ref: 876f21808513d24b82838b0558c71dce9a414f0e
dir: /src/lowp.c/
/*
* Sound Tools Low-Pass effect file.
*
* (C) 2000 Chris Bagwell <cbagwell@sprynet.com>
* See License file for further copyright information.
*
* Algorithm: Recursive single pole lowpass filter
*
* Reference: The Scientist and Engineer's Guide to Digital Signal Processing
*
* output[N] = input[N] * A + output[N-1] * B
*
* X = exp(-2.0 * pi * Fc)
* A = 1 - X
* B = X
* Fc = cutoff freq / sample rate
*
* Mimics an RC low-pass filter:
*
* ---/\/\/\/\----------->
* |
* --- C
* ---
* |
* |
* V
*
*/
#include <math.h>
#include "st.h"
/* Private data for Lowpass effect */
typedef struct lowpstuff {
float cutoff;
double A, B;
double outm1;
} *lowp_t;
/*
* Process options
*/
int st_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->cutoff))
{
st_fail("Usage: lowp cutoff");
return (ST_EOF);
}
return (ST_SUCCESS);
}
/*
* Prepare processing.
*/
int st_lowp_start(effp)
eff_t effp;
{
lowp_t lowp = (lowp_t) effp->priv;
if (lowp->cutoff > effp->ininfo.rate / 2)
{
st_fail("Lowpass: cutoff must be < sample rate / 2 (Nyquest rate)\n");
return (ST_EOF);
}
lowp->B = exp((-2.0 * M_PI * (lowp->cutoff / effp->ininfo.rate)));
lowp->A = 1 - lowp->B;
lowp->outm1 = 0.0;
return (ST_SUCCESS);
}
/*
* Processed signed long samples from ibuf to obuf.
* Return number of samples processed.
*/
int st_lowp_flow(effp, ibuf, obuf, isamp, osamp)
eff_t effp;
LONG *ibuf, *obuf;
LONG *isamp, *osamp;
{
lowp_t lowp = (lowp_t) effp->priv;
int len, done;
double d;
LONG l;
len = ((*isamp > *osamp) ? *osamp : *isamp);
for(done = 0; done < len; done++) {
l = *ibuf++;
d = lowp->A * l + lowp->B * lowp->outm1;
if (d < -2147483647L)
d = -2147483647L;
else if (d > 2147483647L)
d = 2147483647L;
lowp->outm1 = l;
*obuf++ = d;
}
*isamp = len;
*osamp = len;
return (ST_SUCCESS);
}
/*
* Do anything required when you stop reading samples.
* Don't close input file!
*/
int st_lowp_stop(effp)
eff_t effp;
{
/* nothing to do */
return (ST_SUCCESS);
}