dit_niso.cpp

Digital filter designer s handbook C++ code source

//+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
//
//  File = dit_niso.cpp
//
//  Decimation-In-Time FFT
//
//  Naturally-ordered Input / Scrambled Output   
//

#include <math.h>
#include "d_cmplx.h"
#include "log2.h"
#include "bitrev.h"
#include "cbitrev.h"
#include "misdefs.h"
#include "dit_niso.h"

void FftDitNiso( double_complex *array,
                 int fft_size)
{
double trig_arg;
int log2_size;
double_complex twiddle;
double_complex temp;
int pts_in_lft_grp, pts_in_rgt_grp;
int stage, grp_pos, grp_cntr;
int top_node, bot_node;

log2_size = ilog2(fft_size); 

pts_in_rgt_grp = fft_size;
for( stage=1; stage <=log2_size; stage++)
  {   
   grp_cntr = -1;
   
   pts_in_lft_grp = pts_in_rgt_grp;  // set pts_in_left_dft = N/(2**(stage-1))
   pts_in_rgt_grp /= 2;               // set pts_in_right_dft = N/(2**stage)

   for( grp_pos =0; grp_pos < fft_size; grp_pos += pts_in_lft_grp)
     {
      grp_cntr++;
                
      trig_arg = (TWO_PI*bitrev(grp_cntr, log2_size-1))/fft_size;
      twiddle = double_complex(cos(trig_arg), -sin(trig_arg));
                                  
      for( top_node = grp_pos; top_node < grp_pos+pts_in_rgt_grp; 
                               top_node++)
        {                              
         bot_node = top_node + pts_in_rgt_grp;
         temp = array[bot_node] * twiddle;
         array[bot_node] = array[top_node] - temp;
         array[top_node] += temp;
        }  // end of loop over top_node
        
     } // end of loop over grp_pos
  } // end of loop over stage

ComplexBitReverse(array, fft_size);  

return;
}
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