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/*=============================================================================
//
// This software has been released under the terms of the GNU General Public
// license. See http://www.gnu.org/copyleft/gpl.html for details.
//
// Copyright 2002 Anders Johansson ajh@watri.uwa.edu.au
//
//=============================================================================
*/
/* This filter adds a sub-woofer channels to the audio stream by
averaging the left and right channel and low-pass filter them. The
low-pass filter is implemented as a 4th order IIR Butterworth
filter, with a variable cutoff frequency between 10 and 300 Hz. The
filter gives 24dB/octave attenuation. There are two runtime
controls one for setting which channel to insert the sub-audio into
called AF_CONTROL_SUB_CH and one for setting the cutoff frequency
called AF_CONTROL_SUB_FC.
*/
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include "af.h"
#include "dsp.h"
// Q value for low-pass filter
#define Q 1.0
// Analog domain biquad section
typedef struct{
float a[3]; // Numerator coefficients
float b[3]; // Denominator coefficients
} biquad_t;
// S-parameters for designing 4th order Butterworth filter
static biquad_t sp[2] = {{{1.0,0.0,0.0},{1.0,0.765367,1.0}},
{{1.0,0.0,0.0},{1.0,1.847759,1.0}}};
// Data for specific instances of this filter
typedef struct af_sub_s
{
float w[2][4]; // Filter taps for low-pass filter
float q[2][2]; // Circular queues
float fc; // Cutoff frequency [Hz] for low-pass filter
float k; // Filter gain;
int ch; // Channel number which to insert the filtered data
}af_sub_t;
// Initialization and runtime control
static int control(struct af_instance_s* af, int cmd, void* arg)
{
af_sub_t* s = af->setup;
switch(cmd){
case AF_CONTROL_REINIT:{
// Sanity check
if(!arg) return AF_ERROR;
af->data->rate = ((af_data_t*)arg)->rate;
af->data->nch = max(s->ch+1,((af_data_t*)arg)->nch);
af->data->format = AF_FORMAT_FLOAT_NE;
af->data->bps = 4;
// Design low-pass filter
s->k = 1.0;
if((-1 == af_filter_szxform(sp[0].a, sp[0].b, Q, s->fc,
(float)af->data->rate, &s->k, s->w[0])) ||
(-1 == af_filter_szxform(sp[1].a, sp[1].b, Q, s->fc,
(float)af->data->rate, &s->k, s->w[1])))
return AF_ERROR;
return af_test_output(af,(af_data_t*)arg);
}
case AF_CONTROL_COMMAND_LINE:{
int ch=5;
float fc=60.0;
sscanf(arg,"%f:%i", &fc , &ch);
if(AF_OK != control(af,AF_CONTROL_SUB_CH | AF_CONTROL_SET, &ch))
return AF_ERROR;
return control(af,AF_CONTROL_SUB_FC | AF_CONTROL_SET, &fc);
}
case AF_CONTROL_SUB_CH | AF_CONTROL_SET: // Requires reinit
// Sanity check
if((*(int*)arg >= AF_NCH) || (*(int*)arg < 0)){
af_msg(AF_MSG_ERROR,"[sub] Subwoofer channel number must be between "
" 0 and %i current value is %i\n", AF_NCH-1, *(int*)arg);
return AF_ERROR;
}
s->ch = *(int*)arg;
return AF_OK;
case AF_CONTROL_SUB_CH | AF_CONTROL_GET:
*(int*)arg = s->ch;
return AF_OK;
case AF_CONTROL_SUB_FC | AF_CONTROL_SET: // Requires reinit
// Sanity check
if((*(float*)arg > 300) || (*(float*)arg < 20)){
af_msg(AF_MSG_ERROR,"[sub] Cutoff frequency must be between 20Hz and"
" 300Hz current value is %0.2f",*(float*)arg);
return AF_ERROR;
}
// Set cutoff frequency
s->fc = *(float*)arg;
return AF_OK;
case AF_CONTROL_SUB_FC | AF_CONTROL_GET:
*(float*)arg = s->fc;
return AF_OK;
}
return AF_UNKNOWN;
}
// Deallocate memory
static void uninit(struct af_instance_s* af)
{
if(af->data)
free(af->data);
if(af->setup)
free(af->setup);
}
#ifndef IIR
#define IIR(in,w,q,out) { \
float h0 = (q)[0]; \
float h1 = (q)[1]; \
float hn = (in) - h0 * (w)[0] - h1 * (w)[1]; \
out = hn + h0 * (w)[2] + h1 * (w)[3]; \
(q)[1] = h0; \
(q)[0] = hn; \
}
#endif
// Filter data through filter
static af_data_t* play(struct af_instance_s* af, af_data_t* data)
{
af_data_t* c = data; // Current working data
af_sub_t* s = af->setup; // Setup for this instance
float* a = c->audio; // Audio data
int len = c->len/4; // Number of samples in current audio block
int nch = c->nch; // Number of channels
int ch = s->ch; // Channel in which to insert the sub audio
register int i;
// Run filter
for(i=0;i<len;i+=nch){
// Average left and right
register float x = 0.5 * (a[i] + a[i+1]);
IIR(x * s->k, s->w[0], s->q[0], x);
IIR(x , s->w[1], s->q[1], a[i+ch]);
}
return c;
}
// Allocate memory and set function pointers
static int open(af_instance_t* af){
af_sub_t* s;
af->control=control;
af->uninit=uninit;
af->play=play;
af->mul.n=1;
af->mul.d=1;
af->data=calloc(1,sizeof(af_data_t));
af->setup=s=calloc(1,sizeof(af_sub_t));
if(af->data == NULL || af->setup == NULL)
return AF_ERROR;
// Set default values
s->ch = 5; // Channel nr 6
s->fc = 60; // Cutoff frequency 60Hz
return AF_OK;
}
// Description of this filter
af_info_t af_info_sub = {
"Audio filter for adding a sub-base channel",
"sub",
"Anders",
"",
AF_FLAGS_NOT_REENTRANT,
open
};
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