Re: Lower WAV pitch problem
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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