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I'm new to DSP and I would like to make an SDR receiver. For this I want to shift bins in the frequency domain in order to bring to 0Hz any portion of the base band spectrum (I know that there are also other methods). I got this idea from SDR for the masses page 4 figure 7

I use a stereo 192KHz 24bit audio codec connected to a dual channel audio generator with 90 degrees phase shift between channels. I use the complex FFT/IFFT from Paul Bourke FFT with 8192 samples length and no window.

If I do FFT and then IFFT I can't notice any signal degradation (I tested it with a single tone and with music). However, if I try to rotate to left with 100 the bins in the first half of I and Q array and to right the second half of array then I can hear a cyclic noise. The single tone output in this case has some distortions at both ends (second picture), as opposed to the first picture where I did not shift the bins.

enter image description here enter image description here

Also if I shift the bins in the other direction the single tone frequency also seems increase, but not so much(see bellow)! What am I doing wrong? Thanks enter image description here

void rotate_left(SAMPLE_DATA_TYPE *source, SAMPLE_DATA_TYPE *dest, unsigned short 
    int x, unsigned short int len) {
        memcpy(dest, source + x, (len - x) * sizeof(SAMPLE_DATA_TYPE));
        memcpy(dest + len - x, source, x * sizeof(SAMPLE_DATA_TYPE));
    }

void rotate_right(SAMPLE_DATA_TYPE *source, SAMPLE_DATA_TYPE *dest, unsigned short int x, unsigned short int len) {
            memcpy(dest, source + len - x, x * sizeof(SAMPLE_DATA_TYPE));
            memcpy(dest + x, source, (len - x) * sizeof(SAMPLE_DATA_TYPE));
        }

main() {

................

#define SAMPLES                 8192
#define FFT_SIZE                13
#define SAMPLE_DATA_TYPE        double
shift = 100;

while (1) {

    //Get I and Q from interleaved audio stream
    ..........

    FFT(1, FFT_SIZE, I, Q);
    //I and Q have the DC bin at first and last position. At the center is the highest frequency bin

    rotate_right(I, temp_buffer_I, shift, SAMPLES / 2);
    rotate_left(I + SAMPLES / 2, temp_buffer_I + SAMPLES / 2, shift, SAMPLES / 2);
    rotate_right(Q, temp_buffer_Q, shift, SAMPLES / 2);
    rotate_left(Q + SAMPLES / 2, temp_buffer_Q + SAMPLES / 2, shift, SAMPLES / 2);

    memcpy(I, temp_buffer_I, SAMPLES * sizeof(SAMPLE_DATA_TYPE));
    memcpy(Q, temp_buffer_Q, SAMPLES * sizeof(SAMPLE_DATA_TYPE));

    FFT(0, FFT_SIZE, I, Q);

    //Output I and Q
    ..........

}

short FFT(short int dir,long m,SAMPLE_DATA_TYPE  *x,SAMPLE_DATA_TYPE *y)
{
   long  n,i,i1,j,k,i2,l,l1,l2;
   SAMPLE_DATA_TYPE c1,c2,tx,ty,t1,t2,u1,u2,z;

   /* Calculate the number of points */
   n = 1;
   for (i=0;i<m;i++) 
      n *= 2;

   /* Do the bit reversal */
   i2 = n >> 1;
   j = 0;
   for (i=0;i<n-1;i++) {
      if (i < j) {
         tx = x[i];
         ty = y[i];
         x[i] = x[j];
         y[i] = y[j];
         x[j] = tx;
         y[j] = ty;
      }
      k = i2;
      while (k <= j) {
         j -= k;
         k >>= 1;
      }
      j += k;
   }

   /* Compute the FFT */
   c1 = -1.0; 
   c2 = 0.0;
   l2 = 1;
   for (l=0;l<m;l++) {
      l1 = l2;
      l2 <<= 1;
      u1 = 1.0; 
      u2 = 0.0;
      for (j=0;j<l1;j++) {
         for (i=j;i<n;i+=l2) {
            i1 = i + l1;
            t1 = u1 * x[i1] - u2 * y[i1];
            t2 = u1 * y[i1] + u2 * x[i1];
            x[i1] = x[i] - t1; 
            y[i1] = y[i] - t2;
            x[i] += t1;
            y[i] += t2;
         }
         z =  u1 * c1 - u2 * c2;
         u2 = u1 * c2 + u2 * c1;
         u1 = z;
      }
      c2 = sqrt((1.0 - c1) / 2.0);
      if (dir == 1) 
         c2 = -c2;
      c1 = sqrt((1.0 + c1) / 2.0);
   }

   /* Scaling for forward transform */
   if (dir == 1) {
      for (i=0;i<n;i++) {
         x[i] /= n;
         y[i] /= n;
      }
   }

   return(1);
}
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  • $\begingroup$ Hi BlackSun! Can you show your code as well please... $\endgroup$ – Fat32 Sep 10 '18 at 0:33
  • 1
    $\begingroup$ Hi, I add the most relevant parts of the code. Thank you $\endgroup$ – BlackSun Sep 10 '18 at 8:10
  • $\begingroup$ This is C code! It could be great if you had Matlab / Octave or at least Python versions... Anyway, lets see if people can debug your code :-) $\endgroup$ – Fat32 Sep 10 '18 at 11:32
  • $\begingroup$ But I need it to work in C. I never use Matlab ... $\endgroup$ – BlackSun Sep 10 '18 at 12:05
  • $\begingroup$ You better develop your algorithms using Matlab / Octave / Python / Mathematica / Mapple and port them to C / C++ / Java etc for best results and fastest deployment (on the average). $\endgroup$ – Fat32 Sep 10 '18 at 12:33

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