From 60f07fadd390d4d3c04e723759c32b2a97ec99ca Mon Sep 17 00:00:00 2001
From: Justin Ruggles <justin.ruggles@gmail.com>
Date: Sat, 4 Aug 2007 01:13:08 +0000
Subject: [PATCH] dither zero-bit mantissas by default. remove dithering only
if it's explicitly turned off.
Originally committed as revision 9886 to svn://svn.ffmpeg.org/ffmpeg/trunk
---
libavcodec/ac3dec.c | 54 ++++++++++++++++++++++++++++++++++++---------
1 file changed, 44 insertions(+), 10 deletions(-)
diff --git a/libavcodec/ac3dec.c b/libavcodec/ac3dec.c
index 98e2ad07938..7a3d36b0cd7 100644
--- a/libavcodec/ac3dec.c
+++ b/libavcodec/ac3dec.c
@@ -92,6 +92,7 @@ typedef struct {
int blksw[AC3_MAX_CHANNELS];
int dithflag[AC3_MAX_CHANNELS];
+ int dither_all;
int cplinu;
int chincpl[AC3_MAX_CHANNELS];
int phsflginu;
@@ -460,27 +461,24 @@ typedef struct { /* grouped mantissas for 3-level 5-leve and 11-level quantizati
static int get_transform_coeffs_ch(AC3DecodeContext *ctx, int ch_index, mant_groups *m)
{
GetBitContext *gb = &ctx->gb;
- int i, gcode, tbap, dithflag, start, end;
+ int i, gcode, tbap, start, end;
uint8_t *exps;
uint8_t *bap;
float *coeffs;
if (ch_index >= 0) { /* fbw channels */
- dithflag = ctx->dithflag[ch_index];
exps = ctx->dexps[ch_index];
bap = ctx->bap[ch_index];
coeffs = ctx->transform_coeffs[ch_index + 1];
start = 0;
end = ctx->endmant[ch_index];
} else if (ch_index == -1) {
- dithflag = 0;
exps = ctx->dlfeexps;
bap = ctx->lfebap;
coeffs = ctx->transform_coeffs[0];
start = 0;
end = 7;
} else {
- dithflag = 0;
exps = ctx->dcplexps;
bap = ctx->cplbap;
coeffs = ctx->transform_coeffs_cpl;
@@ -493,12 +491,7 @@ static int get_transform_coeffs_ch(AC3DecodeContext *ctx, int ch_index, mant_gro
tbap = bap[i];
switch (tbap) {
case 0:
- if (!dithflag) {
- coeffs[i] = 0;
- }
- else {
coeffs[i] = (av_random(&ctx->dith_state) & 0xFFFF) * LEVEL_MINUS_3DB;
- }
break;
case 1:
@@ -551,6 +544,39 @@ static int get_transform_coeffs_ch(AC3DecodeContext *ctx, int ch_index, mant_gro
return 0;
}
+/**
+ * Removes random dithering from coefficients with zero-bit mantissas
+ * reference: Section 7.3.4 Dither for Zero Bit Mantissas (bap=0)
+ */
+static void remove_dithering(AC3DecodeContext *ctx) {
+ int ch, i;
+ int end=0;
+ float *coeffs;
+ uint8_t *bap;
+
+ for(ch=1; ch<=ctx->nfchans; ch++) {
+ if(!ctx->dithflag[ch-1]) {
+ coeffs = ctx->transform_coeffs[ch];
+ bap = ctx->bap[ch-1];
+ if(ctx->chincpl[ch-1])
+ end = ctx->cplstrtmant;
+ else
+ end = ctx->endmant[ch-1];
+ for(i=0; i<end; i++) {
+ if(bap[i] == 0)
+ coeffs[i] = 0.0f;
+ }
+ if(ctx->chincpl[ch-1]) {
+ bap = ctx->cplbap;
+ for(; i<ctx->cplendmant; i++) {
+ if(bap[i] == 0)
+ coeffs[i] = 0.0f;
+ }
+ }
+ }
+ }
+}
+
/* Get the transform coefficients.
* This function extracts the tranform coefficients form the ac3 bitstream.
* This function is called after bit allocation is performed.
@@ -592,6 +618,10 @@ static int get_transform_coeffs(AC3DecodeContext * ctx)
}
}
+ /* if any channel doesn't use dithering, zero appropriate coefficients */
+ if(!ctx->dither_all)
+ remove_dithering(ctx);
+
return 0;
}
@@ -708,8 +738,12 @@ static int ac3_parse_audio_block(AC3DecodeContext *ctx, int blk)
for (i = 0; i < nfchans; i++) /*block switch flag */
ctx->blksw[i] = get_bits1(gb);
- for (i = 0; i < nfchans; i++) /* dithering flag */
+ ctx->dither_all = 1;
+ for (i = 0; i < nfchans; i++) { /* dithering flag */
ctx->dithflag[i] = get_bits1(gb);
+ if(!ctx->dithflag[i])
+ ctx->dither_all = 0;
+ }
if (get_bits1(gb)) { /* dynamic range */
dynrng = get_sbits(gb, 8);
--
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