Digital Audio & Mastering

by Nikolozi Meladze

The above video has been doing rounds on the internet for the past few weeks. It demonstrates importance of sampling rate and bit depth. After watching it I was inspired and started watching MIT’s Signals and Systems lectures (which are very good, by the way) to learn more about audio signal processing.

The timing was perfect, as I was in the middle of mastering my new album. I was trying to get all the tracks to sound similar and have the same perceived loudness. But that’s not where digital mastering process ends, as I soon learned.

I’ve had Mastered for iTunes tools installed on my system for a while now. Time to time I used AURoundTripAAC plug-in to check what my final tracks would sound like once converted to iTunes Plus format (AAC encoding) and used the Master for iTunes Droplet to convert my files to m4a’s. During my mastering process, I revisited the Mastered for iTunes document. Which reminded me existence of afclip and afconvert command line tools, which, up until this point, I’ve never touched. They turned out to be very useful and powerful.

I ran my mastered tracks through afclip and boy, they were clipping a lot. Situation, was even worse when I tried them on encoded files. You maybe be surprised to know that many commercially available tracks clip a lot. I took a relatively popular EDM track released this year, that was encoded in 320kbps mp3 format, and ran it through afclip. It clipped over 200k times on each channel.

So make sure you run afclip on both lossless and encoded formats before you submit them to music stores. Lower bit rate encoded files clip more. Also, songs that are overdriven for loudness are more likely to clip.

Tarekith’s has a nice overview of what Mastering for iTunes is all about. But I highly recommend reading the the whole Mastering for iTunes document yourself. It’s written in easy-to-read language and you don’t need to be technical to get it. Also, the tools provided by Apple are invaluable for the mastering process. Use them. And if you are serious about mastering I highly recommend Bob Katz book iTunes Music. It's very well written and covers all aspects of digital mastering.

If you find yourself using afclip and afconvert a lot, remember Automator is your friend. For instance, I've created afclip workflow that let’s me run afclip command line on selected audio files in Finder. I also have various afconvert workflows. afconvert is very useful for down-sampling, bit rate conversion, encoding and even dithering your audio files.