diff options
Diffstat (limited to '')
| -rw-r--r-- | src/dsd2pcm/dsd2pcm.c | 188 |
1 files changed, 188 insertions, 0 deletions
diff --git a/src/dsd2pcm/dsd2pcm.c b/src/dsd2pcm/dsd2pcm.c new file mode 100644 index 000000000..31f9a106d --- /dev/null +++ b/src/dsd2pcm/dsd2pcm.c @@ -0,0 +1,188 @@ +#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; +} + |