+ int i,j;
+
+ int * output = (int* ) malloc(sizeof(int) * len);
+ memset(output, 0x00, len);
+ float fc = 0.1125f; // center frequency
+ size_t adjustedLen = len;
+
+ // create very simple low-pass filter to remove images (2nd-order Butterworth)
+ float complex iir_buf[3] = {0,0,0};
+ float b[3] = {0.003621681514929, 0.007243363029857, 0.003621681514929};
+ float a[3] = {1.000000000000000, -1.822694925196308, 0.837181651256023};
+
+ float sample = 0; // input sample read from array
+ float complex x_prime = 1.0f; // save sample for estimating frequency
+ float complex x;
+
+ for (i=0; i<adjustedLen; ++i) {
+
+ sample = data[i];
+
+ // remove DC offset and mix to complex baseband
+ x = (sample - 127.5f) * cexpf( _Complex_I * 2 * M_PI * fc * i );
+
+ // apply low-pass filter, removing spectral image (IIR using direct-form II)
+ iir_buf[2] = iir_buf[1];
+ iir_buf[1] = iir_buf[0];
+ iir_buf[0] = x - a[1]*iir_buf[1] - a[2]*iir_buf[2];
+ x = b[0]*iir_buf[0] +
+ b[1]*iir_buf[1] +
+ b[2]*iir_buf[2];
+
+ // compute instantaneous frequency by looking at phase difference
+ // between adjacent samples
+ float freq = cargf(x*conjf(x_prime));
+ x_prime = x; // retain this sample for next iteration
+
+ output[i] =(freq > 0)? 10 : -10;
+ }
+
+ // show data
+ for (j=0; j<adjustedLen; ++j)
+ data[j] = output[j];
+
+ free(output);
+}
+
+float complex cexpf (float complex Z)