FFT

[Sean Costello]

Hi Paul, Dan:

I don't have any formulas handy, but there are at least two ways to
"morph" between two waveforms using FFTs. To start with, you will want
to perform an FFT analysis of both waveforms. They should be at the same
frequency, and you should use the same parameters (window size, number
of frames analyzed, etc). Ideally, you can have a "pitch-synchronous"
analysis, where the "hop size" between frames is synced to the period of
the waveform.

Once this is done, you can do one of the following:

- Take the phase values for one of the waveforms (i.e. the phase value
in each FFT bin) and interpolate the magnitude values for each bin to
the desired point in between.

- Do the same, but interpolate the phase values, too.

You may have to do some conversion (i.e. convert the FFT output to polar
form). I would highly recommend using an existing utility, like the
phase vocoder routines in Csound, to perform such actions. Richard
Karpen has written some pv routines for Csound that will perform what
you are looking for.  

Sean Costello


Dan Gendreau wrote:
> 
> I am not sure I fully understand your question.
> 
> I am guessing you have two sample roms and you want to "morph" (vague term,
> i knwo) between the two.
> 
> I dont know if it is of any use, but I would just do a linear interpolation
> between them.
> -Dan G.
> 
> > -----Original Message-----
> > From: owner-synth-diy at mailhost.bpa.nl
> > [mailto:owner-synth-diy at mailhost.bpa.nl]On Behalf Of Paul Maddox
> > Sent: Thursday, October 21, 1999 10:24 AM
> > To: synthdiy
> > Subject: FFT
> >
> >
> > dear all,
> >
> >   I know we've touched on this before but... does anyone have
> > or know of, a formula for doing a FFT with two waveforms?
> >
> >   It seems the waveterm/ppg/wave breakdown the eprom stored
> > wave and perform a FFT transform of some kind to get the
> > waves inbetween the two ROM waves...
> >
> >    Paul
> >
> >
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