topic Re: 1D - FFT in Intel® oneAPI Math Kernel Library

1D - FFT

sicb0161 — Thu, 19 Feb 2009 18:47:49 GMT

Hi,

I am a newbie using the FFT, I have some question about the input and output data format using the FFT Functions.

[cpp]  DFTI_DESCRIPTOR *my_desc_handle;
  long status;
  status = DftiCreateDescriptor( &my_desc_handle, DFTI_SINGLE, DFTI_REAL, 1, N);
  status = DftiCommitDescriptor( my_desc_handle);
  status = DftiComputeForward( my_desc_handle, y);
  status = DftiFreeDescriptor( &my_desc_handle );[/cpp]

Well, my input data y is real. After the transformation y contains not only real but also imaginary data. Is there a workaround ?

thx

Re: 1D - FFT

Dmitry_B_Intel — Fri, 20 Feb 2009 04:24:50 GMT

Hi,

It depends on what you need from the spectrum y. If you want to discard the phase and use the power, you should use the magnitude (sqrt(re^2+im^2)) of the y's complex numbers.
Alternatively, if you needa real-to-real transform, look into trigonometric transforms section of MKL Refernce Manual.

Dima

Re: 1D - FFT

Dmitry_B_Intel — Fri, 20 Feb 2009 04:25:07 GMT

Re: 1D - FFT

ArturGuzik — Fri, 20 Feb 2009 04:27:15 GMT

Quoting - sicb0161

Hi,

I am a newbie using the FFT, I have some question about the input and output data format using the FFT Functions.

[cpp]  DFTI_DESCRIPTOR *my_desc_handle;
  long status;
  status = DftiCreateDescriptor( &my_desc_handle, DFTI_SINGLE, DFTI_REAL, 1, N);
  status = DftiCommitDescriptor( my_desc_handle);
  status = DftiComputeForward( my_desc_handle, y);
  status = DftiFreeDescriptor( &my_desc_handle );[/cpp]

Well, my input data y is real. After the transformation y contains not only real but also imaginary data. Is there a workaround ?

thx

Hi,

if I recall correctly you need to use DFTISetValue and select (from many) options you need, say, something along the lines:

[cpp]! set packed format for output complex conjugate-symmetric data
status%flag=DFTISetValue(desc_handle, ....., .....)[/cpp]

where ..... go for named constants. (for example DFTI_REAL_REAL)

Take a look at Named Constants and storage schemes. I believe you'll find many good examples on disk.

A.

Re: 1D - FFT

sicb0161 — Fri, 20 Feb 2009 11:34:37 GMT

Quoting - Dmitry Baksheev (Intel)

Thx Dima ,

Hi, yes I could do so, but I only want to have the real part of y. After computing the FFT, y contains imaginary and real data. To be more precise :

[cpp]y = (real part) if i = even,  
       (imag part) if i = odd.[/cpp]

But y is only of length n , thus only containing half the data.

Thx

Re: 1D - FFT

Dmitry_B_Intel — Fri, 20 Feb 2009 15:12:43 GMT

Right, for size=N real-to complex transform the y contains only N/2+1 complex values. The remaining values can be restored using conjugate-even symmetry of y: y[N-n]=conj(y).
Dima