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ac3dec: simplify zero-bit mantissa dithering by calculating it

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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
This commit is contained in:
Justin Ruggles 2009-08-05 02:30:34 +00:00
parent b972c06a8f
commit 25dcd1823c
1 changed files with 10 additions and 21 deletions
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31
libavcodec/ac3dec.c
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@ -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;
}
}
}