# A crazy problem with GNURadio fft

**URL:** <https://www.ruby-forum.com/t/a-crazy-problem-with-gnuradio-fft/231722>\
**Category:** GNU Radio\
**Created:** [August 26, 2014, 2:49pm UTC](https://www.ruby-forum.com/t/a-crazy-problem-with-gnuradio-fft/231722 "2014-08-26T14:49:59Z")\
**Posts on this page:** 4\
**Page:** 1

<div class="post-metadata">

**Author:** ![addis\_a](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/a/c4cdca/32.png) [@addis\_a](https://www.ruby-forum.com/u/addis_a)\
**Post date:** [August 26, 2014, 2:49pm UTC](https://www.ruby-forum.com/t/a-crazy-problem-with-gnuradio-fft/231722/1 "2014-08-26T14:49:59Z")

</div>

Hi all,

I was wonder why my codes in Matlab aren’t compatible with GNURadio  
ones!  
After all, I found the fft output in GNURadio is different to Matlab!!

Here is my simple code for fft testing with the real inputs of 0 to 63:

#include

#include \<gnuradio/fft/fft.h\>

using namespace std;

using namespace gr;

int main()

{

```
int fft_size = 64;

gr::fft::fft_complex *ifft = new gr::fft::fft_complex (fft_size, 

```

true, 1);

```
// making complex data

gr_complex in[fft_size];

for (int i=0; i<fft_size; i++)

{

    in[i] = i;

}

memcpy(ifft->get_inbuf(), in, fft_size);

ifft->execute();

cout << "output" << endl;

for (int i=0; i<fft_size; i++)

{

    cout << ifft->get_outbuf()[i] << endl;

}

cout << "THE END!" << endl;

return 0;

```

}

the output in GNURadio:

(28,0)

(24.3326,-13.0209)

(14.591,-22.0217)

(2.07356,-24.4501)

(-9.13707,-20.1094)

(-15.692,-11.1259)

(-16.1531,-1.03318)

(-11.3785,6.63407)

(-4,9.65685)

(2.75122,7.8416)

(6.35844,2.87496)

(5.93264,-2.55936)

(2.38009,-5.98642)

(-2.18793,-6.14396)

(-5.4952,-3.37881)

(-6.11148,0.684235)

(-4,4)

(-0.398263,5.06997)

(2.86434,3.58162)

(4.26461,0.45671)

(3.27677,-2.67271)

(0.553035,-4.28344)

(-2.45947,-3.67857)

(-4.25578,-1.27436)

(-4,1.65685)

(-1.899,3.64105)

(0.940382,3.727)

(3.08336,1.93207)

(3.48022,-0.795649)

(1.9727,-3.06732)

(-0.646391,-3.74757)

(-3.04078,-2.51227)

(-4,0)

(-3.04078,2.51227)

(-0.646391,3.74757)

(1.9727,3.06732)

(3.48022,0.795649)

(3.08336,-1.93207)

(0.940382,-3.727)

(-1.899,-3.64105)

(-4,-1.65685)

(-4.25578,1.27436)

(-2.45947,3.67857)

(0.553035,4.28344)

(3.27677,2.67271)

(4.26461,-0.45671)

(2.86434,-3.58162)

(-0.398263,-5.06997)

(-4,-4)

(-6.11148,-0.684235)

(-5.4952,3.37881)

(-2.18793,6.14396)

(2.38009,5.98642)

(5.93264,2.55936)

(6.35844,-2.87496)

(2.75122,-7.8416)

(-4,-9.65685)

(-11.3785,-6.63407)

(-16.1531,1.03318)

(-15.692,11.1259)

(-9.13707,20.1094)

(2.07356,24.4501)

(14.591,22.0217)

(24.3326,13.0209)

the output of the Matlab:  
1.0e+03 \*

2.0160  
-0.0320 + 0.6514i  
-0.0320 + 0.3249i  
-0.0320 + 0.2157i  
-0.0320 + 0.1609i  
-0.0320 + 0.1278i  
-0.0320 + 0.1055i  
-0.0320 + 0.0894i  
-0.0320 + 0.0773i  
-0.0320 + 0.0677i  
-0.0320 + 0.0599i  
-0.0320 + 0.0534i  
-0.0320 + 0.0479i  
-0.0320 + 0.0431i  
-0.0320 + 0.0390i  
-0.0320 + 0.0353i  
-0.0320 + 0.0320i  
-0.0320 + 0.0290i  
-0.0320 + 0.0263i  
-0.0320 + 0.0237i  
-0.0320 + 0.0214i  
-0.0320 + 0.0192i  
-0.0320 + 0.0171i  
-0.0320 + 0.0151i  
-0.0320 + 0.0133i  
-0.0320 + 0.0114i  
-0.0320 + 0.0097i  
-0.0320 + 0.0080i  
-0.0320 + 0.0064i  
-0.0320 + 0.0047i  
-0.0320 + 0.0032i  
-0.0320 + 0.0016i  
-0.0320  
-0.0320 - 0.0016i  
-0.0320 - 0.0032i  
-0.0320 - 0.0047i  
-0.0320 - 0.0064i  
-0.0320 - 0.0080i  
-0.0320 - 0.0097i  
-0.0320 - 0.0114i  
-0.0320 - 0.0133i  
-0.0320 - 0.0151i  
-0.0320 - 0.0171i  
-0.0320 - 0.0192i  
-0.0320 - 0.0214i  
-0.0320 - 0.0237i  
-0.0320 - 0.0263i  
-0.0320 - 0.0290i  
-0.0320 - 0.0320i  
-0.0320 - 0.0353i  
-0.0320 - 0.0390i  
-0.0320 - 0.0431i  
-0.0320 - 0.0479i  
-0.0320 - 0.0534i  
-0.0320 - 0.0599i  
-0.0320 - 0.0677i  
-0.0320 - 0.0773i  
-0.0320 - 0.0894i  
-0.0320 - 0.1055i  
-0.0320 - 0.1278i  
-0.0320 - 0.1609i  
-0.0320 - 0.2157i  
-0.0320 - 0.3249i  
-0.0320 - 0.6514i

Please help me!! I’m totally confused.

Best

---

<div class="post-metadata">

**Author:** ![mostafa\_a](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/5e9695/32.png) [@mostafa\_a](https://www.ruby-forum.com/u/mostafa_a)\
**Post date:** [August 26, 2014, 3:37pm UTC](https://www.ruby-forum.com/t/a-crazy-problem-with-gnuradio-fft/231722/2 "2014-08-26T15:37:01Z")

</div>

> ```
> memcpy(ifft->get_inbuf(), in, fft_size);
> 
> ```

fft\_size \* sizeof(gr\_complex)

---

<div class="post-metadata">

**Author:** ![mostafa\_a](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/5e9695/32.png) [@mostafa\_a](https://www.ruby-forum.com/u/mostafa_a)\
**Post date:** [August 26, 2014, 4:03pm UTC](https://www.ruby-forum.com/t/a-crazy-problem-with-gnuradio-fft/231722/3 "2014-08-26T16:03:19Z")

</div>

Thanks Sylvain,  
not finding that drove me crazy (more crazy than usual); the zeroth  
element must be sum(range(fftlen))=(63+0)_(64/2) = 63_32 = 2048 - 32 =  
2016, like in the matlab result, yet I did not find where the data  
stopped flowing… now,  
sum(range(fftlen/sizeof(gr\_complex)))=sum(range(8))=(7+0)\*8/2=28  
explains that 🙂

Also, Mostafa, you call your transform ifft, yet you set forward=true,  
which might be a bit confusing in the future.  
For the IFFT, Matlab does something different than the pure FFTW that  
GNU Radio uses: it divides by fft\_size, compare [1] to [2].

Greetings,  
Marcus

[1][http://www.mathworks.de/de/help/matlab/ref/fft.html](http://www.mathworks.de/de/help/matlab/ref/fft.html)  
[2][The 1d Discrete Fourier Transform (DFT) (FFTW 3.3.10)](http://www.fftw.org/doc/The-1d-Discrete-Fourier-Transform-_0028DFT_0029.html#The-1d-Discrete-Fourier-Transform-_0028DFT_0029)

---

<div class="post-metadata">

**Author:** ![mostafa\_a](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/5e9695/32.png) [@mostafa\_a](https://www.ruby-forum.com/u/mostafa_a)\
**Post date:** [August 26, 2014, 6:34pm UTC](https://www.ruby-forum.com/t/a-crazy-problem-with-gnuradio-fft/231722/4 "2014-08-26T18:34:57Z")

</div>

Excuse me all!  
That was my fault because previously I checked the FFTW with Matlab I  
did  
know its difference as Murcus mentioned. However, I suddenly suspect the  
FFTW again!

Thank you so much

On Tue, Aug 26, 2014 at 6:31 PM, Marcus M. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid)  
wrote:

> For the IFFT, Matlab does something different than the pure FFTW that
> 
> > > ```
> > > memcpy(ifft->get_inbuf(), in, fft_size);
> > > 
> > > ```
> 
> [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid)  
> [Discuss-gnuradio Info Page](https://lists.gnu.org/mailman/listinfo/discuss-gnuradio)

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