Re: How to compute FFT and IFFT in C++ OOT?

[George Edwards]

Hi Ron, 

Thank you very much! 

I am sure I will have questions later on how to use FFT with windows (Harris Blackman, Kaiser, Hamming, etc.) but that is another story and I will do a new posting on that when I reach that point.

Thank you very much!

Happy Holiday Season!

George

On Thu, Dec 23, 2021 at 6:18 PM Ron Economos <w6rz@comcast.net> wrote:

You have to copy your samples into get_inbuf() first. That's where FFTW expects the data. *dst is just a convenience variable to allow for less typing. Instead of:

memcpy(ofdm_fft.get_inbuf(), in, sizeof(gr_complex) * ofdm_fft_size);

You can use:

memcpy(dst, in, sizeof(gr_complex) * ofdm_fft_size);

Ron

On 12/23/21 10:42 AM, George Edwards wrote:

Hi Ron,

Sorry for bothering some more!

In Debugging it appears that dst is not pointing to the input data (input_items[0]), which we pointed to with pointer variable in at the start of the work(...) method

 (const gr_complex *in = (const gr_complex *) input_items[0];

Here is a snippet of my Debugging code and the values I see in ge_in = dst[i] while single stepping through the code are all zeros.

 

gr_complex * dst;                // copy from your code:

dst = ofdm_fft.get_inbuf();   //copy from your code

 //****** my debug code starts **********

gr_complex  ge_in;

for(int i = 0; i < 8; i++)          // I am inputting 8 samples

     {

           ge_in = dst[i];            // extracting the values in dst shows zero's

      }

// ******my debug code ends *********

It appears that setting dst = ofdm_fft.get_inbuf() is not pointing to the actual input samples values.

I did not see anywhere in your code where there is an explicit binding between in and get_inbuf().

I will appreciate it if you can help me here!

Thank you!

George

On Wed, Dec 15, 2021 at 12:32 AM Ron Economos <w6rz@comcast.net> wrote:

FFT support is built in to GNU Radio with FFTW. Here's how it's done. First, define it in your foo_impl.h file. The options are fft_complex_fwd, fft_complex_rev, fft_real_fwd and fft_real_rev.

https://github.com/drmpeg/gr-paint/blob/master/lib/paint_bc_impl.h#L25

https://github.com/drmpeg/gr-paint/blob/master/lib/paint_bc_impl.h#L41

Then initialize it in your foo_impl.cc constructor.

https://github.com/drmpeg/gr-paint/blob/master/lib/paint_bc_impl.cc#L47

Then execute it.

https://github.com/drmpeg/gr-paint/blob/master/lib/paint_bc_impl.cc#L175-L179

You'll need to add the component in the top level CMakeLists.txt.

https://github.com/drmpeg/gr-paint/blob/master/CMakeLists.txt#L78

And link with it in lib/CMakeLists.txt

https://github.com/drmpeg/gr-paint/blob/master/lib/CMakeLists.txt#L25

If you need a window, you can look at the block implementation file for details.

https://github.com/gnuradio/gnuradio/blob/master/gr-fft/lib/fft_v_fftw.cc

Ron

On 12/14/21 7:53 PM, George Edwards wrote:

 Dear GNURadio Community:

I am writing a C++ OOT block where the signal processing requires the computation of both fft and ifft. Is there any Gnuradio C++ functions for the fft and ifft? If not, is there any way to wrap in Python's libraries with these methods into C++ OOT?

Thank you!

Regards,

George