MultiTrack file conversion question

[Anorton]

I'm curious about what actually happens when MultiTrack converts an m4a file. Is the file actually decompressed when converted to .wav or is it essentially still an m4a with a different file extension? GarageBand for iPad only seems to export m4as through email, and I'm wondering how much sound quality I might be losing. Thanks, in advance, for any insights on this.

[pwnified]

It's fully converted from lossy (m4a) to lossless (wav) and stored in the Bin library for each song. I think more than one conversion is ok, as long as you use a high bitrate (AAC of 160 or higher). All copy/paste and everything else is maintained as lossless wav (linear pcm).

Most of the mp3 type compression algorithms use perceptual coding as the main bit reduction technique, so multiple recompressions are usually ok because the physchoacoustic model is mostly maintained. If you hear any artifacts with a particular bitrate after multiple reencodes, you'll want to raise the bitrate until the artifacts are gone. I'm not sure if garageband can change the bitrate for export though.

[Anorton]

Thank you! Add teacher to your resume. Very useful information.