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
181
182
183
184
|
#include "util/bit_reverse.h"
#include <stdlib.h>
#include <string.h>
#include "dsd2pcm.h"
#define HTAPS 48 /* number of FIR constants */
#define FIFOSIZE 16 /* must be a power of two */
#define FIFOMASK (FIFOSIZE-1) /* bit mask for FIFO offsets */
#define CTABLES ((HTAPS+7)/8) /* number of "8 MACs" lookup tables */
#if FIFOSIZE*8 < HTAPS*2
#error "FIFOSIZE too small"
#endif
/*
* Properties of this 96-tap lowpass filter when applied on a signal
* with sampling rate of 44100*64 Hz:
*
* () has a delay of 17 microseconds.
*
* () flat response up to 48 kHz
*
* () if you downsample afterwards by a factor of 8, the
* spectrum below 70 kHz is practically alias-free.
*
* () stopband rejection is about 160 dB
*
* The coefficient tables ("ctables") take only 6 Kibi Bytes and
* should fit into a modern processor's fast cache.
*/
/*
* The 2nd half (48 coeffs) of a 96-tap symmetric lowpass filter
*/
static const double htaps[HTAPS] = {
0.09950731974056658,
0.09562845727714668,
0.08819647126516944,
0.07782552527068175,
0.06534876523171299,
0.05172629311427257,
0.0379429484910187,
0.02490921351762261,
0.0133774746265897,
0.003883043418804416,
-0.003284703416210726,
-0.008080250212687497,
-0.01067241812471033,
-0.01139427235000863,
-0.0106813877974587,
-0.009007905078766049,
-0.006828859761015335,
-0.004535184322001496,
-0.002425035959059578,
-0.0006922187080790708,
0.0005700762133516592,
0.001353838005269448,
0.001713709169690937,
0.001742046839472948,
0.001545601648013235,
0.001226696225277855,
0.0008704322683580222,
0.0005381636200535649,
0.000266446345425276,
7.002968738383528e-05,
-5.279407053811266e-05,
-0.0001140625650874684,
-0.0001304796361231895,
-0.0001189970287491285,
-9.396247155265073e-05,
-6.577634378272832e-05,
-4.07492895872535e-05,
-2.17407957554587e-05,
-9.163058931391722e-06,
-2.017460145032201e-06,
1.249721855219005e-06,
2.166655190537392e-06,
1.930520892991082e-06,
1.319400334374195e-06,
7.410039764949091e-07,
3.423230509967409e-07,
1.244182214744588e-07,
3.130441005359396e-08
};
static float ctables[CTABLES][256];
static int precalculated = 0;
static void precalc(void)
{
int t, e, m, k;
double acc;
if (precalculated) return;
for (t=0; t<CTABLES; ++t) {
k = HTAPS - t*8;
if (k>8) k=8;
for (e=0; e<256; ++e) {
acc = 0.0;
for (m=0; m<k; ++m) {
acc += (((e >> (7-m)) & 1)*2-1) * htaps[t*8+m];
}
ctables[CTABLES-1-t][e] = (float)acc;
}
}
precalculated = 1;
}
struct dsd2pcm_ctx_s
{
unsigned char fifo[FIFOSIZE];
unsigned fifopos;
};
extern dsd2pcm_ctx* dsd2pcm_init(void)
{
dsd2pcm_ctx* ptr;
if (!precalculated) precalc();
ptr = (dsd2pcm_ctx*) malloc(sizeof(dsd2pcm_ctx));
if (ptr) dsd2pcm_reset(ptr);
return ptr;
}
extern void dsd2pcm_destroy(dsd2pcm_ctx* ptr)
{
free(ptr);
}
extern dsd2pcm_ctx* dsd2pcm_clone(dsd2pcm_ctx* ptr)
{
dsd2pcm_ctx* p2;
p2 = (dsd2pcm_ctx*) malloc(sizeof(dsd2pcm_ctx));
if (p2) {
memcpy(p2,ptr,sizeof(dsd2pcm_ctx));
}
return p2;
}
extern void dsd2pcm_reset(dsd2pcm_ctx* ptr)
{
int i;
for (i=0; i<FIFOSIZE; ++i)
ptr->fifo[i] = 0x69; /* my favorite silence pattern */
ptr->fifopos = 0;
/* 0x69 = 01101001
* This pattern "on repeat" makes a low energy 352.8 kHz tone
* and a high energy 1.0584 MHz tone which should be filtered
* out completely by any playback system --> silence
*/
}
extern void dsd2pcm_translate(
dsd2pcm_ctx* ptr,
size_t samples,
const unsigned char *src, ptrdiff_t src_stride,
int lsbf,
float *dst, ptrdiff_t dst_stride)
{
unsigned ffp;
unsigned i;
unsigned bite1, bite2;
unsigned char* p;
double acc;
ffp = ptr->fifopos;
lsbf = lsbf ? 1 : 0;
while (samples-- > 0) {
bite1 = *src & 0xFFu;
if (lsbf) bite1 = bit_reverse(bite1);
ptr->fifo[ffp] = bite1; src += src_stride;
p = ptr->fifo + ((ffp-CTABLES) & FIFOMASK);
*p = bit_reverse(*p);
acc = 0;
for (i=0; i<CTABLES; ++i) {
bite1 = ptr->fifo[(ffp -i) & FIFOMASK] & 0xFF;
bite2 = ptr->fifo[(ffp-(CTABLES*2-1)+i) & FIFOMASK] & 0xFF;
acc += ctables[i][bite1] + ctables[i][bite2];
}
*dst = (float)acc; dst += dst_stride;
ffp = (ffp + 1) & FIFOMASK;
}
ptr->fifopos = ffp;
}
|
