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News

Mastered for iTunes - Part I

André Engelhardt

masteredituneslogo20120217After coming across an interesting thread on the gearslutz forums on the Mastering for iTunes program, I decided to take a closer look at what this program actually entails. Starting off with Apple's official documentation which can be found here. The document opens with a bang:

If you follow the guidelines outlined in this document and audition sample AAC encodes on Apple devices, you can achieve dynamic range that's superior to red book audio and a final product that's virtually indistinguishable from the original recording. - Apple's "Mastered for iTunes"

Now the document isn't exactly free of marketing (albeit pleasantly low key), so I picked out the important bits and compiled them into a list which is as follows (my comments are in italics).

  • 16 billion downloads encoded as AAC to date worldwide. Pretty sure that includes pre-AAC Plus downloads.
  • iTunes Plus: AAC 256kbps VBR
  • Two step encoding process: First SRC (Sample Rate Conversion) of master file to 44.1kHz, output is 32bit to prevent aliasing or clipping, the output file is then input to the encoder. No dither noise needed.
  • "Ideal master" would be 24 bit, 96kHz "any resolution above that will benefit from our encoding process"
  • Don't provide up or down-sampled files.
  • "As technology advances and bandwidth, storage, battery life, and processor power increase, keeping the highest quality masters available in our systems allows for full advantage of future improvements to your music. Also, though it may not be apparent because there may not always be a physical, tangible master created in LP or CD format, the iTunes catalog forms an important part of the world's historical and cultural record."
  • "You're being provided with all the tools you'll need to encode your masters precisely the same way the iTunes Store does so that you can audition exactly what they'll sound like as iTunes Plus AAC files." This, in my opinion, is a big thing. With many other stores or websites you have no way of knowing what happens to your submitted masters and how they're going to sound until they're actually up on the site and you can buy a copy for yourself. Those tools can be found here.
  • "A growing consensus is emerging that digital masters should have a small amount of headroom (roughly 1dB) in order to avoid [clipping from oversampling in the D/A conversion process]." Something that has been done for a long time now; initially a level of -0.3dB was considered safe but over time with better understanding of the whole issue of inter-sample peaks it went down to -1dBFS.
  • "The effect of Sound Check, as well as other volume-controlling technologies, is that songs that have been mastered to be too loud will be played back at a lower volume, letting listeners more easily notice any artifacts or unintentional distortion." Great feature when you shuffle your library by songs.
  • "Because many such technologies are available to listeners, you should always mix and master your tracks in a way that captures your intended sound regardless of playback volume."
  • "[...] in order to qualify as Mastered for iTunes remastered content must begin with a high resolution digitization of the original analog source and must sound noticeably superior to the previously released version. Songs and albums submitted to the iTunes Store as remastered content will be reviewed to ensure that the sound quality shows discernible improvement." I wonder who does the reviewing and if they have exact criteria. Then again not disclosing those criteria might be beneficial to the program so that it can't be abused as a marketing quick fix.
  • "iTunes won't reject a master file based on the number of clips the file contains." Which shows that they don't see themselves as a gatekeeper on account of technical details.
  • Use afclip command line tool to check for clipped samples, also includes inter-sample clipping. Free inter-sample clipping check tool, works great and fast!
  • This one's interesting: "You might decide to take the listener on a dynamic journey through an album as a complete work, raising and lowering the volume level across the sequence of tracks to increase the music's emotional impact."
  • But then: "Sound Check is a feature in iTunes and all iOS devices that lets listeners hear all their songs at approximately the same volume. It first determines the loudness of a track and then stores that information in the files' metadata. [...]. The metadata is then used to raise or lower the volume of each track to prevent jarring volume changes while a device is shuffling songs."
  • This is the really big thing: "Sound Check can also correctly set the volume per album, rather than per song, allowing albums that rely on volume differences between tracks, such as "The Dark Side of the Moon by Pink Floyd, to maintain their intended volumes.)"

Now this last thing is something worth taking a closer look at. Bob Ludwig, in an interview from June 2012, said:

"If Apple would only allow the possibility to have an entire album carry a single Sound Check value. If Sound Check were to work in this manner and if it was turned on by default, dynamic music will sound much better than squished recordings and yet an album would maintain the correct level from song to song - the quiet middle movements of a concerto would remain quiet in comparison to the louder movements. Without the ability to have a unified ‘Sound Check’ level for an album it presently undoes part of my job to make the levels flow correctly." - Bob Ludwig, CEPro Interview

Apple's document is from January 2012 so one would think that by now Apple would have included the album Sound Check feature in iTunes. In my own testing by ear I couldn't conclusively say that this feature had indeed been implemented. I certainly could not find any information whatsoever on either Apple's webpages, nor on the rest of the internets on how and if Sound Check across a whole album is already available and if so how it is used/configured. Considering that Apple got in touch with Bob Ludwig and other mastering engineers, one would assume that this was indeed brought to their attention and that we'll see this feature coming soon.

I really hope we'll hear more about this new feature shortly. Apple was set to release the new iTunes at the end of October, however the new iTunes webpage now states it'll be coming in November (usually with Apple that means 2-3 days before December). We'll know more shortly, hopefully.

Obviously this is a hugely important topic to us audio engineers but sadly until iTunes' Sound Check isn't turned on by default on all devices (iPods, iPhones etc.) and iTunes software as well as other platforms and playback software, we as engineers can't fully take advantage of the amazing dynamic range we could achieve and still have to find a balance between the loudness war and dynamic range. To this extent, do check out the K-System by Bob Katz and www.dynamicrange.de.

So while I wait for my copy of Bob Katz's book, "iTunes Music: Mastering High Resolution Audio Delivery: Produce Great Sounding Music with Mastered for iTunes", to arrive and on which I'll post a review on this site, I leave you with this:

“The sound of a kiss is not so loud as that of a cannon, but its echo lasts a great deal longer.” - Oliver Wendell Holmes