Apple Audio Mastering Tools
Earlier last year, Apple announced their “Mastered For iTunes” initiative, along with releasing accompanying Apple Audio Mastering Tools. In brief, Apple are now accepting higher resolution (up to 24/96) masters for upload/conversion to the iTunes Store. These files are kept in their original format on the Apple servers, and from these the Variable Bit Rate 256kbit AAC’s which go on sale are created. As well as “keeping the highest quality masters available in our systems [that] allows for full advantage of future improvements to your music”, Apple claim that “you can achieve dynamic range that’s superior to red book audio and a final product that’s virtually indistinguishable from the original recording” by submitting higher than CD resolution wav’s for conversion to iTunes Plus format.
This MFiT concept has generated a fair amount of online chatter with many opinions around the claim of higher quality , so instead of re-hashing this, I wanted to take a look at the tools that Apple are providing to support the initiative. Regardless of opinion, iTunes has well over 80% market share in music sales, and as the most likely source that music will be sold from, perhaps the tools can be of some benefit in QC’ing and A/B’ing masters before they go to the iTunes Store for sale.
The Audio Mastering Tools are available as a free of charge download on the Apple website, and comprise of 5 parts: afconvert, Master For iTunes Droplet, AURoundTripAAC, afclip and Audio to Wave droplet.
Afconvert is a command line utility that is part of the Mac OS Core Audio framework, which converts wav or aif files to iTunes Plus (256kbit VBR AAC) files. The conversion is done in two steps: first to a 32 bit floating point CAF file whilst adding iTunes sound check metadata to the file and, if necessary, doing a sample-rate conversion. The CAF file is then compressed into an AAC file.
The Master For iTunes Droplet is an easier to use drag-and-drop application, utilizing afconvert. Remembering that Apple accept full resolution wav files to ingest into the iTunes Store, and not AAC conversions, I was curious to know if this conversion process was any different to the “store” conversion… And after blind testing Mastering Engineer Leon Zervos with a song bought from the store, and the original master converted with the Droplet, he found that there was a subtle difference, but not enough to be a concern from a mastering perspective.
The AURoundTripAAC Audio Unit is a plugin for comparing iTunes Plus compression to the uncompressed audio in real time. Via the Audition tab, you can switch the monitor path between “source” and “encoded” to compare before and after the codec on the fly. There is also a clip indicator, which goes into more detail by displaying the peak value, and tallying each sample and inter-sample that clips, both before and after encoding. Interestingly, a full resolution file with a peak at 0dBfs, but no clips, produces clipping in the AAC version, as well as more frequent inter-sample peaks (which are peaks that lie between samples, that can become apparent in the up-sampling process of DAC’s).
Moving over to the Listening Test tab in the AU is where things get entertaining. The plugin presents “Source” and randomly selected “A” and “B” paths – one being the unadulterated source, and the other being the compressed codec signal. Below these buttons, selection is required by the user to determine whether “A is Source” or “B is Source” (known as an A-B-X test). After 20 selections, you are prompted with your test results. Do it, test yourself! At first, I found myself having to listen with much more concentration than I expected, and even then my results were a little embarrassing. After further listening, and demonstrating this to Leon, the differences became more apparent – particularly in the transient on the snare drum in our test track.
Moving on through the tools, afclip is a command line utility for checking various audio files for clipping. Once a file is run through afclip, a readout is presented to show where clipping samples are present in the file (similar to the detailed clip indicator in the AURoundTripAAC plugin).
Lastly, and the simplest of the Apple Audio Mastering Tools, the Audio to Wave Droplet also utilises afconvert and will convert any Mac OS X natively recognised audio format to a wav file. Presumable this is to aid in the comparison of various formats and/or codecs, and to appease my curiosity, I compared various AAC to wav conversions in Logic to those done via the droplet. There was absolutely no difference (determined via a null-test).
As a separate download, Apple also offer AU Lab, which is a stand alone Audio Unit host. If you don’t run Logic, this is an option for setting up real-time AAC auditioning using the AURoundTripAAC plugin. With most Macs equipped with optical input and outputs, this could be an easy way to use the RoundTripAAC plugin on a spare mac as a digital insert, to test the results of iTunes Plus compression. However, within the delicate constrains of mastering, the opinions given to me by engineers advise not to use this as a black and white test. Optical connections are more prone to jitter and other digital side effects, meaning the sound is coloured simply by passing through the Optical I/O, irrespective of any extra processing.
As a final note, and complete aside to this use, AU Lab is a simple, low overhead AU host that loads AU effects, instruments and generators with flexible routing and preset management – I’m sure it has a few uses in some setups where extra effects or instruments are needed!
Mastered for iTunes documentation, the Apple Audio Mastering Tools and AU Lab are available at: