Re: fftw and derivative. once again

On Sunday, January 9, 2011 6:52:09 PM UTC-5, Tim Wescott wrote:

  ...

> You're not using any frequency limiting -- are you sure you're not just 
> amplifying the heck out of your noise?
> 
> In Scilab:
> 
> -->N = 4096; n = 0:N-1;
> 
> -->nn = pmodulo(n + N/2, N) - N/2;
> 
> -->x = exp(-nn.^2 / 100000); xd = ifft(fft(x) .* (-nn .^ 2));
> 
> -->clf; plot2d(n, [x' xd'])
> 
> gives me what looks like a very clean double derivative of the input 
> Gaussian.

I suppose the double differentiation simply negates the signal, avoiding the need for phase shifts.

Jerry