Re: [Discuss-gnuradio] volk atan

[West, Nathan]

Hi Mostafa,

On Thu, Feb 26, 2015 at 8:26 AM, Mostafa Alizadeh <address@hidden> wrote:

Hi everybody, 

I'm using "volk_32fc_s32f_atan2_32f_a" to compute phase of signal.

If you are calling the _a version make sure that you *know* your buffer is aligned because you have allocated it yourself or have done an alignment check. Otherwise it is best to just call the dispatcher, volk_32fc_s32f_atan2_32f, and it will do the alignment check for you.

 

It's important to know: what is the output phase interval of this function? Is it within [-pi/2,pi/2] as an standard atan function?

It's the same as the standard atan2f (from your C implementations math.h) which is a 2-argument atan. atan2's usually return [-pi,pi] which is also the case with volk_32fc_s32f_atan2_32f. You can verify this yourself with a sample that follows.

 

This shows its importance when we're dealing with quadrature modulations when you need to wrap the phase between [0,2pi] interval instead of [-pi/2,pi/2].

As I see the output of this function, it also has the phase greater than pi/2, so I guess it automatically wrap the phase. I didn't see any hint about this on the comments, so to be sure, I asked this. 

Good point. It's worth documenting that the output is [-pi, pi]. I'll add that in. Thanks for the suggestion!

 

Best, 

Mostafa  

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#include <stdio.h>
#include <volk/volk.h>

int main()
{
   int N = 10;
   unsigned int alignment = volk_get_alignment();
   lv_32fc_t* in  = (lv_32fc_t*)volk_malloc(sizeof(lv_32fc_t)*N, alignment);
   float* out = (float*)volk_malloc(sizeof(float)*N, alignment);
   float scale = 1.f; // we want unit circle

   float delta = 2.f*M_PI/(float)N;
   for(unsigned int ii = 0; ii < N; ++ii){
       // Generate points around the unit circle
       float real = std::cos((float)ii * delta);
       float imag = std::sin((float)ii * delta);
       in[ii] = lv_cmake(real, imag);
   }

   volk_32fc_s32f_atan2_32f(out, in, scale, N);

   for(unsigned int ii = 0; ii < N; ++ii){
       printf("atan2(%1.2f, %1.2f) = %1.2f\n",
           lv_cimag(in[ii]), lv_creal(in[ii]), out[ii]);
   }

   volk_free(in);
   volk_free(out);
   return 0;
}

Nathan