[ot] Re: [music-dsp] Wondering if this has been tried yet . . .

douglas irving repetto douglas at music.columbia.edu
Mon Jun 10 19:46:12 EDT 2002


nice post, chris.

isn't the inverse problem also the basis for a lot of crypto? vis-a-vis 
a "one way function":

http://www.c3.lanl.gov/mega-math/gloss/compute/oneway.html


douglas


Christopher Weare wrote:

> You do not have enough information from the data to actually separate
> the instruments by looking at the spectrum.  What you are attempting to
> perform is a member of the nasty inverse problem class.  A very simple
> example:
> 
> Given 42, determine uniquely the two numbers added together to produce
> that result.  
> 
> While it is simple enough to calculate 40 + 2 = 42, the inverse problem,
> 42 = ? + ?, can not be solved uniquely with the information given.
> 
> To solve the audio inversion problem you must have an a priori model for
> the spectrum each source will produce.  To make this remotely tractable
> you need to have some clue as to what instruments are even present in
> the first place.
> 
> The brain is able to localize so well because it does have internal
> models of expected sounds it uses to solve the inverse problem.  The
> brain uses a similar tact to solve the equally vexing inverse optics
> problem.
> 
> But don't feel discouraged.  Inverse problems are probably some of the
> most difficult challenges left in engineering so it will probably take
> some extra effort to make some progress.
> 
> 
> Have fun,
> -chris
> 
> 
> 
> 
> 
> -----Original Message-----
> From: Keith Handy [mailto:keith at indierecords.com] 
> Sent: Monday, June 10, 2002 2:45 PM
> To: music-dsp at shoko.calarts.edu
> Subject: [music-dsp] Wondering if this has been tried yet . . .
> 
> Just a brainstorm I had the other night -- if you continuously analyze
> the 
> full audio spectrum of a stereo signal, couldn't you break the signal
> down 
> into all its frequency components and figure out where they're all
> coming 
> from in the stereo field?  And in doing so, couldn't you conceiveably 
> identify and isolate the "center material" (or material panned to any
> given 
> location) in a stereo mix, and filter out anything that is known to be 
> panned elsewhere (in the same way that noise can be filtered out using a
> 
> noise profile)?
> 
> (Or maybe one would use resynthesis instead of filtering, or even some 
> "intelligent" combination of both.)
> 
> I'm thinking a kind of "inverted Karaoke" effect where instead of just 
> dumbly cancelling out one location, you actually could isolate a
> position 
> in the field -- and if this were possible, it follows that you could
> break 
> a stereo field up into *several* independent tracks, though I realize
> there 
> are probably a lot of good reasons why this wouldn't work spectacularly
> well.
> 
> Maybe you would have to analyze phase relationships between the channel,
> in 
> case there's energy in the same band coming from two instruments panned 
> opposite each other.  Or maybe it would be a matter of analyzing (L-R)
> and 
> using that profile to filter (L+R).
> 
> I'm thinking this is one of those things that would be heavily flawed,
> in 
> the same way that time stretching is flawed -- but I'd be interested to 
> hear what would come out regardless.  I'm also sure I'm not the first to
> 
> think of it, as I never am, but I figure some of you on this list would 
> know more about the logistics of practical implication.  It's beyond my 
> skill level to actually attempt this, but it's been a fun thing to think
> about.
> 
> -Keith
> 
> 
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> 
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> 
> 



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