filter.c

SDL_mixer 是一个基于 SDL 的混音器

/*

    TiMidity -- Experimental MIDI to WAVE converter
    Copyright (C) 1995 Tuukka Toivonen <[e-mail]>

    This program is free software; you can redistribute it and/or modify
    it under the terms of the GNU General Public License as published by
    the Free Software Foundation; either version 2 of the License, or
    (at your option) any later version.

    This program is distributed in the hope that it will be useful,
    but WITHOUT ANY WARRANTY; without even the implied warranty of
    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
    GNU General Public License for more details.

    You should have received a copy of the GNU General Public License
    along with this program; if not, write to the Free Software
    Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.

   filter.c: written by Vincent Pagel ( [e-mail] ) 
 
   implements fir antialiasing filter : should help when setting sample
   rates as low as 8Khz.

   April 95
      - first draft

   22/5/95
      - modify "filter" so that it simulate leading and trailing 0 in the buffer
   */

#include <stdio.h>
#include <string.h>
#include <math.h>
#include <stdlib.h>
#include "config.h"
#include "common.h"
#include "ctrlmode.h"
#include "instrum.h"
#include "filter.h"

/*  bessel  function   */
static float ino(float x)
{
    float y, de, e, sde;
    int i;
    
    y = x / 2;
    e = 1.0;
    de = 1.0;
    i = 1;
    do {
	de = de * y / (float) i;
	sde = de * de;
	e += sde;
    } while (!( (e * 1.0e-08 - sde > 0) || (i++ > 25) ));
    return(e);
}	

/* Kaiser Window (symetric) */
static void kaiser(float *w,int n,float beta)
{
    float xind, xi;
    int i;
    
    xind = (float)((2*n - 1) * (2*n - 1));
    for (i =0; i<n ; i++) 
	{
	    xi = (float)(i + 0.5);
	    w[i] = ino((float)(beta * sqrt((double)(1. - 4 * xi * xi / xind))))
		/ ino((float)beta);
	}
}

/*
 * fir coef in g, cuttoff frequency in fc
 */
static void designfir(float *g , float fc)
{
    int i;
    float xi, omega, att, beta ;
    float w[ORDER2];
    
    for (i =0; i < ORDER2 ;i++) 
	{
	    xi = (float) (i + 0.5);
	    omega = (float)(PI * xi);
	    g[i] = (float)(sin( (double) omega * fc) / omega);
	}
    
    att = 40.; /* attenuation  in  db */
    beta = (float) (exp(log((double)0.58417 * (att - 20.96)) * 0.4) + 0.07886 
	* (att - 20.96));
    kaiser( w, ORDER2, beta);
    
    /* Matrix product */
    for (i =0; i < ORDER2 ; i++)
	g[i] = g[i] * w[i];
}

/*
 * FIR filtering -> apply the filter given by coef[] to the data buffer
 * Note that we simulate leading and trailing 0 at the border of the 
 * data buffer
 */
static void filter(sample_t *result,sample_t *data, int32 length,float coef[])
{
    int32 sample,i,sample_window;
    int16 peak = 0;
    float sum;

    /* Simulate leading 0 at the begining of the buffer */
     for (sample = 0; sample < ORDER2 ; sample++ )
	{
	    sum = 0.0;
	    sample_window= sample - ORDER2;
	   
	    for (i = 0; i < ORDER ;i++) 
		sum += (float)(coef[i] *
		    ((sample_window<0)? 0.0 : data[sample_window++])) ;
	    
	    /* Saturation ??? */
	    if (sum> 32767.) { sum=32767.; peak++; }
	    if (sum< -32768.) { sum=-32768; peak++; }
	    result[sample] = (sample_t) sum;
	}

    /* The core of the buffer  */
    for (sample = ORDER2; sample < length - ORDER + ORDER2 ; sample++ )
	{
	    sum = 0.0;
	    sample_window= sample - ORDER2;

	    for (i = 0; i < ORDER ;i++) 
		sum += data[sample_window++] * coef[i];
	    
	    /* Saturation ??? */
	    if (sum> 32767.) { sum=32767.; peak++; }
	    if (sum< -32768.) { sum=-32768; peak++; }
	    result[sample] = (sample_t) sum;
	}
    
    /* Simulate 0 at the end of the buffer */
    for (sample = length - ORDER + ORDER2; sample < length ; sample++ )
	{
	    sum = 0.0;
	    sample_window= sample - ORDER2;
	    
	    for (i = 0; i < ORDER ;i++) 
		sum += (float)(coef[i] *
		    ((sample_window>=length)? 0.0 : data[sample_window++])) ;
	    
	    /* Saturation ??? */
	    if (sum> 32767.) { sum=32767.; peak++; }
	    if (sum< -32768.) { sum=-32768; peak++; }
	    result[sample] = (sample_t) sum;
	}

    if (peak)
	ctl->cmsg(CMSG_ERROR, VERB_NORMAL, 
		  "Saturation %2.3f %%.", 100.0*peak/ (float) length);
}

/***********************************************************************/
/* Prevent aliasing by filtering any freq above the output_rate        */
/*                                                                     */
/* I don't worry about looping point -> they will remain soft if they  */
/* were already                                                        */
/***********************************************************************/
void antialiasing(Sample *sp, int32 output_rate )
{
    sample_t *temp;
    int i;
    float fir_symetric[ORDER];
    float fir_coef[ORDER2];
    float freq_cut;  /* cutoff frequency [0..1.0] FREQ_CUT/SAMP_FREQ*/
 

    ctl->cmsg(CMSG_INFO, VERB_NOISY, "Antialiasing: Fsample=%iKHz",
	      sp->sample_rate);
 
    /* No oversampling  */
    if (output_rate>=sp->sample_rate)
	return;
    
    freq_cut= (float) output_rate / (float) sp->sample_rate;
    ctl->cmsg(CMSG_INFO, VERB_NOISY, "Antialiasing: cutoff=%f%%",
	      freq_cut*100.);

    designfir(fir_coef,freq_cut);
    
    /* Make the filter symetric */
    for (i = 0 ; i<ORDER2 ;i++) 
	fir_symetric[ORDER-1 - i] = fir_symetric[i] = fir_coef[ORDER2-1 - i];
    
    /* We apply the filter we have designed on a copy of the patch */
    temp = safe_malloc(sp->data_length);
    memcpy(temp,sp->data,sp->data_length);
    
    filter(sp->data,temp,sp->data_length/sizeof(sample_t),fir_symetric);
    
    free(temp);
}