Mastered for iTunes: How Audio Engineers Tweak Music for the iPod Age
High quality sound can be had on modern audio devices, you just have to work at it.
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.