[PD] Spectrum graphing amplitude problem

Jason Plumb jason at noisybox.net
Mon Oct 22 01:37:40 CEST 2007

Ed Kelly wrote:
> It is really just the pixel-resolution of the graph. The graph size is 
> 252, but the FFT size is 4096 ( giving 2049 points within the Nyquist 
> frequency ).

Hmmm....I think I follow...but if that reason holds, if I expand the 
graph to 256 and use a block size of 512, it should be pixel perfect eh? 
  I tried that earlier today, but saw the same behavior...

> So you will see magnitude peaks between the pixels as lower amplitude 
> because the screen doesn't display pixels between pixels.
> I hope this makes sense,

That *does* make sense.  It's for display reasons that I ultimately want 
to build a little scaling control into the abstraction...but I gotta get 
the basics going first.

Any other ideas?

I'm a bit new to FFT in the pd context, but I think I grok Nyquist -- 
Sampling at S can, at best, yield the S/2 frequency (where S is the 
sampling rate).  But how does this relate to block size in the DFT? 
Your original statement sounds like the max frequency out of the fft is 
(block size)/2...but that can't be right, eh?



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