+
+void iceIIR_Butterworth(int *data, const size_t len){
+
+ int i,j;
+
+ int * output = (int* ) malloc(sizeof(int) * len);
+ if ( !output ) return;
+
+ // clear mem
+ memset(output, 0x00, len);
+
+ size_t adjustedLen = len;
+ float fc = 0.1125f; // center frequency
+
+ // 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) ? 127 : -127;
+ }
+
+ // show data
+ //memcpy(data, output, adjustedLen);
+ for (j=0; j<adjustedLen; ++j)
+ data[j] = output[j];
+
+ free(output);
+}
+
+void iceSimple_Filter(int *data, const size_t len, uint8_t k){
+// ref: http://www.edn.com/design/systems-design/4320010/A-simple-software-lowpass-filter-suits-embedded-system-applications
+// parameter K
+#define FILTER_SHIFT 4
+
+ int32_t filter_reg = 0;
+ int16_t input, output;
+ int8_t shift = (k <=8 ) ? k : FILTER_SHIFT;
+
+ for (int i = 0; i < len; ++i){
+
+ input = data[i];
+ // Update filter with current sample
+ filter_reg = filter_reg - (filter_reg >> shift) + input;
+
+ // Scale output for unity gain
+ output = filter_reg >> shift;
+ data[i] = output;
+ }
+}
+
+float complex cexpf (float complex Z)
+{
+ float complex Res;
+ double rho = exp (__real__ Z);
+ __real__ Res = rho * cosf(__imag__ Z);
+ __imag__ Res = rho * sinf(__imag__ Z);
+ return Res;
+}