Re: [Discuss-gnuradio] How to make an FFT block in c++

Thanks a lot. I do have a question about the 6.67 that you pass to the gr::firdes::window(gr::filter::firdes::WIN_BLACKMAN_HARRIS, SIZE, 6.67);

What is the importance of this value? Is this the default BETA? I found that when creating a BLACKMANHARRIS window in python and c++, I got two different results. 

Sincerely,

Tommy James Tracy II

Ph.D Student

High Performance Low Power Lab

University of Virginia

Phone: 913-775-2241

On Oct 18, 2013, at 5:20 PM, Alexandru Csete <oz9aec@gmail.com> wrote:

On Fri, Oct 18, 2013 at 10:05 PM, Tommy Tracy II <tjt7a@virginia.edu> wrote:

Hello,

I'm trying to make an FFT block in my hierarchical block, but I cannot seem
to find the correct syntax.
I'm trying to make it with a BLACKMAN_HARRIS windowing function, and it's
easy in a python program:

                       fft_forward = fft.fft_vfc(window_size, True,
(fft.blackmanharris(window_size)), 1)
                       fft_backward = fft.fft_vcc(window_size, False,
(fft.blackmanharris(window_size)), False, 1)


But in a c++ program it gets hairy. Does anyone have any experience with
this?

   gr::fft::fft_vfc::sptr fft_forward = gr::fft::fft_vfc::make(SIZE, true,
gr::filter::firdes::WIN_BLACKMAN_HARRIS, num_threads);
   gr::fft::fft:vcc::sptr fft_reverse = gr::fft::fft_vcc::make(SIZE, false,
gr::filter::firdes::WIN_BLACKMAN_HARRIS, false, num_threads);

I believe what I have wrong is the Windowing, which is of the form:

   const std::vector< float > & window

I have only used gr::fft::fft_complex in C++ but I think it uses the
same window type. If that's true you can create the window using:

window = gr::filter::firdes::window(gr::filter::firdes::WIN_BLACKMAN_HARRIS,
SIZE, 6.76);

Alex