Thread Subject: fft in MatLab

Hi,

I am trying to reconcile the differences between Matlab and fftw. And I would like to know how Matlab *plans* it's signal, i.e., what does it use in "fftw_plan"?


if data is an N-dimensional 1D array that holds amplitudes that make up a wave;

in Matlab, I am using -
Y=fft(data(:,2),NFFT);
where 2^(NFFT) = N

and in fftw, I am using:
  static fftw_plan p = fftw_plan_dft_1d(len, data,out,FTW_REDFT01,FFTW_MEASURE);
  fftw_execute(p);

My numbers are off by a non-constant factor. The shapes are similar to each other, but not entirely offset by a constant factor.

Also, I get N/2 results from my Matlab fft output; but get N results from my fftw output.

I could post the two different histograms if people want to take a look - but I am not sure if I can upload graphics here?

Thanks!

- Sujeewa

On Nov 10, 8:09 pm, "humpty Kumaratunga" <suje...@lps.umontreal.ca>
wrote:
> Hi,
>
> I am trying to reconcile the differences between Matlab and fftw. And I would like to know how Matlab *plans* it's signal, i.e., what does it use in "fftw_plan"?
>
> if data is an N-dimensional 1D array that holds amplitudes that make up a wave;
>
> in Matlab, I am using -
> Y=fft(data(:,2),NFFT);
> where 2^(NFFT) = N

That last statement makes no sense.

Hope this helps.

Greg

"humpty Kumaratunga" <sujeewa@lps.umontreal.ca> wrote in message
news:hdd2rf$4ts$1@fred.mathworks.com...
> Hi,
>
> I am trying to reconcile the differences between Matlab and fftw. And I
> would like to know how Matlab *plans* it's signal, i.e., what does it use
> in "fftw_plan"?

Take a look at the FFTW function.

http://www.mathworks.com/access/helpdesk/help/techdoc/ref/fftw.html

I'm not certain if there will be a 1-to-1 correlation between what you refer
to as "fftw_plan" and the FFTW wisdoms, but you may still find it useful.

--
Steve Lord
slord@mathworks.com
comp.soft-sys.matlab (CSSM) FAQ: http://matlabwiki.mathworks.com/MATLAB_FAQ

On Nov 10, 8:09 pm, "humpty Kumaratunga" <suje...@lps.umontreal.ca>
wrote:
> if data is an N-dimensional 1D array that holds amplitudes that make up a wave;
>
> in Matlab, I am using -
> Y=fft(data(:,2),NFFT);
> where 2^(NFFT) = N
>
> and in fftw, I am using:
>   static fftw_plan  p = fftw_plan_dft_1d(len, data,out,FFTW_REDFT01,FFTW_MEASURE);
>   fftw_execute(p);

First, by passing NFFT in Matlab you are zero padding the array. In
FFTW, it doesn't look like you are zero-padding. Don't zero-pad in
Matlab if you want to get the same results.

Second, it makes no sense whatsoever to pass FFTW_REDFT01 to
fftw_plan_dft_1d. You should be passing FFTW_FORWARD to match the
Matlab FFT function. Moreover, FFTW_REDFT01 is only valid as an
argument to fftw_plan_r2r, and it corresponds to a type of discrete
cosine transform, which is an entirely different operation than the
discrete Fourier transform the Matlab fft function computes.

Regards,
Steven G. Johnson.