OpenEuphoria

Re: Lower WAV pitch problem

• Posted by daryl_vdb at hotmail.com Jun 03, 2003
• 437 views

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)

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