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Suppose I have an mp3 of a song. That is, I have a final mixdown; I don't have access to the constituent tracks.

Now, suppose I want to isolate parts of this song according to pan position. For example, I want to isolate something that's mixed hard left or hard right in the pan, or really at any point in between.

What are good software tools for doing this? I'm a bit out of the loop on audio software, so I'm not sure what the best tool for the job would be --- Logic, Soundtrack, Live etc.

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I believe this would apply for any audio, not just music, and I believe you'll get a better answer on the Audio site. so I'm going to migrate this. –  Matthew Read Sep 3 '11 at 21:51
    
Thanks, I agree. Better served over here! –  Peter Skirko Sep 4 '11 at 7:07
    
To answer the question simplified (but at least 80% correct): you can't. Isolating parts from a mixed down track needs a lot more information than just the pan position, unless it's a very simple mix. –  leftaroundabout Sep 5 '11 at 9:58
    
@leftaroundabout: I don't know, but I imagine this should be possible somehow. A simple approach could be: Analyze left/right channel into 10 frequency bands. Then compare the ratio of left/right for each band. Ampflify the frequencies that match the pan position best, "mute" the rest... Maybe interesting: "Center Pan Remover" plugin for Audacity. –  Chris Lercher Sep 6 '11 at 9:37
    
@Chris: even with a lot more than 10 bands to do this (not a problem in principle) it will work only if the signal you want to isolate has almost no frequency overlap with any other signal in the mix. But you're not likely to find this in many mixes. — As for this nyquist plugin: it does little but take the difference of the L and R channels. This way you can eliminate anything that's panned center without any stereo effects, but you can't isolate anything (unless it happens to be the only signal that's not mono at center). –  leftaroundabout Sep 6 '11 at 13:47
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2 Answers 2

Realistically, I could see this working where your original mix only has instruments panned hard left or right.

The problem is that any final mix will have a blend across both/all channels which not only introduces frequencies but phase/time shift - for example a vocal may be through one mic but have a small element through another mic with a delay (this happens a lot in live recordings)

It's not even a case of using maths to pull the information out - with a mixdown you are permanently losing information, so although you may be able to broadly block out a frequency range (eg vocals) when you try to extract it from the original you will have other instruments in there as well.

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This is correct. It's like asking what is the value of 'b' in the following problem: a + b = 100 –  d-_-b Feb 22 '12 at 1:28
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Considering your situation you have only a few options:

From a stereo mix you may extract 4 audio tracks that compose the stereo mix:

Left/Right/Mid/Side

Left and Right can be extracted using PAN

From the classic M/S Matrix:

Left = Mid + Side Right = Mid − Side

So, Left - Right = Side Left + Right - Side = Mid

Using Ableton you can use EQ8 and set it up to M/S Still over at the utilies section there's a diference and invert plug in to switch the channels.

In other DAWs:

1.) Extract the audio into 2 Mono Channels.

2.) Invert the audio from one of the channels.

3.) The Result is the SIDE audio.

4.) Using the SIDE audio, invert it and add it to the right + left side - or even to the single left or right.

5.) The result is the MID audio.

But to be 100% accurate and honest. On the market, on the moment, there's only one solution that will be able to do what you want with hi quality.

Melodyne.

If you never heard of it you should check youtube for a melodyne dna demo or read this.

It's possible to extract ANY piece of audio from a Mixdown, you can even change keys completly or generate a new instrument from a sample.

Check this video:

It shows how you may extract some information from Lucy in the Sky with Diamonds.

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