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