People have talked about this forever. Lots of programs have midi based chord detection and it's the same with everyone. Midi is simply computer commands. If the midi file has bass, guitar, piano and horns and you look at any one instant in the timeline on that chart, musicians themselves would have a hard time agreeing on what that chord should be. Say the actual chord should be a G7 but the guitar happens to have a Db in it and one of the horns has a Bb for a quick 16th note. Do you want the whole band to play the b5#9? Probably not, it's just that the guitar and one horn happened to throw those notes in there briefly as passing tones. This happens all the time in bands and of course with midi the more instruments, the worse it gets. Sometimes you can mute some instruments like the bass if it's playing lots of harmonies and get a better result or cut the horns and guitar and just try using the piano to do midi chord detection.

Remember it's just a dumb computer algo and it's easy to fool, a human musician knows what it is but the computer is just seeing what's there and attempting to apply some "smarts" to it based on the key sig. Oh, that's another thing, your midi file has to have the proper key sig or that will throw the midi detection off too. Lots of files are set to the default C because the author never bothered to set it properly.

Bob

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