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/* rtbuf
 * Copyright 2018-2023 kmx.io <contact@kmx.io>
 *
 * Permission is hereby granted to use this software excepted
 * on Apple computers granted the above copyright notice and
 * this permission paragraph are included in all copies and
 * substantial portions of this software.
 *
 * THIS SOFTWARE IS PROVIDED "AS-IS" WITHOUT ANY GUARANTEE OF
 * PURPOSE AND PERFORMANCE. IN NO EVENT WHATSOEVER SHALL THE
 * AUTHOR BE CONSIDERED LIABLE FOR THE USE AND PERFORMANCE OF
 * THIS SOFTWARE.
 */
/* Butterworth lowpass second order filter

B2: s^2+sqrt(2)*s+1 $
H: 1/B2 $

*/
#include <math.h>
#include "../../librtbuf/rtbuf.h"
#include "../../librtbuf/lib.h"
#include "../signal.h"
#include "../signal_type.h"

s_rtbuf_lib_proc_in rtbuf_signal_lowpass2_in[] =
  {{ "signal", RTBUF_SIGNAL_TYPE, 0.0, -1.0, 1.0, 1.0 },
   { "cutoff", RTBUF_SIGNAL_TYPE, 400.0, 1.0, RTBUF_SIGNAL_SAMPLERATE / 2.0, 2.0 },
   { 0, 0, 0.0, 0.0, 0.0, 0.0 }};

s_rtbuf_lib_proc_out rtbuf_signal_lowpass2_out[] =
  {{ "signal", RTBUF_SIGNAL_TYPE },
   { "x1", RTBUF_SIGNAL_SAMPLE_TYPE },
   { "x2", RTBUF_SIGNAL_SAMPLE_TYPE },
   { "y1", RTBUF_SIGNAL_SAMPLE_TYPE },
   { "y2", RTBUF_SIGNAL_SAMPLE_TYPE },
   { 0, 0 }};

int rtbuf_signal_lowpass2_start (s_rtbuf *rtb)
{
  s_rtbuf_signal_lowpass2_data *data;
  assert(rtb->proc->out_bytes == sizeof(*data));
  data = (s_rtbuf_signal_lowpass2_data*) rtb->data;
  data->x1 = 0.0;
  data->x2 = 0.0;
  data->y1 = 0.0;
  data->y2 = 0.0;
  return 0;
}

int rtbuf_signal_lowpass2 (s_rtbuf *rtb)
{
  s_rtbuf_signal_fun in;
  s_rtbuf_signal_fun cutoff;
  s_rtbuf_signal_lowpass2_data *data;
  unsigned int i = 0;
  rtbuf_signal_fun(rtb, RTBUF_SIGNAL_LOWPASS_IN_SIGNAL, &in);
  rtbuf_signal_fun(rtb, RTBUF_SIGNAL_LOWPASS_IN_CUTOFF, &cutoff);
  data = (s_rtbuf_signal_lowpass2_data*) rtb->data;
  while (i < RTBUF_SIGNAL_SAMPLES) {
    double x = in.sample_fun(in.signal, i);
    double fc = cutoff.sample_fun(cutoff.signal, i);
    double k = RTBUF_SIGNAL_SAMPLERATE / (M_PI * fc);
    double k2 = k * k;
    double k_sqrt2 = M_SQRT2 * k;
    double z = 1.0 / (k2 + k_sqrt2 + 1);
    data->signal[i] = z * (x
                           + data->x1 * 2.0
                           + data->x2
                           + data->y1 * 2.0 * (k2 - 1.0)
                           + data->y2 * (k_sqrt2 - (k2 + 1.0)));
    data->x2 = data->x1;
    data->x1 = x;
    data->y2 = data->y1;
    data->y1 = data->signal[i];
    i++;
  }
  return 0;
}

s_rtbuf_lib_proc rtbuf_lib_proc =
  { "lowpass2",
    rtbuf_signal_lowpass2,
    rtbuf_signal_lowpass2_start,
    0,
    rtbuf_signal_lowpass2_in,
    rtbuf_signal_lowpass2_out };

unsigned long rtbuf_lib_ver = RTBUF_LIB_VER;
rtbuf/rtbuf/lib/signal/lowpass2.c

Commit

lib/signal/lowpass2.c
