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."