[sdiy] Harmonic bandwidth

[Andre Majorel]

On 2008-01-06 11:47 +0000, Tom Wiltshire wrote:

> What I was most interested in was the use of IFFT to get from a
> non-perfect harmonic spectrum with spread harmonics (and even
> noisy smearing at the high end) to a loopable sample that can be
> used for a wavetable.

It's going to be a long loop.

I wouldn't be surprised if the amount of spread in plucked strings
instruments evolved over time (highest just after the string is
plucked).

> This spread increases as the frequency goes up. This could be
> the difference Karl is talking about between 'harmonics' and
> 'overtones'.

As far as I know, the word is "partial". Harmonics are partials
whose frequency happens to be an integer multiple of the
fundamental.

> At first, I thought this would be the same as simply using many
> detuned oscillators, since if an oscillator is detuned by (say)
> 1Hz, then its 2nd harmonic is detuned by 2Hz, and the 3rd by
> 3Hz, etc, giving the frequency spread effect that we're after.
> But this isn't quite right. Doing it that way is going to give
> you as many sinewaves at the fundamental (and as much smearing,
> although not as detuned) as at some higher harmonic. The IFFT
> technique will give you clearer lower harmonics (with less
> sines) and more smearing at higher frequencies (with many
> sines). This _does_ mimic what you see if you run an FFT on an
> acoustic instrument.

In the analogue domain, you could use a single oscillator into a
frequency shifter. Negative shift = spread harmonics, positive
shift = narrowed harmonics. It might or might not be the same kind
of spread as that found in physical instruments.

> I'm interested to know if anyone has tried/knows of an
> instrument that has tried this technique of using IFFT to
> produce wavetable samples. Is this chap alone?

You could ask on music-dsp. I'm sure it's common place with Csound
and tools like that.

-- 
André Majorel <URL:http://www.teaser.fr/~amajorel/>
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