dsd2pcm.c

#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;
}