shithub: sox

ref: 71b37b7c4f39520902a0e61e31a44f74c36d8598
dir: /src/highpass.c/

/*

    High-pass effect file for SoX
    Copyright (C) 1999 Jan Paul Schmidt <jps@fundament.org>

    This source code is free software; you can redistribute it and/or modify
    it under the terms of the GNU General Public License as published by
    the Free Software Foundation; either version 2 of the License, or
    (at your option) any later version.

    This source code 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 General Public License for more details.

    You should have received a copy of the GNU Library General Public
    License along with this library; if not, write to the Free
    Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA

    Code based on the high-pass implementation in

    Sound Processing Kit - A C++ Class Library for Audio Signal Processing
    Copyright (C) 1995-1998 Kai Lassfolk

    as described in

    Computer music: synthesis, composition, and performance
    Charles Dodge, Thomas A. Jerse
    [2nd ed.]
    Page 215

 */

#include <math.h>

#include "st.h"
#include "btrworth.h"



void
highpass_getopts (effp, n, argv) 
eff_t effp;
int n;
char **argv;
{
  butterworth_t butterworth = (butterworth_t)effp->priv;

  if (n != 1) {
    fail("Usage: highpass FREQUENCY");
  }

  butterworth_start (effp);

  if (!(sscanf (argv [0], "%lf", &butterworth->frequency))) {
    fail("highpass: illegal frequency");
  }
}



void
highpass_start (effp)
eff_t effp;
{
  butterworth_t butterworth = (butterworth_t) effp->priv;
  double c;

  c = 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 * (c * c - 1.0) * butterworth->a[0];
  butterworth->b [1] = (1.0 - sqrt(2.0) * c + c * c) * butterworth->a [0];
}