OpenEuphoria

Re: Lower WAV pitch problem

• Posted by gertie at visionsix.com Jun 04, 2003
• 446 views

On 4 Jun 2003, at 9:16, daryl_vdb at hotmail.com wrote:

 >
 >
 >
 > What David Cuny wrote was very interesting, but I don't know anything about
 > fourier analysis.  I would love to find out about how to do fourier analysis
 > because it would open up a lot of new possibilities (this "changing duration
 > without changing pitch" would be just one of them.)
 >
> Who knows about fourier analysis, and where can I find more information on
 > it?
 > (OTOH, a google search revealed heaps of information, thanks anyway)

I know enough to say you will want to do *fast* fourier analysis, not the long
version. I dunno about now, but way back when, mere mortals could not
buy a puter that would do a long version in real time. See:
http://www.google.com/search?hl=en&lr=&ie=ISO-8859-1&q=FFT+analysis
there's some pdf's on how to do it.

Kat

 > thanks,
 > Daryl Van Den Brink
 >
 >
 > >Daryl Van Den Brink wrote:
 > >
 > > > I don't know what you could be doing to double the duration
 > > > and make it's pitch stay the same, but I'd love to find out.
 > >
> >This turns out to be solvable, but non-trivial. Sound basically decomposes
 > >into
 > >two types: sound carrying harmonic content, and noise.
 > >
 > >First, you chop up the sound into sufficiently small chunks. If you make
 > >the
 > >chunks to small, you don't capture enough harmonic information. If you make
 > >them too large, you end up getting "pre echo" because you're including
 > >information that doesn't belong in that timeframe.
 > >
 > >To derive the harmonic content, you do a fourier analysis on each chunk.
 > >
 > >To derive noise content, once you decide that a chunk contains noise, you do
 > >bark banding on it. Noise doesn't have to be pitch shifted in the
 > >reconstruction.
 > >
 > >You also need to look at the volume, so you can build a volume envelope
 > >when
 > >you rebuild the sound.
 > >
 > >Now you've got enough information to reconstruct the sound. Take the chunks
> >that have harmonic content, and rebuild their harmonics to the new pitch -
 > >just
 > >reverse the fourier process. The noise chunks are rebuilt out of the bark
> >bands. Join all the chunks together and recreate the volume envelope to
 > >match
 > >the original sound.
 > >
 > >Easy, huh?
 > >
 > >Sorry, I don't have the references available. That's the point I threw up my
 > >hands and decided to try something easier, like herding cats or juggling
 > >knives...
 > >
 > >-- David Cuny
 >
 >
 >
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 >
 >
 >
 >