1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
|
/*=============================================================================
//
// 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 af_open(af_instance_t* af){
af_sub_t* s;
af->control=control;
af->uninit=uninit;
af->play=play;
af->mul=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,
af_open
};
|
