I am not sure if this is the right forum for me to float this crazy idea - and wish to also give warning that it is quite technical, and also may be completely impractical..
Over on the olympics thread the topic of backing tracks became quite dominant - I had not ever really thought about this "problem" - I have seldom wanted to play someone elses song, deluding myself by believing I am a composer (LOL) - and when I have wanted backing for a song, found a MIDI file, butchered it, and produced a backing that way..
However - GordonC put a link to "Hills of the North, Rejoice" and I love the arrangement of this song - none of the MIDI files I found were in any way usable, and it is beyond my ability to produce a backing of the quality in that link.
So I have been fretting and pondering about how to remove the lead on that track - knowing full well that one cannot - its like trying to take the sugar out of a baked cake so that one can replace this with low GI Agave nectar..
Then I woke early this morning with a complete, and probably completely crazy, "solution". This "solution" uses the tools I have available - but (if there is any merit in it) could probably be tailored for other set-ups.
Take a piece of music, load the audio file/s into a multi-track recorder, Ideally have a theremin which outputs CV, even more ideally your theremin outputs MIDI control data in CC format, giving CC for pitch, or you have a CV to MIDI converter which allows you to do this.
Play your theremin "over" the instrument you want to "delete" (or at least greatly attenuate) and record your theremin and the MIDI generated by it or its CV to seperate tracks in your multi-track recorder, so that both the music you are playing back (and playing along with) and the Theremin audio and (ideally) MIDI are in sync on seperate tracks.
Now - What you need to do is to feed the original music through a processor controlled by the theremin pitch - This processor would be a notch filter at least, which tracked the theremin pitch or MIDI data, and "notched out" the same frequencies from the original music.. This could be an external processor driven by MIDI or CV data from the recording of the theremin (one could, for example, just feed the recording of the theremin into a pitch-CV converter, and use the CV to drive a notch VCF, and record the output from the VCF back on a seperate track - and do this for both tracks of a stereo 'backing' seperately if one only has one VCF)
The above process could be repeated to remove harmonics from the instrumen one is removing, by tuning the VCF to succsessive harmonics from the instrument one is removing..
But ideally, I think it should be possible to develop a software "plug in" which could do the whole job simply by taking the recorded theremin (audio) track, analysing this (it does not need to be "real time" - so it could accurately determine the pitch without any constraints which bug real-time pitch analysis) and which could allow one to set the harmonic levels of the instrument to be removed, and process the original music in one 'hit' - one would then not need the theremin to output CV or MIDI, would not need a pitch-CV or pitch-MIDI converter - all you would need is a theremin and the ability to play the theremin accurately over the instrument you want to remove.
It might even be possible to make the "plug-in" a bit more intellegent - it could, for example, find the instrument in the original which was closest to the theremins pitch, analyse this instrument, and tailor its filters so that it tracked this instruments dynamics and harmonic content and almost completely removed it whilst doing minimum damage to other audio.
Alas, I have no expierience of developing audio "plug-ins" and this subject is well outside of my "comfort zone" - I will be looking into what is available in terms of development "kits" to see if I can do this job -
I would be really interested to hear from anyone who understands the above, and can see either flaws in the idea, or can point me in the right direction, or who (dare I hope?) has the ability and interest in developing such a plug-in or whatever.
ps - I have just downloaded both Cakewalk and Sony Direct-X Plug-in development kits, I have visual C++.. now its down to trying to understand the darn monsterous collection of 'objects' one needs to work with, and the windows interface, and the maths - OMG.. there goes the next year of my life! ;-)