[LAD] Paul's Extreme Sound Stretch

Robin Gareus robin at gareus.org
Thu Sep 30 11:53:44 UTC 2010

On 09/30/10 13:15, Dave Phillips wrote:
> Robin Gareus wrote:
>> In layman terms:
>> There's a smart French guy by the name of Joseph F. sitting inside it:
>> If you play him some audio: He thinks: "Hey, this is actually just a few
>> simple sine-waves added together (superpositioned)", he quickly
>> calculates their frequencies and amplitudes and asks "Now, you want to
>> change the duration?" easy: "I'll generate some new sine-waves with
>> these frequencies and amplitudes, how long did you say you want?"
>> (The smart thing about this French guy is that he actually speaks fluent
>> English - Sorry I could not resist :)
> I recall a brief exchange on a DSP list that went something like this:
> Q: Can anyone explain the FFT in simple terms ?
> A. No.


basically, Fourier proved that any signal can be represented a sum of

(well, that's not entirely true: it needs to be a periodic signal, but
the period length can approach infinity...)

FFT is "just" the implementation of that theorem (or Principle?!)

> I have read and understood some articles and descriptions of the FFT.
> Mark Dolson's original article on the phase vocoder (in the Computer
> Music Journal V10,#4, Winter 1986), is still a good introduction for
> computer-based musicians, though it may be a bit too technical by
> contemporary standards.
> Louigi, take a look at some descriptions of what an FFT does. It does
> use "windows" but you'll have to do some homework to find out what's
> meant by the term in this context.
> http://en.wikipedia.org/wiki/FFT
> Beware, that page is a maths page, it's not directly concerned with the
> use of the FFT in music/sound applications.

It may help to make some connection with the equations on
http://en.wikipedia.org/wiki/Fourier_transform if you know that

  sin (x) = ImaginaryPartOf ( e^(i x) )
  cos (x) = RealPartOf ( e^(i x) )
  where i = sqrt(-1);

> And btw, I agree wrt Paul's Extreme Stretch, it's a great tool.

It is indeed. Kudos to Paul.

FWIW, http://arss.sourceforge.net/ works similarly. It provides
sound-to-image and image-to-sound functionality using FFT.

You can also use it to do time-stretching but it's actually more fun to
toy around with it: http://arss.sourceforge.net/examples.shtml

> Best,
> dp
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