How Audio Engineers Compensate for Compression with Apple's Mastered for iTunes Initiative

By Wesley Fenlon

Apple provides audio engineers with guidelines for remastering tracks for the conversion process to AAC.

At the D: Dive Into Media conference earlier this month, rock legend Neil Young revealed that he'd been working with Steve Jobs to develop a music player for 24-bit audio. He claimed that the project hadn't continued after Steve's death and lamented the quality lost in today's highly compressed digital audio. While Apple isn't unveiling a new FLAC initiative or a terabyte iPod, they still care about pushing audio quality forward. First came the step up to iTunes Plus, their name for the 256kbps AAC files that now make up the iTunes library. Next comes Mastered for iTunes.

Mastered for iTunes presents audio engineers with a set of guidelines for re-mastering music with digital distribution in mind. The white paper calls for the highest quality masters as possible, but it's more specific than that. Mastering for iTunes requires ensuring music will sound good in earbuds, an iPod dock, in a car, or hooked up to a stereo system--and that means no more boosting the low end to add fake bass to earbuds and ruin playback on better audio gear.

Ars Technica spoke to an audio engineer who recently remastered Rush's back catalog. In the process, he created masters specifically tuned for Apple's AAC format. As he explains, mastering for digital delivery is completely different than mastering for vinyl, or even a CD:

"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."

It's important for audio engineers to master iTunes tracks with a variety of sound systems in mind--tuning them specifically to earbuds or headphones will only make them sound bad on higher end equipment. But no matter how you prepare the music, an AAC encode is still going to throw away a massive amount of data. 24-bit 96khz recording averages about 100MB of data per minute. The average song on iTunes, by comparison, is only a few megabytes.

Engineers can't predict exactly how the AAC algorithm will affect a song. For a perfect master--or at least as close to perfect as they can get with a lossy format--the engineers use Apple's tools to convert tracks to AAC, listen to the results, and make special adjustments to compensate for the conversion. They then repeat the process up to half a dozen times.

Apple's guidelines suggests awareness of dynamic range, clipping and volume control technologies. The iTunes Store now devotes a section to music Mastered for iTunes. It includes albums from Beck, U2, Metallica, Nirvana, Pink Floyd, and many other artists.