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 <[email protected]> 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?):
> [image: df0555eb-9839-457b-ac3d-0a4c9d17c662.png]
>
> 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 <[email protected]> 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
>>
>>
