[PD] Colored (fractal, 1/f^a) noise generator for PD

alberto.zin at poste.it alberto.zin at poste.it
Tue Aug 22 09:36:14 CEST 2006


Another interesting source of info on this subject 
is a paragraph in the book "Elements of Computer Music" 
by F.R. Moore, a classic in Computer Music. 
There the generation method for 1/f^alpha noise is 
"spectrum shaping"(1), which at the end is not very
different from the Kasdin method implemented in cnoise patch

(1): noise -> fft -> "shape" the spectrum by multiplication 
on the desired function (1/x, 1/x^2 etc.) -> ifft ->
normalize in audio range (+1, -1)

Alberto




On Mon, 21 Aug 2006, alberto.zin at poste.it wrote:

> if I'm not wrong, as long as it is white it is flat in
spectral terms 
> but the distribution of amplitudes (probability density
function, pdf) 
> of white noise can be gaussian, cauchy, uniform, etc.
Therefore I'm not 
> sure that the spectrum is proportional to the pdf..

Sorry, my mistake. I was completely confused. The spectrum
that we're 
talking about is actually the expectation of a random
spectrum, which is 
the expectation of amplitude of each frequency separately,
which doesn't 
say much about which pdf each frequency has.

Btw, the spectrum of white noise is not flat, it's just that 
the expectation of the spectrum is flat.

What does "uniform" mean in a situation where things are
understood in 
terms of amplitude and phase? are the amplitudes selected in
polar 
coordinates with the correct jacobian? if instead cos and
sin components 
are selected uniformly, then the maximum amplitude will only
be possible 
for four phases, which are the four corners of a square shape.

  _ _ __ ___ _____ ________ _____________
_____________________ ...
| Mathieu Bouchard - tél:+1.514.383.3801 -
http://artengine.ca/matju
| Freelance Digital Arts Engineer, Montréal QC Canada





More information about the Pd-list mailing list