#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 unsigned char bitreverse[256];
static int precalculated = 0;
static void precalc()
{
int t, e, m, k;
double acc;
if (precalculated) return;
for (t=0, e=0; t<256; ++t) {
bitreverse[t] = e;
for (m=128; m && !((e^=m)&m); m>>=1)
;
}
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()
{
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 = bitreverse[bite1];
ptr->fifo[ffp] = bite1; src += src_stride;
p = ptr->fifo + ((ffp-CTABLES) & FIFOMASK);
*p = bitreverse[*p & 0xFF];
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;
}
