ref: a48b568a48019dfbaba46df760c27bf434667031
dir: /src/lowpass.c/
/* Low-pass effect file for SoX Copyright (C) 1999 Jan Paul Schmidt <jps@fundament.org> This library is free software; you can redistribute it and/or modify it under the terms of the GNU Lesser General Public License as published by the Free Software Foundation; either version 2 of the License, or (at your option) any later version. This library is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU Lesser General Public License for more details. You should have received a copy of the GNU Lesser General Public License along with this library; if not, write to the Free Software Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA */ #include <math.h> #include "st.h" #include "btrworth.h" int st_lowpass_getopts (effp, n, argv) eff_t effp; int n; char **argv; { butterworth_t butterworth = (butterworth_t)effp->priv; if (n != 1) { st_fail("Usage: lowpass FREQUENCY"); return (ST_EOF); } st_butterworth_start (effp); if (!(sscanf (argv [0], "%lf", &butterworth->frequency))) { st_fail("lowpass: illegal frequency"); return (ST_EOF); } return (ST_SUCCESS); } int st_lowpass_start (effp) eff_t effp; { butterworth_t butterworth = (butterworth_t) effp->priv; double c; c = 1.0 / tan (M_PI * butterworth->frequency / effp->ininfo.rate); butterworth->a [0] = 1.0 / (1.0 + sqrt(2.0) * c + c * c); butterworth->a [1] = 2.0 * butterworth->a [0]; butterworth->a [2] = butterworth->a [0]; butterworth->b [0] = 2 * (1.0 - c * c) * butterworth->a[0]; butterworth->b [1] = (1.0 - sqrt(2.0) * c + c * c) * butterworth->a [0]; return (ST_SUCCESS); }