Fix the downsampling noise problem

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>

diff --git a/src/pcm/pcm_rate.c b/src/pcm/pcm_rate.c
index a6bc6443be8bd0fecabfa0954b31e43732f8d75e..27fe4aa520b22c3861ea3e90619e64fa5d694a97 100644 (file)
--- a/src/pcm/pcm_rate.c
+++ b/src/pcm/pcm_rate.c
@@ -45,7 +45,8 @@ const char *_snd_module_pcm_rate = "";
 #ifndef DOC_HIDDEN
 
 /* 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 {
        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;
                char *dst;
                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;
                //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;
                src = snd_pcm_channel_area_addr(src_area, src_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"
 #undef GET16_END
                after_get:
+                       new_sample = sample;
                        src += src_step;
                        src_frames1++;
                        pos += get_increment;
                        if (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;
 #define PUT16_END after_put
 #include "plugin_ops.h"
@@ -237,6 +245,7 @@ static void snd_pcm_rate_shrink(const snd_pcm_channel_area_t *dst_areas,
                                dst_frames1++;
                                assert(dst_frames1 <= dst_frames);
                        }
+                       old_sample = new_sample;
                }
 
                states->u.linear.sum = sum;