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I have two audio tracks which are mostly identical. I want to isolate the differences between the two. I can use Audition, Audacity, or really anything else.

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Whichever app you use, you'll want to line them up and then invert the phase on one of them. Depending on how identical, and how they were made, it might easy to line them up and get an accurate result, otherwise you'll have to spend some time trying to line it up as best you can by zooming in on the samples.

The idea being that if you have two things that are the same, but have opposite phases will cancel each other out. Imagine a 2 samples being at 1. They are the same. Invert phase on one, and you have 1 and -1. Add them together and you have 0, aka nothing.

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    Note that this may fail even on tracks that sound almost exactly the same. For instance, if you record an orchestra with two identical microphones located at different spots in the room (but both far, and equally far, from the stage) then both signals will appear almost indistinguishable on hearing. But they'll have completely different phase relations between the instruments, so no matter how you align them you can never achieve sound cancellation.
    – leftaroundabout
    Dec 23, 2013 at 21:27
  • @leftaroundabout - very good point. I was imagining a more in the box situation, but you bring up a very good point!
    – elburzs
    Dec 24, 2013 at 0:44
  • Good point to note. My purpose is to isolate the vocals given a standard and instrumental version of a song, which would work but the two versions seem to have been slightly different for other reasons so it didn't work. Another question: will MP3 compression also make this not work?
    – Keavon
    Dec 24, 2013 at 1:22
  • @Keavon - It could, but I've created acapellas from mp3s before. They weren't perfect, but worked for what I needed them for. Is the instrumental actually from the band? It won't work if someone else made the instrumental, like from a karaoke mix or similar. The other thing I'd suggest is line them up as best you can, play it, see what you get, then nudge it the tinniest bit forward or back, and see if the results get any better.
    – elburzs
    Dec 24, 2013 at 1:26
  • It's an official instrumental. The two different songs are of different total length (a couple seconds different) so I assume some things are messed up too badly to be fixed.
    – Keavon
    Dec 24, 2013 at 4:26

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