Isolating an instrument from a mix
1) Take two tracks differing by just one instrument (consecutive mixes) and load them both into Audacity (I think you can do this with most recording programmes)
2) Timeshift both tracks VERY CAREFULLY to get an as close match as possible
3) Invert the track NOT containing the instrument to be isolated.
4) Merge (mix) both tracs into a single one. In audacity take copies of both tracs before doing this because the original tracs will be deleted by the merging process
5) Compare parts of the new tracs where the instruement to be isolated is quiet with the two original tracs. Reduce the volume of the track mostly in phase with the result and redo the process.
Here is how it works: By inverting one of two identical tracks they should theoretically cancel each other out when merging them. If these tracks are not identical but differ by one instrument this instrument should be the only thing remaining. For this to work the tracks must be as identical as possible so timeshifting and volume is critical. Also all processing done on either of the tracks (EQ, compression, reverb etc) will diminish the result
I knew this is the way to do it technically, but I would be really interested to know if your result is worth all that effort...
I'd imagine the problems you named yourself like added EQs and effects that will create differences that will not vanish by inversion will smudge up the result so bad it wont be worth it... what is your experience?
two more interesting things on the topic:
Did you know karaoke machines remove the vocals by this trick?
And - did you know you can actually do the trick in your living room with a set of stereo speakers, too? I once ran a setup, playing a mono signal to both speakers. I used a mixing console, so I had different volume faders for both speakers.
Now, by inverting one of the signals, the two speakers really canceled each other - if I had one fader up, I could hear the sound, when pushing both faders up, the volume lowered and it went silent when both were at the same level! Sound cancelling feels a little magic when it happens right in front of your ears - and its being used in many, many context, p.e. to remove surrounding noise in expensive headphones. Its a nice physical experiment, and it will teach you too love your inversion buttons... happy experimenting!
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