From 25dcd1823c6fd0be6b41a36c4909a2aa78e8096f Mon Sep 17 00:00:00 2001
From: Justin Ruggles <justin.ruggles@gmail.com>
Date: Wed, 5 Aug 2009 02:30:34 +0000
Subject: [PATCH] ac3dec: simplify zero-bit mantissa dithering by calculating
 it conditionally during mantissa decoding, then only removing it from the
 coupling range for coupled channels which do not use dithering.

Originally committed as revision 19588 to svn://svn.ffmpeg.org/ffmpeg/trunk
---
 libavcodec/ac3dec.c | 31 ++++++++++---------------------
 1 file changed, 10 insertions(+), 21 deletions(-)

diff --git a/libavcodec/ac3dec.c b/libavcodec/ac3dec.c
index 80b4decd45..56024bb7a0 100644
--- a/libavcodec/ac3dec.c
+++ b/libavcodec/ac3dec.c
@@ -452,6 +452,7 @@ static void ac3_decode_transform_coeffs_ch(AC3DecodeContext *s, int ch_index, ma
     uint8_t *baps = s->bap[ch_index];
     int8_t *exps = s->dexps[ch_index];
     int *coeffs = s->fixed_coeffs[ch_index];
+    int dither = (ch_index == CPL_CH) || s->dither_flag[ch_index];
     GetBitContext *gbc = &s->gbc;
     int freq;
 
@@ -460,7 +461,10 @@ static void ac3_decode_transform_coeffs_ch(AC3DecodeContext *s, int ch_index, ma
         int mantissa;
         switch(bap){
             case 0:
+                if (dither)
                 mantissa = (av_lfg_get(&s->dith_state) & 0x7FFFFF) - 0x400000;
+                else
+                    mantissa = 0;
                 break;
             case 1:
                 if(m->b1){
@@ -517,33 +521,18 @@ static void ac3_decode_transform_coeffs_ch(AC3DecodeContext *s, int ch_index, ma
 }
 
 /**
- * Remove random dithering from coefficients with zero-bit mantissas
+ * Remove random dithering from coupling range coefficients with zero-bit
+ * mantissas for coupled channels which do not use dithering.
  * reference: Section 7.3.4 Dither for Zero Bit Mantissas (bap=0)
  */
 static void remove_dithering(AC3DecodeContext *s) {
     int ch, i;
-    int end=0;
-    int *coeffs;
-    uint8_t *bap;
 
     for(ch=1; ch<=s->fbw_channels; ch++) {
-        if(!s->dither_flag[ch]) {
-            coeffs = s->fixed_coeffs[ch];
-            bap = s->bap[ch];
-            if(s->channel_in_cpl[ch])
-                end = s->start_freq[CPL_CH];
-            else
-                end = s->end_freq[ch];
-            for(i=0; i<end; i++) {
-                if(!bap[i])
-                    coeffs[i] = 0;
-            }
-            if(s->channel_in_cpl[ch]) {
-                bap = s->bap[CPL_CH];
-                for(; i<s->end_freq[CPL_CH]; i++) {
-                    if(!bap[i])
-                        coeffs[i] = 0;
-                }
+        if(!s->dither_flag[ch] && s->channel_in_cpl[ch]) {
+            for(i = s->start_freq[CPL_CH]; i<s->end_freq[CPL_CH]; i++) {
+                if(!s->bap[CPL_CH][i])
+                    s->fixed_coeffs[ch][i] = 0;
             }
         }
     }