mirror of https://github.com/ericonr/sndio.git
120 lines
2.7 KiB
C
120 lines
2.7 KiB
C
/*
|
|
* Copyright (c) 2021 Alexandre Ratchov <alex@caoua.org>
|
|
*
|
|
* Permission to use, copy, modify, and distribute this software for any
|
|
* purpose with or without fee is hereby granted, provided that the above
|
|
* copyright notice and this permission notice appear in all copies.
|
|
*
|
|
* THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
|
|
* WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
|
|
* MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
|
|
* ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
|
|
* WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
|
|
* ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
|
|
* OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
|
|
*/
|
|
|
|
/*
|
|
* This program generates coefficients for the resampling low-pass
|
|
* filter used in sndiod and aucat.
|
|
*
|
|
* The coefficients correspond to the filter impulse response sampled
|
|
* with 64 points per time unit; they are represented as 24-bit
|
|
* fixed-point numbers.
|
|
*
|
|
* The filter is an ideal low-pass (sinc function), multiplied by a 8
|
|
* time units long Blackman window. The filter cut-off frequency is
|
|
* set at 0.75 of the Nyquist frequency (for instance, at 48kHz, the
|
|
* cut-off will be at 18kHz, beyond audible frequencies).
|
|
*
|
|
* References:
|
|
* https://en.wikipedia.org/wiki/Sinc_filter
|
|
* https://en.wikipedia.org/wiki/Window_function
|
|
* https://ccrma.stanford.edu/~jos/resample/
|
|
*/
|
|
|
|
#include <math.h>
|
|
#include <stdio.h>
|
|
#include <stdlib.h>
|
|
#include <string.h>
|
|
#include <unistd.h>
|
|
|
|
int order = 8, factor = 64, bits = 24;
|
|
|
|
/*
|
|
* convert double to fixed-point integer
|
|
*/
|
|
int
|
|
mkfp(double x)
|
|
{
|
|
long long r, unit;
|
|
|
|
unit = 1LL << (bits - 1);
|
|
r = x * unit + ((x >= 0) ? 0.5 : -0.5);
|
|
if (r < -unit || r >= unit) {
|
|
fprintf(stderr, "%g: sample out of range\n", x);
|
|
exit(1);
|
|
}
|
|
return r;
|
|
}
|
|
|
|
/*
|
|
* Blackman window function
|
|
*/
|
|
double
|
|
win_blackman(double a, double x)
|
|
{
|
|
if (x < 0)
|
|
x = -x;
|
|
return (x >= 0.5) ? 0 :
|
|
0.5 * (1 - a) +
|
|
0.5 * cos(2 * M_PI * x) +
|
|
0.5 * a * cos(4 * M_PI * x);
|
|
}
|
|
|
|
/*
|
|
* Ideal low-pass filter response
|
|
*/
|
|
double
|
|
lowpass_sinc(double cutoff, double t)
|
|
{
|
|
double arg;
|
|
|
|
if (t == 0)
|
|
return cutoff;
|
|
arg = M_PI * t;
|
|
return sin(cutoff * arg) / arg;
|
|
}
|
|
|
|
/*
|
|
* Convert table index to time
|
|
*/
|
|
double
|
|
timeof(int i)
|
|
{
|
|
return (double)(i - order * factor / 2) / factor;
|
|
}
|
|
|
|
int
|
|
main(void)
|
|
{
|
|
double t;
|
|
double h;
|
|
double cutoff = 3. / 4.;
|
|
double param = 0.16;
|
|
int i;
|
|
|
|
printf("{");
|
|
for (i = 0; i <= order * factor; i++) {
|
|
t = timeof(i);
|
|
h = lowpass_sinc(cutoff, t) * win_blackman(param, t / order);
|
|
if (i != 0)
|
|
printf(",");
|
|
printf("%s", (i % 8 == 0) ? "\n\t" : " ");
|
|
printf("%7d", mkfp(h));
|
|
}
|
|
printf("\n};\n");
|
|
|
|
return 0;
|
|
}
|