Inside Apple's New iTunes Mastering Tool

Mixing engineers, producers and mastering engineers have faced a dilema for a number of years now. That is, "Should digital files intended for Internet distribution be mastered separately?" This has been a sore spot with many veteran mastering guys, some who believe a separate master is a must, while others feel that an MP3 or AAC file made from a well-made master is more than sufficient.

It seems that Apple has finally made the decision for us all, as they've just released their Mastering For iTunes guidelines. I learned of this through an email from legendary mastering engineer Bob Ludwig of Gateway Mastering, who I believe was instrumental in Apple getting their mastering thing together (more on this later).

Among the things that the guideline recommends is:

• Always use the highest resolution file available. That means if your project started out at 96kHz/24 bit, that's what you should send to iTunes for encoding.

• Compressing the living hell out of the track to make it as loud as possible actually works against you. That's because of a circuit called Sound Check in iTunes (it's called different things on different players and services) that lets the listener hear all his songs at about the same level. A highly compressed master actually pulls the level down, so it sounds quiet and less punchy. This is a mantra that Bob and many other top mastering engineers have been singing for quite some time. 

• It's best to leave a dB or so of headroom when mastering a data-compressed online file like an AAC or MP3, as I've state in both The Mastering Engineer's Handbook and Mixing And Mastering With T-RackS. Some encoders actually output a little hot, and mastering at -1 or -2dB instead of -.1 dB can make the difference between clipping or not.

• Bandwidth is becoming less and less of an issue, so data-compressing is becoming less and less necessary.  That means that larger file sizes of greater fidelity will be less of an issue in the future. That's why Apple is pushing its AAC Plus lossless hi-res format. Soon it will become more and more possible to hear your glorious hi-res files as they were intended.

Apple has gone as far as to make a number of tools available to mastering engineers or anyone who plans on distributing via iTunes. This is so you can monitor your file before sending it to iTunes to determine how it will sound. These utilities include:

• Master for iTunes Droplet. The Master for iTunes Droplet is a simple, standalone drag-and-drop tool that can be used to quickly and easily encode your masters in iTunes Plus (the hi-res version of iTunes) format.

• afconvert. The afconvert command-line utility can be used to encode your masters in iTunes Plus format.

• afclip. The afclip command-line utility can be used to check any audio file for clipping.=

• AURoundTripAAC Audio Unit. The AURoundTripAAC Audio Unit can be used to compare an iTunes Plus file to the original source file.

By the way, you have to be on at least Mac OS 10.6.8 to run these tools. If you have anything to do with distributing via iTunes, be sure that you check out the iTunes mastering guidelines.

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