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Which solutions (standalone or plugins) exist for unmixing/de-mixing sources?
Lets say I have a track with acoustic guitar and string synth in mono, and I want each source separated. Which tools allow me to do this?
(note: before you say this is impossible, check out the research field of source separation algorithms)
Quick answer - No I haven't heard of any but I've heard about research being done on the field.
The greatest theoretical problem with de-mixing is knowing what to subtract.
When we mix, we add A and B which are both known signals. But when we try to do the opposite, we need to know in advance what B is. This would render the process useless in most cases since, using your case as an example, if we already had the isolated guitar signal, we wouldn't need to de-mix it.
When we do know what B is, then it is a process as straightforward as mixing is. For example, there are plugins that will analyze 'hiss', clicks or any regular noise and subtract it (de-mix) it from the original signal.
I believe that's the reason we haven't seen much progress in de-mixing technology. It would take a lot of very complex self-correlation (comparing a signal to itself) to adequately identify complex sounds like voices or instruments.
There's actually one software company that claims to provide credible unmixing : audionamix.
I am not affiliated with them, neither can I confirm their claims but there are demos on their website.
In concurrence with other answers; generally "not really feasible" is the answer. I've had a play with iZotope RX5, a very interesting editor; draws the signal as a 2D frequency graph and has tools for selecting regions (including changing regions) to isolate certain sounds and amplify or subdue them, but in the context I was operating it proved impossible to feasibly remove damage to an original recording of mixed sources (a telephone interview where a screaming, playing baby, a microwave and a background television had polluted it)
Your ears can "hear just the piano" so you assume that someone must have been able to make software that can "just hear the piano" and isolate it into its own track. No software does this well, or automatically; computationally it's a massive problem, largely rooted in sounds that aren't there in raw numbers form in the source stream, thanks to psychoacoustics and how your brain interprets the audio from your ears
More discussion: Can any software effectively remove one person/intermittent noises from an audio recording of a three party conversation?
You don't state how your sources were mixed. If we are talking about multitrack session with sources panned in place (the kind of studio mix popular in the seventies), separation works very well.
If we are talking about a single multitrack session with master tape available where we want to deal with bleedover into the "wrong" microphones, stuff is still doable.
If you just have some "acoustic" recording of the whole band where the "mix" happens by being in the same room, stuff is pretty hopeless for enjoyable results: you can still achieve improvements if the aim is preprocessing for the sake of recognition tasks.
"Song with acoustic guitar and string synth" tells very little: does "song" imply "singing" as well? How many microphones were used? Is the synth mixed in as a digital track, as an analog input track, or was it also present in the acoustic signal? How much digital and redigitized and analog post-processing was done?
At any rate: in our current time copyright law is so abysmally stupid that you need to negotiate for every trivial sampling of everything, so if you are thinking about commercial remixing, you can try just to negotiate for access to the original master tapes as well if that would help.
So it might make more sense to restate your question in reference to a much more specific scenario: in "I have a song with acoustic guitar and string synth" is too unspecific with regard to "song" and too unspecific with regard to "I have": what kind of material/media do you actually have, and what kind of legal situation do you actually have?
