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

@@ -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;
             }
         }
     }