On Thu, Jul 23, 2009 at 03:32:15PM -0400, Stefan Monnier wrote: > How can I tell (from a script) if some *.m4a file is using the AAC codec > or the ALAC codec? Unpack it and compare it to the "original" unpacked file? Perhaps the packer leaves a signature somewhere in the packed file to tell an unpacker what it is. If you can find the signature or take a look at the source of the unpacker, you might be able to find out how to check what it is.