pgw wrote:

> I used FFTW only one time, so I'm not familiar with it.
> Try use fftw_plan_dft_r2c_1d and then compute magnitude as:
> sqrt(real_of_outpute^2 + imaginary_of_outpute^2). That will have to work.

I tried that, but i think there's something about the complex order that 
i wasn't getting.
I have to admint that with r2c_1d also real numbers were different from 
matlab..


--

Clyde

Scott Seidman :

>> Don't you need to deal with the "half complex" format that fftw is
>> asked to produce before you deal with the magnitude?

Yes, you are right.
I missed that in half complex format complex is in reversed order, 
starting from the end -2 (!)
Now it seem to work!
I'll do some more analysis with windows.

Thanks _A LOT_ you all


--

Clyde

Clyde wrote:

> pgw wrote:
> 
>> How it look without windowing?
> 
> They are different, and the problem still remains.
> 
>> You can try to generate one harmonic signal and do FFTW (without windowing) 
>> on it and then show us the result.
> 
> Here is the result:
> http://i45.photobucket.com/albums/f77/yesitookmypills/FFT.png
> (blue is FFTW, red is Matlab FFT)
> 
> I used fftw_plan_r2r_1d(dim, in[0], out[0], FFTW_R2HC, FFTW_ESTIMATE)
> and i compute the magnitude in this way:
> magnitude[i] = sqrt(out[i]*out[i]);

I used FFTW only one time, so I'm not familiar with it.
Try use fftw_plan_dft_r2c_1d and then compute magnitude as:
sqrt(real_of_outpute^2 + imaginary_of_outpute^2). That will have to work.

-- 
pgw

Clyde <iloveuppercaseEREKOSE@EMAIL.IT> writes:

> pgw wrote:
>
>> How it look without windowing?
>
> They are different, and the problem still remains.
>
>> You can try to generate one harmonic signal and do FFTW (without
>> windowing) on it and then show us the result.
>
> Here is the result:
> http://i45.photobucket.com/albums/f77/yesitookmypills/FFT.png
> (blue is FFTW, red is Matlab FFT)
>
> I used fftw_plan_r2r_1d(dim, in[0], out[0], FFTW_R2HC, FFTW_ESTIMATE)
> and i compute the magnitude in this way:
> magnitude[i] = sqrt(out[i]*out[i]);
>
> Here's the data with a small script in matlab:
>
> http://rapidshare.com/files/25643904/FFTWvsFFTMatlab.zip.html

One difference I can see is that you use abs() in Matlab but
your own magnitude function in your other code (is it C?).

Are you using doubles in the C code? Can we see your C code?
-- 
%  Randy Yates                  % "Watching all the days go by...    
%% Fuquay-Varina, NC            %  Who are you and who am I?"
%%% 919-577-9882                % 'Mission (A World Record)', 
%%%% <yates@ieee.org>           % *A New World Record*, ELO
http://home.earthlink.net/~yatescr

Scott Seidman <namdiesttocs@mindspring.com> wrote in
news:Xns99109030D964Dscottseidmanmindspri@130.133.1.4: 

> Clyde <iloveuppercaseEREKOSE@EMAIL.IT> wrote in news:461e738c$0$4786
> $4fafbaef@reader4.news.tin.it:
> 
>> fftw_plan_r2r_1d
> 
> 
> Don't you need to deal with the "half complex" format that fftw is
> asked to produce before you deal with the magnitude?
> 


See http://www.fftw.org/fftw3_doc/Real_002dto_002dReal-Transform-
Kinds.html#Real_002dto_002dReal-Transform-Kinds


-- 
Scott
Reverse name to reply

Clyde <iloveuppercaseEREKOSE@EMAIL.IT> wrote in news:461e738c$0$4786
$4fafbaef@reader4.news.tin.it:

> fftw_plan_r2r_1d


Don't you need to deal with the "half complex" format that fftw is asked to 
produce before you deal with the magnitude?

-- 
Scott
Reverse name to reply

Andor wrote:
> Matlab also uses FFTW, so you must be doing something wrong.

Yeah, i agree.
But what i'm trying to underlying is not "who's the best".
I just want to understand what differs, and where i do something wrong.

--

Clyde

Scott Seidman wrote:

> Ditto.  I haven't used FFTW directly, so I'm grasping, but I do recall some 
> people posting about having problems getting the argument order right, or 
> possibly the shape of the inputs, in FFTW.  Double and triple check your 
> function call.  Also, Matlab scales the FFT a little funny, so you might be 
> off by a constant 1/N or something, but both answers should have the same 
> shape.


I'm not a FFT guru, and FFTW has so many option that it can be trivial..
I just used my complex analysis book, ant a little of intuition.
With FFTW you can compute 11 kind of real to real fft, but i have big 
troubles understanding the differences between them.

The scale should be the same. The same "feature" is "documented" in fftw 
  (at least they warn you).

--

Clyde

pgw wrote:

> How it look without windowing?

They are different, and the problem still remains.

> You can try to generate one harmonic signal and do FFTW (without windowing) 
> on it and then show us the result.

Here is the result:
http://i45.photobucket.com/albums/f77/yesitookmypills/FFT.png
(blue is FFTW, red is Matlab FFT)

I used fftw_plan_r2r_1d(dim, in[0], out[0], FFTW_R2HC, FFTW_ESTIMATE)
and i compute the magnitude in this way:
magnitude[i] = sqrt(out[i]*out[i]);

Here's the data with a small script in matlab:

http://rapidshare.com/files/25643904/FFTWvsFFTMatlab.zip.html

--

Clyde

Clyde wrote:

> Hi,
> i'm using FFTW to computer determine the SNR of a recorded sinusoidal 
> signal. (1khz sinus for 5seconds @ 48khz)

> If i try to do the same with matlab or any other software (like wavelab 
> or audacity) i can see that with FFTW i obtain higher magnitudinae at 
> low frequencies ( < 200hz ~).

Can you show us the shape of the outpute data? Maybe it'll help.

> I tried 6 different windowing  methods, but it didn't worked.

How it look without windowing?

> I tried to compute a
> fftw_plan_dft_1d(dim, in, out, FFTW_FORWARD, FFTW_ESTIMATE) but it 
> didn't worked as well.
> Is there something i'm missing?

You can try to generate one harmonic signal and do FFTW (without windowing) 
on it and then show us the result.

-- 
pgw