shithub: sox

ref: 00071341d288c17fd063d92b29cbe45f0feff564
dir: /src/band.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 Bandpass effect file.
 *
 * Algorithm:  2nd order recursive filter.
 * Formula stolen from MUSIC56K, a toolkit of 56000 assembler stuff.
 * Quote:
 *   This is a 2nd order recursive band pass filter of the form.                
 *   y(n)= a * x(n) - b * y(n-1) - c * y(n-2)   
 *   where :    
 *        x(n) = "IN"           
 *        "OUT" = y(n)          
 *        c = EXP(-2*pi*cBW/S_RATE)             
 *        b = -4*c/(1+c)*COS(2*pi*cCF/S_RATE)   
 *   if cSCL=2 (i.e. noise input)               
 *        a = SQT(((1+c)*(1+c)-b*b)*(1-c)/(1+c))                
 *   else       
 *        a = SQT(1-b*b/(4*c))*(1-c)            
 *   endif      
 *   note :     cCF is the center frequency in Hertz            
 *        cBW is the band width in Hertz        
 *        cSCL is a scale factor, use 1 for pitched sounds      
 *   use 2 for noise.           
 *
 *
 * July 1, 1999 - Jan Paul Schmidt <jps@fundament.org>
 *
 *   This looks like the resonator band pass in SPKit. It's a
 *   second order all-pole (IIR) band-pass filter described
 *   at the pages 186 - 189 in
 *     Dodge, Charles & Jerse, Thomas A. 1985: 
 *       Computer Music -- Synthesis, Composition and Performance.
 *       New York: Schirmer Books.  
 *   Reference from the SPKit manual.
 */

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

static st_effect_t st_band_effect;

/* Private data for Bandpass effect */
typedef struct bandstuff {
        float   center;
        float   width;
        double  A, B, C;
        double  out1, out2;
        short   noise;
        /* 50 bytes of data, 52 bytes long for allocation purposes. */
} *band_t;

/*
 * Process options
 */
int st_band_getopts(eff_t effp, int n, char **argv) 
{
        band_t band = (band_t) effp->priv;

        band->noise = 0;
        if (n > 0 && !strcmp(argv[0], "-n")) {
                band->noise = 1;
                n--;
                argv++;
        }
        if ((n < 1) || !sscanf(argv[0], "%f", &band->center))
        {
                st_fail(st_band_effect.usage);
                return (ST_EOF);
        }
        band->width = band->center / 2;
        if ((n >= 2) && !sscanf(argv[1], "%f", &band->width))
        {
                st_fail(st_band_effect.usage);
                return (ST_EOF);
        }
        return (ST_SUCCESS);
}

/*
 * Prepare processing.
 */
int st_band_start(eff_t effp)
{
        band_t band = (band_t) effp->priv;
        if (band->center > effp->ininfo.rate/2)
        {
                st_fail("Band: center must be < minimum data rate/2\n");
                return (ST_EOF);
        }

        band->C = exp(-2*M_PI*band->width/effp->ininfo.rate);
        band->B = -4*band->C/(1+band->C)*
                cos(2*M_PI*band->center/effp->ininfo.rate);
        if (band->noise)
                band->A = sqrt(((1+band->C)*(1+band->C)-band->B *
                        band->B)*(1-band->C)/(1+band->C));
        else
                band->A = sqrt(1-band->B*band->B/(4*band->C))*(1-band->C);
        band->out1 = band->out2 = 0.0;
        return (ST_SUCCESS);
}

/*
 * Processed signed long samples from ibuf to obuf.
 * Return number of samples processed.
 */

int st_band_flow(eff_t effp, st_sample_t *ibuf, st_sample_t *obuf, 
                 st_size_t *isamp, st_size_t *osamp)
{
        band_t band = (band_t) effp->priv;
        int len, done;
        double d;
        st_sample_t l;

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

        /* yeah yeah yeah registers & integer arithmetic yeah yeah yeah */
        for(done = 0; done < len; done++) {
                l = *ibuf++;
                d = (band->A * l - band->B * band->out1) - band->C * band->out2;
                band->out2 = band->out1;
                band->out1 = 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_band_stop(eff_t effp)
{
        return (ST_SUCCESS);    /* nothing to do */
}

static st_effect_t st_band_effect = {
   "band",
   "Usage: band [ -n ] center [ width ]",
   0,
   st_band_getopts,
   st_band_start,
   st_band_flow,
   st_effect_nothing_drain,
   st_band_stop
};

const st_effect_t *st_band_effect_fn(void)
{
    return &st_band_effect;
}