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/*
Copyright 2009, 2011 Sebastian Gesemann. All rights reserved.
Copyright 2020 Max Kellermann <max.kellermann@gmail.com>
Redistribution and use in source and binary forms, with or without modification, are
permitted provided that the following conditions are met:
1. Redistributions of source code must retain the above copyright notice, this list of
conditions and the following disclaimer.
2. Redistributions in binary form must reproduce the above copyright notice, this list
of conditions and the following disclaimer in the documentation and/or other materials
provided with the distribution.
THIS SOFTWARE IS PROVIDED BY SEBASTIAN GESEMANN ''AS IS'' AND ANY EXPRESS OR IMPLIED
WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND
FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL SEBASTIAN GESEMANN OR
CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON
ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING
NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF
ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
The views and conclusions contained in the software and documentation are those of the
authors and should not be interpreted as representing official policies, either expressed
or implied, of Sebastian Gesemann.
*/
#include "Dsd2Pcm.hxx"
#include "util/bit_reverse.h"
#include "util/GenerateArray.hxx"
#include <stdlib.h>
#include <string.h>
/** number of FIR constants */
static constexpr size_t HTAPS = 48;
/** number of "8 MACs" lookup tables */
static constexpr size_t CTABLES = (HTAPS + 7) / 8;
static_assert(Dsd2Pcm::FIFOSIZE * 8 >= HTAPS * 2, "FIFOSIZE too small");
/*
* 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 constexpr 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 constexpr float
CalculateCtableValue(size_t t, int k, int e) noexcept
{
double acc = 0;
for (int m = 0; m < k; ++m) {
acc += (((e >> (7 - m)) & 1) * 2 - 1) * htaps[t * 8 + m];
}
return acc;
}
/* this needs to be a struct because GCC 6 doesn't have constexpr
lambdas (C++17) */
struct GenerateCtableValue {
size_t t;
int k;
constexpr auto operator()(int e) const noexcept {
return CalculateCtableValue(t, k, e);
}
};
static constexpr auto
GenerateCtable(int t) noexcept
{
int k = HTAPS - t*8;
if (k>8) k=8;
return GenerateArray<256>(GenerateCtableValue{CTABLES - 1 - t, k});
}
static constexpr auto ctables = GenerateArray<CTABLES>(GenerateCtable);
void
Dsd2Pcm::Reset() noexcept
{
/* my favorite silence pattern */
std::fill_n(fifo, std::size(fifo), 0x69);
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
*/
}
inline void
Dsd2Pcm::ApplySample(size_t ffp, uint8_t src) noexcept
{
fifo[ffp] = src;
uint8_t *p = fifo + ((ffp-CTABLES) & FIFOMASK);
*p = bit_reverse(*p);
}
inline float
Dsd2Pcm::CalcOutputSample(size_t ffp) const noexcept
{
double acc = 0;
for (size_t i = 0; i < CTABLES; ++i) {
unsigned bite1 = fifo[(ffp -i) & FIFOMASK] & 0xFF;
unsigned bite2 = fifo[(ffp-(CTABLES*2-1)+i) & FIFOMASK] & 0xFF;
acc += ctables[i][bite1] + ctables[i][bite2];
}
return acc;
}
void
Dsd2Pcm::Translate(size_t samples,
const uint8_t *src, ptrdiff_t src_stride,
bool lsbf,
float *dst, ptrdiff_t dst_stride) noexcept
{
size_t ffp = fifopos;
while (samples-- > 0) {
unsigned bite1 = *src & 0xFFu;
src += src_stride;
if (lsbf) bite1 = bit_reverse(bite1);
ApplySample(ffp, bite1);
*dst = CalcOutputSample(ffp);
dst += dst_stride;
ffp = (ffp + 1) & FIFOMASK;
}
fifopos = ffp;
}
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