src/hilbert.c

   1 /* libSoX effect: Hilbert transform filter
   2  *
   3  * First version of this effect written 11/2011 by Ulrich Klauer, using maths
   4  * from "Understanding digital signal processing" by Richard G. Lyons.
   5  *
   6  * Copyright 2011 Chris Bagwell and SoX Contributors
   7  *
   8  * This library is free software; you can redistribute it and/or modify it
   9  * under the terms of the GNU Lesser General Public License as published by
  10  * the Free Software Foundation; either version 2.1 of the License, or (at
  11  * your option) any later version.
  12  *
  13  * This library is distributed in the hope that it will be useful, but
  14  * WITHOUT ANY WARRANTY; without even the implied warranty of
  15  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU Lesser
  16  * General Public License for more details.
  17  *
  18  * You should have received a copy of the GNU Lesser General Public License
  19  * along with this library; if not, write to the Free Software Foundation,
  20  * Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301  USA
  21  */
  22
  23 #include "sox_i.h"
  24 #include "dft_filter.h"
  25
  26 typedef struct {
  27   dft_filter_priv_t base;
  28   double *h;
  29   int taps;
  30 } priv_t;
  31
  32 static int getopts(sox_effect_t *effp, int argc, char **argv)
  33 {
  34   lsx_getopt_t optstate;
  35   int c;
  36   priv_t *p = (priv_t*)effp->priv;
  37   dft_filter_priv_t *b = &p->base;
  38
  39   b->filter_ptr = &b->filter;
  40
  41   lsx_getopt_init(argc, argv, "+n:", NULL, lsx_getopt_flag_none, 1, &optstate);
  42
  43   while ((c = lsx_getopt(&optstate)) != -1) switch (c) {
  44     GETOPT_NUMERIC(optstate, 'n', taps, 3, 32767)
  45     default: lsx_fail("invalid option `-%c'", optstate.opt); return lsx_usage(effp);
  46   }
  47   if (p->taps && p->taps%2 == 0) {
  48     lsx_fail("only filters with an odd number of taps are supported");
  49     return SOX_EOF;
  50   }
  51   return optstate.ind != argc ? lsx_usage(effp) : SOX_SUCCESS;
  52 }
  53
  54 static int start(sox_effect_t *effp)
  55 {
  56   priv_t *p = (priv_t*)effp->priv;
  57   dft_filter_t *f = p->base.filter_ptr;
  58
  59   if (!f->num_taps) {
  60     int i;
  61     if (!p->taps) {
  62       p->taps = effp->in_signal.rate/76.5 + 2;
  63       p->taps += 1 - (p->taps%2);
  64       /* results in a cutoff frequency of about 75 Hz with a Blackman window */
  65       lsx_debug("choosing number of taps = %d (override with -n)", p->taps);
  66     }
  67     lsx_valloc(p->h, p->taps);
  68     for (i = 0; i < p->taps; i++) {
  69       int k = -(p->taps/2) + i;
  70       if (k%2 == 0) {
  71         p->h[i] = 0.0;
  72       } else {
  73         double pk = M_PI * k;
  74         p->h[i] = (1 - cos(pk))/pk;
  75       }
  76     }
  77     lsx_apply_blackman(p->h, p->taps, .16);
  78
  79     if (effp->global_info->plot != sox_plot_off) {
  80       char title[100];
  81       sprintf(title, "SoX effect: hilbert (%d taps)", p->taps);
  82       lsx_plot_fir(p->h, p->taps, effp->in_signal.rate,
  83           effp->global_info->plot, title, -20., 5.);
  84       free(p->h);
  85       return SOX_EOF;
  86     }
  87     lsx_set_dft_filter(f, p->h, p->taps, p->taps/2);
  88   }
  89   return lsx_dft_filter_effect_fn()->start(effp);
  90 }
  91
  92 sox_effect_handler_t const *lsx_hilbert_effect_fn(void)
  93 {
  94   static sox_effect_handler_t handler;
  95   handler = *lsx_dft_filter_effect_fn();
  96   handler.name = "hilbert";
  97   handler.usage = "[-n taps]";
  98   handler.getopts = getopts;
  99   handler.start = start;
 100   handler.priv_size = sizeof(priv_t);
 101   return &handler;
 102 }