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

[George Edwards]

Hi Ron, 

Thank you very much!

I really appreciate your help and patience.

Best Regards,

George

On Fri, Dec 17, 2021 at 12:57 AM Ron Economos <w6rz@comcast.net> wrote:

Take a look at the FFT block code.

https://github.com/gnuradio/gnuradio/blob/master/gr-fft/lib/fft_v_fftw.cc#L83-L85

Ron

On 12/16/21 7:19 PM, George Edwards wrote:

Hi Ron,

Thank you very much! I understand most of it, except for how to index properly into the values returned by the method ofdm_fft.get_outbuf() to do a forward FFT per your instruction to do the shift after the FFT.

Let me write down the code snippet based on my understanding and you can instruct me on how to get the 2 nd half of the values returned by ofdm_fft.get_outbuf():

    gr_complex* dst;

    dst = ofdm_fft.get_inbuf();
    memcpy(&dst[0], &in[0], sizeof(gr_complex) * ofdm_fft_size);     //transfer the ofdm_fft_size amount of data from input to dst

    ofdm_fft.execute();                 //compute the FFT

    memcpy(&out[ ofdm_fft_size / 2  ], ofdm_fft.get_outbuf(), sizeof(gr_complex) * ofdm_fft_size / 2);    //this I think is correct

    memcpy(&out[ 0 ], ofdm_fft.get_outbuf(XXXX), sizeof(gr_complex) * ofdm_fft_size / 2);  // Problem here, how to access ofdm_fft.get_outbuf() 

                                                                                                                                                //starting at address ofdm_fft_size/2

Thank you!

Best Regards,

George

On Thu, Dec 16, 2021 at 8:14 PM Ron Economos <w6rz@comcast.net> wrote:

Comments in-line.

Ron

On 12/16/21 12:47 PM, George Edwards wrote:

Hi Ron,

Thanks again for sending the links and annotating the lines of interest. I have looked through links and have some questions:

Q1. The lib/CMakeList.txt file I can identify easily, but I am not sure which is the top level CMakeList.txt file. There are CMakeList.txt files in directories "apps" and "grc" in my project directory and I am not sure which is considered the top level. I do not know if you can help me here (thing is, in my project the contents of these CMakefileList.txt files are small and do not mimic that in the link you sent, so I have no clue).

Let's say you have an OOT in your home directory.

~/gr-something

The top level CMakeLists.txt is in ~/gr-something.

Q2. I am reading in a fixed size vector of floats to compute the fft and output a vector of fft values (complex) the same size as the block of input, so I assumed the OOT block should be a "sync" block, right?

Yes. If you want to use vectors on your input and output pins, change the signature to (where vlength is your vector size):

    gr::io_signature::make(1, 1, sizeof(input_type) * vlength),
    gr::io_signature::make(1, 1, sizeof(output_type) * vlength)),

Then in the block .yml file, use the vlen tag.

inputs:
-   domain: stream
    dtype: complex
    vlen: ${vlength}

outputs:
-   domain: stream
    dtype: complex
    vlen: ${vlength}

Note that when you use vectors, noutput_items will be the number of vectors, not the number of items in each vector.

Q3. Assuming the input vector comes in on the variable "in" and the output vector is on the variable "out" how do I taylor the _expression_ in the link which I copied below to compute the fft and pass it to the output (plus, is volk_32fc_s32fc_multiply_32fc(...) a function in the fft computation module?):

I was using an IFFT in that example, and it's almost always required to normalize the output. So that's what the volk_32fc_s32fc_multiply_32fc() was for. Here's the code for just a simple in to out IFFT. Also, the variable names don't have to be "ofdm_fft". You can use something more descriptive.

    gr_complex* dst;

    dst = ofdm_fft.get_inbuf();
    memcpy(&dst[ofdm_fft_size / 2], &in[0], sizeof(gr_complex) * ofdm_fft_size / 2);
    memcpy(&dst[0], &in[ofdm_fft_size / 2], sizeof(gr_complex) * ofdm_fft_size / 2);

    ofdm_fft.execute();

    memcpy(&out[0], ofdm_fft.get_outbuf(), sizeof(gr_complex) * ofdm_fft_size);

Note that for a forward FFT, you have to do the shift after the FFT. Also, the example is complex in and complex out.

Thank you very much!

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