Yet Another Helpful Tutorial from MacLife

Build Your Own GarageBand Loops From Any Music File
"Loops are wonderful little things. With judicious use of these musical snippets, you can construct entire albums of material from tiny building blocks -- or just scatter a couple over your compositions for flavor. GarageBand comes with a pretty healthy assortment, but spend enough time in the Garage and you may find your songs starting to sound an awful lot like everyone else's.

You're in luck. In a few easy steps, you can create your own loops from nearly any music file you've got lying around."
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