And You May Ask Yourself: Mastered for iTunes? Same As It Ever Was.

Apple's Mastered for iTunes process has been publicly criticized for not producing audio files that were subjectively 'closer to the CD' than standard iTunes. Turns out, the test methodology was flawed and the negative conclusion was the only one possible under the test conditions. The conclusion may be true, but is not interesting or informative.

The Mastered for iTunes toolkit (MfI) is free. Anyone can get it. I thought it would be a more interesting exercise to download the tools and process our 24-96 WAV test set and examine whether the result was closer to the source than standard iTunes. My conclusion from the exercise is interesting and informative, but not in the way I expected.

In all cases, MfI produced an AAF file virtually identical to standard iTunes. Darn near entirely identical.

RMS amplitude measured for the difference wave between the MfI AAF file and the WAV master is numerically identical in all cases to the result reported for standard iTunes. And the difference wave between the two AAF files in each case is virtually silent, modulo an occasional burp or murmur. The two files may not be exactly alike, but the difference does not reflect different approaches to processing the audio source.

What do we make of this?

1. The afconvert program supplied with Mastered for iTunes is the same afconvert program under the hood at iTunes. The Mastered for iTunes tools do not a priori produce better-sounding compressed audio files. According to Apple's white paper, the MfI version is instrumented to optionally produce additional information about the output files, most notable the presence of clipping (sample values outside the [-1, +1] range truncated to fit).
2. If you are a professional mastering engineer, you already have tools that can detect the presence of clipping in sound. Heck, I am a non-professional doing analog-to-digital audio archiving and restoration at home, and even I have tools that can detect (and allow me to correct for) the presence of clipping.

So mastering engineers have always had the tools and technique to create master recordings optimized for compression, the same way they have created unique masters for other formats, like CD and SACD. By offering transparency into their mastering process and with the carrot of a premium certification in the world's #1 music retail source, Apple is now encouraging the music industry to do it.

Some in the industry have called this a perfectly normal development, a good thing. But as a consumer, I question the value of a label that says that experts certify they have now done what they may have silently been doing all along. Same as it ever was.

Thanks to my Harman colleagues Jonathan Moss and Rahul Misra for their invaluable assistance in data collection.

            Vinyl-to-Digital Restoration #26           

Title: Remain in Light

Artist: Talking Heads

Genre: Alternative

Year: 1980

David Byrne is one of my musical heroes. Put him together with producer Brian Eno, and it's no surprise that Remain in Light is my favorite Talking Heads studio release. I never saw them live, but the once-in-a-lifetime performances of some of this material in Jonathan Demme's Stop Making Sense are mesmerizing.


© 2012 Thomas G. Dennehy. All rights reserved.

'Mastered for iTunes is BS?' Bad Science Meets Rampant Retweeting

Mastering engineer and writer Ian Shepherd took exception to Apple's new Mastered for iTunes process being characterized in an interview as 'sounding closer to the CD.' So he took to YouTube and his website to prove that it does not. "Not true. Testably not true."

His result was re-blogged and re-tweeted without comment (2000+ times) until it not only became news, it threatens to become fact.

Unfortunately, his test design and logic are flawed, and the false conclusion he draws is the only conclusion possible under the test conditions. Let's take a look. [Disclaimer: I have no connection to Apple or its products.]

Basically, Mr. Sheppard used a well-known test that identifies differences between sound waves to compare a 16-44 WAV file ripped from a CD against two files.

1. File A: Mastered for iTunes AAC of the same track professionally created from a 24-96 master, downloaded from iTunes store.
2. File B: AAC created on his computer from the 16-44 WAV using iTunes with default program settings.

He then listenened to the files and concluded that File B sounded subjectively closer to the CD than File A. So it was "testably not true" (extrapolated from a sample set of 1) that Mastered for iTunes files sound closer to the CD than regular iTunes files. (Viewers of the YouTube video proof have to take his word for it; they can't hear exactly what he is hearing.) Test set size and possible equipment bias aside, I have two hypotheses as to why his conclusion isn't significant or surprising.

• Hypothesis #1. Files A & B were produced from different sources. The basic iTunes WAV-to-AAC converter will create a unique result from any particular master recording of a song (varied by sampling rate, bit depth, equalization, etc.).
• Hypothesis #2. An AAC file will have a stronger statistical correlation to its specific master file than to any other version.

If both hypotheses can be proven objectively, File B sounded closer to the CD because there was no other result possible. Mr. Shepherd's pronouncement is a tautology, a logical statement in which the conclusion is equivalent to the premise. And, as confirmed by the blogosphere and twittershpere,  another definition for tautology is "needless repetition of an idea, without imparting additional force or clearness."

My full test methodology can be found here. Using results from a sample set of three songs, we find the following:

Hypothesis #1. In all cases, the iTunes encoder produced a different AAC file from the 24-96 version of a song than from the 16-44 version, verified with the same null test used by Mr. Shepherd. The titles were chosen essentially at random from my 24-96 WAV collection and cross genres, so there is no reason to believe the result won't be repeated for any 24-96/16-44 file pair that does not have special characteristics.

Hypothesis #2. To get an objective measure of correlation, I mixed aligned versions of all WAV/AAC pairs (one in-phase, the other phase-inverted) and calculated the RMS volume of the resulting difference wave. That is, on average, how measurable the difference is between the WAV file and AAC file tested. Results below.

(Lower values indicate closer correlation between AAC and WAV files.)

In all cases, the closest correlation was between the 16-44 WAV and its corresponding AAC. This makes sense, as there is less information lost from compressing a 16-44 master to AAC than a 24-96. And in all cases, the off-diagonal files (an AAC mixed with a file other than its master) showed no closer correlation than the main-diagonal files (AAC mixed with its master), to the precision of the measuring tools.

Therefore, we can conclude that Mr. Shepherd's test was unwittingly biased toward hearing a closer match between the CD master and File B (created directly from the CD by standard iTunes) than File A (ceated by Mastered for iTunes from a different master). It is not surprising or significant that he did, in fact, prefer File B.

Nowhere in Apple's technical literature or advertising do they actually say the goal of Mastered for iTunes is to create audio files that sound 'closer to the CD.' Apple does say they set out to create a process and tools that produce audio files that are closer to the source. So we repeated our test method to compare Mastered for iTunes v. standard iTunes to see which produces files closer to a 24-96 master. Come back to see results.


            Vinyl-to-Digital Restoration #25            

Title: Ella Fitzgerald Sings the Cole Porter Songbook

Artist: Ella Fitzgerald

Genre: Jazz

Year: 1956

I didn't become aware of Ella Fitzgerald until the 70s, when her TV commercials. ("Is it live, or is it Memorex?") made her almost seem a caricature of herself. I'm very glad to have restored an original-edition copy of this LP from my father-in-law's collection to get insight into her earlier career. It reveals Ms. Fitzgerald delivering interpretations of Cole Porter songs that the composer himself must have thought true to his intent.

© 2012 Thomas G. Dennehy. All rights reserved.