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Fix the downsampling noise problem

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Description: The patch replaces the nearest neighbor downsampling method
in the sample rate conversion plugin by linear interpolation of the closest
two samples, thus significantly reducing the resampling noise.

Signed-off-by: Markus Grabner <grabner@icg.tu-graz.ac.at>
This commit is contained in:
Markus Grabner 2004-09-16 10:02:12 +00:00 • committed by Takashi Iwai
parent cc17f92972
commit 3a8bfd43d7
1 changed files with 11 additions and 2 deletions
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13
src/pcm/pcm_rate.c
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@ -45,7 +45,8 @@ const char *_snd_module_pcm_rate = "";
#ifndef DOC_HIDDEN #ifndef DOC_HIDDEN
/* LINEAR_DIV needs to be large enough to handle resampling from 192000 -> 8000 */ /* LINEAR_DIV needs to be large enough to handle resampling from 192000 -> 8000 */
#define LINEAR_DIV (1<<19) #define LINEAR_DIV_SHIFT 19
#define LINEAR_DIV (1<<LINEAR_DIV_SHIFT)
enum rate_type { enum rate_type {
RATE_TYPE_LINEAR, /* linear interpolation */ RATE_TYPE_LINEAR, /* linear interpolation */
@ -206,9 +207,12 @@ static void snd_pcm_rate_shrink(const snd_pcm_channel_area_t *dst_areas,
const char *src; const char *src;
char *dst; char *dst;
int src_step, dst_step; int src_step, dst_step;
int16_t old_sample = 0;
int16_t new_sample = 0;
int old_weight, new_weight;
sum = states->u.linear.sum; sum = states->u.linear.sum;
//int16_t old_sample = states->u.linear.old_sample; //int16_t old_sample = states->u.linear.old_sample;
pos = LINEAR_DIV/2; /* Start at 0.5 */ pos = LINEAR_DIV - get_increment; /* Force first sample to be copied */
states->u.linear.init = 0; states->u.linear.init = 0;
src = snd_pcm_channel_area_addr(src_area, src_offset); src = snd_pcm_channel_area_addr(src_area, src_offset);
dst = snd_pcm_channel_area_addr(dst_area, dst_offset); dst = snd_pcm_channel_area_addr(dst_area, dst_offset);
@ -223,11 +227,15 @@ static void snd_pcm_rate_shrink(const snd_pcm_channel_area_t *dst_areas,
#include "plugin_ops.h" #include "plugin_ops.h"
#undef GET16_END #undef GET16_END
after_get: after_get:
new_sample = sample;
src += src_step; src += src_step;
src_frames1++; src_frames1++;
pos += get_increment; pos += get_increment;
if (pos >= LINEAR_DIV) { if (pos >= LINEAR_DIV) {
pos -= LINEAR_DIV; pos -= LINEAR_DIV;
old_weight = (pos << (32 - LINEAR_DIV_SHIFT)) / (get_increment >> (LINEAR_DIV_SHIFT - 16));
new_weight = 0x10000 - old_weight;
sample = (old_sample * old_weight + new_sample * new_weight) >> 16;
goto *put; goto *put;
#define PUT16_END after_put #define PUT16_END after_put
#include "plugin_ops.h" #include "plugin_ops.h"
@ -237,6 +245,7 @@ static void snd_pcm_rate_shrink(const snd_pcm_channel_area_t *dst_areas,
dst_frames1++; dst_frames1++;
assert(dst_frames1 <= dst_frames); assert(dst_frames1 <= dst_frames);
} }
old_sample = new_sample;
} }
states->u.linear.sum = sum; states->u.linear.sum = sum;