One frame IFFT tone?

[Zizoo]

If I want to make a tone by just performing an IFFT on a single frame and looping the result, what would be the simplest way to do that?

[Roger Dannenberg]

First, make an array with the FFT coefficients. The length must be a
power of 2. Convert the array into a sound with SND-IFFT. A wavetable is
just a list of the form (sound nominal-pitch-of-the-sound-in-steps t).
Make the wavetable list using LIST. Finally, call OSC, passing in the
wavetable.

Doing this is probably a bad idea. Unless you take special care, the
result is likely to have a discontinuity between the last samples and
the first samples, introducing some high frequencies that aren't
explicitly included in the coefficients. Secondly, the result could
easily alias.

You might be better off looking at BUILD-HARMONIC. By calling this once
per desired harmonic and summing the results, you can create a wavetable
with specific harmonics.

-Roger

Zizoo wrote:
> If I want to make a tone by just performing an IFFT on a single frame and
> looping the result, what would be the simplest way to do that?
>  


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[Zizoo]

Wouldn't any IFFT have a specific peroid though before it just loops? Would I have to ascertain that the period falls within or close the sampling rate or something similar to prevent issues like aliasing, or could I just find the period and set the duration of the period to that value when performing the IFFT?

>.< Do you happen to know how I would find the duration of an IFFT I'm trying to implement?

Mostly I'm just goofing around and trying to do things like figure out what an IFFT of the bell-curve sounds like, and similar such nonsense. ^^;

Roger Dannenberg wrote:

Doing this is probably a bad idea. Unless you take special care, the
result is likely to have a discontinuity between the last samples and
the first samples, introducing some high frequencies that aren't
explicitly included in the coefficients. Secondly, the result could
easily alias.

[Roger Dannenberg]

Some javascript/style in this post has been disabled (why?)

FFT and IFFT compute the STFT and ISTFT -- short term FT. Since the duration is finite, it does not make sense to talk about periodicity. I think you have a conceptual model that doesn't match the actual properties of these. People *do* use FT and IFT to analyze and synthesize signals, but exactly how you go from short term estimates to long-term signals depends on the analysis/synthesis model. -Roger

Zizoo wrote:

Wouldn't any IFFT have a specific peroid though before it just loops? Would I
have to ascertain that the period falls within or close the sampling rate or
something similar to prevent issues like aliasing, or could I just find the
period and set the duration of the period to that value when performing the
IFFT?

  

.< Do you happen to know how I would find the duration of an IFFT I'm
    

trying to implement?

Mostly I'm just goofing around and trying to do things like figure out what
an IFFT of the bell-curve sounds like, and similar such nonsense. ^^;


Roger Dannenberg wrote:
  

Doing this is probably a bad idea. Unless you take special care, the 
result is likely to have a discontinuity between the last samples and 
the first samples, introducing some high frequencies that aren't 
explicitly included in the coefficients. Secondly, the result could 
easily alias.
    

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