OpenEuphoria

Re: Lower WAV pitch problem

• Posted by gertie at visionsix.com Jun 03, 2003
• 439 views

On 3 Jun 2003, at 0:18, David Cuny wrote:

> 
> 
> 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... 

The olde method really was to chop at a freq way below audible, but zero-
crossing of the lowest fundamental works nicely, if it is sustained. 
Otherwise, the time-space of the chopping will vary, but may not be 
noticeable. Drop every other chop. Stretch the remaining to fit the original 
space. This works without changing the pitch (other than dropping everything 
an octave), if the chops are the same length, hence the fixed chop 
frequency. Peak levels won't be changed, but percieved volume may, 
because of Fletcher-Munson, etc. As you have discovered, your mileage 
may vary.

Kat