ref: 1969e6763675792f743c5299de1d78bbe342da22
dir: /src/pt2_ledfilter.c/
#include <stdint.h> #include <math.h> #include "pt2_rcfilter.h" // DENORMAL_OFFSET definition #include "pt2_ledfilter.h" /* Imperfect Amiga "LED" filter implementation. This may be further improved in the future. ** Based upon ideas posted by mystran @ the kvraudio.com forum. ** ** This filter may not function correctly used outside the fixed-cutoff context here! */ void clearLEDFilterState(ledFilter_t *filterLED) { filterLED->buffer[0] = 0.0; // left channel filterLED->buffer[1] = 0.0; filterLED->buffer[2] = 0.0; // right channel filterLED->buffer[3] = 0.0; } static double sigmoid(double x, double coefficient) { /* aciddose: ** Coefficient from: ** 0.0 to inf (linear) ** -1.0 to -inf (linear) */ return x / (x + coefficient) * (coefficient + 1.0); } void calcLEDFilterCoeffs(const double sr, const double hz, const double fb, ledFilter_t *filter) { /* aciddose: ** tan() may produce NaN or other bad results in some cases! ** It appears to work correctly with these specific coefficients. */ const double pi = 4.0 * atan(1.0); // M_PI can not be trusted const double c = (hz < (sr / 2.0)) ? tan((pi * hz) / sr) : 1.0; const double g = 1.0 / (1.0 + c); // aciddose: dirty compensation const double s = 0.5; const double t = 0.5; const double ic = c > t ? 1.0 / ((1.0 - s*t) + s*c) : 1.0; const double cg = c * g; const double fbg = 1.0 / (1.0 + fb * cg*cg); filter->c = c; filter->ci = g; filter->feedback = 2.0 * sigmoid(fb, 0.5); filter->bg = fbg * filter->feedback * ic; filter->cg = cg; filter->c2 = c * 2.0; } void LEDFilter(ledFilter_t *f, const double *in, double *out) { const double in_1 = DENORMAL_OFFSET; const double in_2 = DENORMAL_OFFSET; const double c = f->c; const double g = f->ci; const double cg = f->cg; const double bg = f->bg; const double c2 = f->c2; double *v = f->buffer; // left channel const double estimate_L = in_2 + g*(v[1] + c*(in_1 + g*(v[0] + c*in[0]))); const double y0_L = v[0]*g + in[0]*cg + in_1 + estimate_L * bg; const double y1_L = v[1]*g + y0_L*cg + in_2; v[0] += c2 * (in[0] - y0_L); v[1] += c2 * (y0_L - y1_L); out[0] = y1_L; // right channel const double estimate_R = in_2 + g*(v[3] + c*(in_1 + g*(v[2] + c*in[1]))); const double y0_R = v[2]*g + in[1]*cg + in_1 + estimate_R * bg; const double y1_R = v[3]*g + y0_R*cg + in_2; v[2] += c2 * (in[1] - y0_R); v[3] += c2 * (y0_R - y1_R); out[1] = y1_R; }