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My guess is that they're doing it through ML. The old "cancel out the left track with the inverted right track" trick is probably good enough, but I don't see why Apple wouldn't take it one step further.

> Do any audio file formats support separate tracks with level info?

I don't know what file format audio engineering software uses to store all the track info but my guess is that it's akin to a zip file containing a flac for each track.

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> The old "cancel out the left track with the inverted right track" trick is probably good enough

I've never had this produce good results. It removes major parts of the music, while also not fully removing the vocals.

I found out about this by having a poor headphone jack connection on a cassette walkman as a kid.

I would actually take that and record out from that to another tape, so I could have 'instrumental' versions of the songs.

And, no - it doesn't work very well at all; it sounds like a poor 64kbps MP3 with bizarre artifacts and; yes - many missing instruments.

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