Here is a more in-depth article on Mastered For iTunes, well worth
your time.
From ArsTechnica, posted February 22, 2012
http://tiny.cc/bu4rew
<<By Chris Foresman
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."
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."
Vlado Meller, engineer at Masterdisk, described mastering for iTunes
"like polishing your Bentley in total darkness, then turning on the
lights to see where you missed."
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.">>
TO BE CONTINUED NEXT POST...