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audio dithering: Fix some problems

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* Replace bit shift with integer division - Shifting doesn't behave
nicely for negative numbers, for example -2 >> 8 equals -1. This
introduces distortion

* Use rounding from 0.5 up - Truncating introduces distortion, because
the interval of numbers that round to 0 is disproportionaly big (-1, 1).

* Replace random number generation with a faster algorithm

With all theese changes, the code is also a bit faster.
This commit is contained in:
Martin Piatka
2022-02-09 11:56:42 +01:00
parent f59ca12da4
commit ef0806da14
1 changed files with 16 additions and 12 deletions
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28
src/audio/utils.cpp
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@@ -206,15 +206,14 @@ void audio_frame_write_desc(struct audio_frame *f, struct audio_desc desc)
}
int32_t downshift_with_dither(int32_t val, int shift){
static thread_local std::uint_fast32_t last_rand = 1;
static thread_local uint32_t last_rand = 0;
const int mask = (1 << shift) - 1;
//Pseudorandom number generation, same parameters as std::minstd_rand
last_rand = (last_rand * 48271) % 2147483647;
int triangle_dither = last_rand & mask;
last_rand = (last_rand * 48271) % 2147483647;
triangle_dither -= last_rand & mask; //triangle probability distribution
//Quick and dirty random number generation (ranqd1)
//Numerical Recipes in C, page 284
last_rand = (last_rand * 1664525) + 1013904223L;
int triangle_dither = last_rand >> (32 - shift);
last_rand = (last_rand * 1664525) + 1013904223L;
triangle_dither -= last_rand >> (32 - shift); //triangle probability distribution
/* Prevent over/underflow when val is big.
*
@@ -222,15 +221,20 @@ int32_t downshift_with_dither(int32_t val, int shift){
* 32-bit pcm is rare and should not contain the value INT32_MIN
* anyway because of symmetry, as specified by AES17 and IEC 61606-3
*/
if(INT32_MAX - abs(val) < mask)
return val >> shift;
if(INT32_MAX - abs(val) < (1 << shift) - 1 + (1 << (shift - 1)))
return val / (1 << shift);
return (val + triangle_dither) >> shift;
if(val > 0)
val += 1 << (shift - 1);
else
val -= 1 << (shift - 1);
return (val + triangle_dither) / (1 << shift);
}
#define NO_DITHER_PARAM "no-dither"
ADD_TO_PARAM(NO_DITHER_PARAM, "* " NO_DITHER_PARAM "\n"
" Disable audio dithering when resampling\n");
" Disable audio dithering when reducing bit depth\n");
void change_bps(char *out, int out_bps, const char *in, int in_bps, int in_len /* bytes */) {
static const bool dither = commandline_params.find(NO_DITHER_PARAM) == commandline_params.end();