ref: 39fa45a7edd0a05898bec9ac9eae80e5e18013d7
dir: /src/lowp.c/
/*
* July 5, 1991
* Copyright 1991 Lance Norskog And Sundry Contributors
* This source code is freely redistributable and may be used for
* any purpose. This copyright notice must be maintained.
* Lance Norskog And Sundry Contributors are not responsible for
* the consequences of using this software.
*/
/*
* Sound Tools Low-Pass effect file.
*
* Algorithm: 1nd order filter.
* From Fugue source code:
*
* output[N] = input[N] * A + input[N-1] * B
*
* A = 2.0 * pi * center
* B = exp(-A / frequency)
*/
#include <math.h>
#include "st.h"
/* Private data for Lowpass effect */
typedef struct lowpstuff {
float center;
double A, B;
double in1;
} *lowp_t;
/*
* Process options
*/
void lowp_getopts(effp, n, argv)
eff_t effp;
int n;
char **argv;
{
lowp_t lowp = (lowp_t) effp->priv;
if ((n < 1) || !sscanf(argv[0], "%f", &lowp->center))
fail("Usage: lowp center");
}
/*
* Prepare processing.
*/
void lowp_start(effp)
eff_t effp;
{
lowp_t lowp = (lowp_t) effp->priv;
if (lowp->center > effp->ininfo.rate*2)
fail("Lowpass: center must be < minimum data rate*2\n");
lowp->A = (M_PI * 2.0 * lowp->center) / effp->ininfo.rate;
lowp->B = exp(-lowp->A / effp->ininfo.rate);
lowp->in1 = 0.0;
}
/*
* Processed signed long samples from ibuf to obuf.
* Return number of samples processed.
*/
void lowp_flow(effp, ibuf, obuf, isamp, osamp)
eff_t effp;
LONG *ibuf, *obuf;
int *isamp, *osamp;
{
lowp_t lowp = (lowp_t) effp->priv;
int len, done;
double d;
LONG l;
len = ((*isamp > *osamp) ? *osamp : *isamp);
/* yeah yeah yeah registers & integer arithmetic yeah yeah yeah */
for(done = 0; done < len; done++) {
l = *ibuf++;
d = lowp->A * (l / 65536L) + lowp->B * (lowp->in1 / 65536L);
d *= 0.8;
if (d > 32767)
d = 32767;
if (d < - 32767)
d = - 32767;
lowp->in1 = l;
*obuf++ = d * 65536L;
}
*isamp = len;
*osamp = len;
}
/*
* Do anything required when you stop reading samples.
* Don't close input file!
*/
void lowp_stop(effp)
eff_t effp;
{
/* nothing to do */
}