shithub: sox

ref: a83636279984823b1d7e04cff5bfaa7588655f66
dir: /src/highp.c/

/*
 * Sound Tools High-Pass effect file.
 *
 * (C) 2000 Chris Bagwell <cbagwell@sprynet.com>
 * See License file for further copyright information.
 *
 * Algorithm:  Recursive single pole high-pass filter
 *
 * Reference: The Scientist and Engineer's Guide to Digital Processing
 *
 * 	output[N] = A0 * input[N] + A1 * input[N-1] + B1 * output[N-1]
 *
 *      X  = exp(-2.0 * pi * Fc)
 *      A0 = (1 + X) / 2
 *      A1 = -(1 + X) / 2
 *      B1 = X
 *      Fc = cutoff freq / sample rate
 *
 * Mimics an RC high-pass filter:
 *
 *        || C
 *    ----||--------->
 *        ||    |
 *              <
 *              > R
 *              <
 *              |
 *              V
 *
 */

#include <math.h>
#include "st_i.h"

/* Private data for Highpass effect */
typedef struct highpstuff {
	float	cutoff;
	double	A0, A1, B1;
	double	inm1, outm1;
} *highp_t;

/*
 * Process options
 */
int st_highp_getopts(eff_t effp, int n, char **argv) 
{
	highp_t highp = (highp_t) effp->priv;

	if ((n < 1) || !sscanf(argv[0], "%f", &highp->cutoff))
	{
		st_fail("Usage: highp cutoff");
		return (ST_EOF);
	}
	return (ST_SUCCESS);
}

/*
 * Prepare processing.
 */
int st_highp_start(eff_t effp)
{
	highp_t highp = (highp_t) effp->priv;
	if (highp->cutoff > effp->ininfo.rate/2)
	{
		st_fail("Highpass: cutoff must be < sample rate / 2 (Nyquest rate)\n");
		return (ST_EOF);
	}

	highp->B1 = exp((-2.0 * M_PI * (highp->cutoff / effp->ininfo.rate)));
	highp->A0 = (1 + highp->B1) / 2;
	highp->A1 = (-1 * (1 + highp->B1)) / 2;
	highp->inm1 = 0.0;
	highp->outm1 = 0.0;
	return (ST_SUCCESS);
}

/*
 * Processed signed long samples from ibuf to obuf.
 * Return number of samples processed.
 */
int st_highp_flow(eff_t effp, st_sample_t *ibuf, st_sample_t *obuf, 
                  st_size_t *isamp, st_size_t *osamp)
{
	highp_t highp = (highp_t) effp->priv;
	int len, done;
	double d;
	st_sample_t l;

	len = ((*isamp > *osamp) ? *osamp : *isamp);

	for(done = 0; done < len; done++) {
		l = *ibuf++;
		d = highp->A0 * l + 
		    highp->A1 * highp->inm1 + 
		    highp->B1 * highp->outm1;
                if (d < -2147483647L)
                    d = -2147483647L;
                else if (d > 2147483647L)
                    d = 2147483647L;
		highp->inm1 = l;
		highp->outm1 = d;
		*obuf++ = d;
	}
	*isamp = len;
	*osamp = len;
	return (ST_SUCCESS);
}

/*
 * Do anything required when you stop reading samples.  
 * Don't close input file! 
 */
int st_highp_stop(eff_t effp)
{
	/* nothing to do */
    return (ST_SUCCESS);
}