastering engineers: Master for iTunes Droplet is a drag-and-drop program that lets you encode uncompressed masters in iTunes Plus format; afclip, which checks any file for clipping distortion; and AURoundTripAAC Audio Unit (catchy!), which compares an iTunes Plus file to the source to check for clipping in a different way.

Apple is relying on its own CAF (Core Audio File) format for these tools, which could pose compatibility issues with some existing digital audio workstations depending on workflow. For example, Apple's Logic Pro 9 and Avid Pro Tools 10 support CAF, but Cakewalk SONAR X1 and Steinberg Cubase 6 don't. That shouldn't be a problem in most situations, since we're talking about the delivery portion of the mastering stage, but it's worth noting.

As audiophiles already know, with compressed 256Kbps AAC or MP3 files, it's impossible to achieve the level of detail previously available on CDs, let alone SACD or DVD Audio. An ideal solution would have been to skip this latest step entirely, and simply make available Apple Lossless (or even 24-bit) digital masters through iTunes directly—files that you can already buy from HDTracks and other online stores, albeit from somewhat limited catalogs.

The guidelines in the PDF are a good rough overview of the challenges today's mastering engineers face, given that so many people now listen regularly to compressed music files instead of uncompressed CDs or vinyl records. Apple insists the original 128Kbps AAC files available when the iTunes Music Store launched in 2003 already "sounded great," which is disingenuous at best, but the rest of the document contains some good information on 24-bit masters, the loudness wars (more on that below), and relatively esoteric (except to recording engineers and musicians) topics like dithering, dynamic range, and oversampling.

We'll take any effort to improve the sound of the files available in iTunes. But sorting out just what's happening here will be tough, as there are several things here that make back-to-back comparisons difficult. For starters, AAC and MP3 encoding algorithms differ. Traditionally, AAC files sounded slightly better than MP3 files encoded at the same bit rate, although recent improvements in MP3 encoding have rendered this largely inaudible. At the same time, different encoders will generate slightly different results; for years, people looked for alternative MP3 encoders (such as LAME) in lieu of the standard conversion tools in iTunes and Windows Media Player, all in an effort to achieve better sound quality.

Dynamics, Sound Check, and Testing

Then there are the loudness wars, which fortunately are getting more and more play in the press as time goes on. This one's actually pretty easy to understand: Digital music files have a maximum loudness ceiling, and since louder music sounds "better" to the human ear, mastering engineers apply all sorts of tricks to squeeze out as much perceived loudness as possible against that hard digital ceiling—much to the detriment of dynamics, punch, warmth, and audible details.

Apple's "Sound Check" feature is actually a great existing solution to this problem: Turn it on, and it looks at the signal level of existing files and just turns the volume up or down to make them all relatively even, which is pretty much what you already do manually as you're listening on your iPod. Spotify also has something similar to Sound Check, while Pandora and Slacker built it into the normal streaming algorithms. Contrary to what some believe, Sound Check doesn't affect the sound quality at all—if anything, it highlights just how bad some of today's brickwall-limited tracks sound, once it puts them on an even keel with older, more dynamic masters from the 1990s and earlier.

On the other hand, mixing styles have evolved over the years. That has resulted in some really good sounding albums, despite any excessive loudness. Some early CD masters in the 1980s also had avoidable flaws because of the newness of the technology. Still, the main point holds that the loudness wars have largely been detrimental to good sound, despite other improvements in recording and mixing that have happened in the past two decades.

This brings us back to Mastered for iTunes. I listened to two Mastered For iTunes tracks this morning, just for an initial comparison: Nirvana's Smells Like Teen Spirit, which just had its 20th birthday recently, and Metallica's Hate Train, from the latter's new Beyond Magnetic EP. While both of these are relatively close in the musical spectrum (hard rock and metal) they're actually recorded and mastered about as differently as they could be.

Hate Train definitely sounds a little punchier and less "smashed" than the audiophile disaster that was 2008's Death Magnetic album, particularly when it comes to the kick drum and cymbals, and the space around Hetfield's voice. But the trick here will be sorting out just what was possible at the original mastering stage, before Apple's Mastered for iTunes guidelines; it's still pretty clear that these aren't lossless files, judging by the lack of depth in the sound stage and the overly crisp hi-hats that don't ring out and decay naturally. Meanwhile, the seminal Nirvana track sounds as warm and round as usual, with a full electric bass tone and plenty of room for Cobain's husky belting, although again there's an artificial, closed-in sound to the reverb around the instruments that's not there on the CD.

In the coming weeks, we'll be taking a closer look at Mastered For iTunes and seeing how it grows with time. For now, though, call it a welcome but still artificially limited improvement (no pun intended) that uncompressed, lossless audio tracks would have avoided entirely. At least everyone gets to benefit from this, and not just people with iPod storage to spare and (presumably, if history is any indication) the higher prices Apple would charge for uncompressed tracks. Shorter files mean Apple can push them easily over its new iCloud and iTunes Match services. But that doesn't mean there can't be high-resolution files in the iTunes Store too.

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For more from Jamie, follow him on Twitter: @jlendino.