I finished the consistent noise generation patch for AC-3 decoding.
Set AVOption "cons_noisegen" to 1 (true) to enable it.
Git repository:
https://github.com/joncampbell123/FFmpeg.git
commit dbd086586f0ad1591ea2013293bbb6e4dbfd0455
Author: Jonathan Campbell <jonat...@castus.tv>
Date: Thu Sep 1 18:46:16 2016 -0700
AC-3 consistent noise generation: avopt -cons_noisegen <number>
When -cons_noisegen 1, the linear feedback generator used for AC-3
dithering is seeded with the contents of the AC-3 frame. Seeding from
the AC-3 frame ensures the dithering noise comes out exactly the same
when given the same AC-3 frame data, which can then be used by
non-linear editing software to reliably decode discontinuous
segments of
an AC-3 bitstream without gaps or discontinuities.
Jonathan Campbell
Patch follows, hope Thunderbird doesn't garble it:
diff --git a/libavcodec/ac3dec.c b/libavcodec/ac3dec.c
index fac189b..28048d7 100644
--- a/libavcodec/ac3dec.c
+++ b/libavcodec/ac3dec.c
@@ -1419,6 +1419,19 @@ static int ac3_decode_frame(AVCodecContext *
avctx, void *data,
(const uint16_t *) buf, cnt);
} else
memcpy(s->input_buffer, buf, FFMIN(buf_size,
AC3_FRAME_BUFFER_SIZE));
+
+ /* if consistent noise generation is enabled, seed the linear
feedback generator
+ * with the contents of the AC-3 frame so that the noise is
identical across
+ * decodes given the same AC-3 frame data, for use with non-linear
edititing software. */
+ if (s->consistent_noise_generation) {
+ const AVCRC *avcrc = av_crc_get_table(AV_CRC_32_IEEE);
+
+ if (avcrc != NULL)
+ av_lfg_init(&s->dith_state, av_crc(avcrc, 0,
s->input_buffer, FFMIN(buf_size, AC3_FRAME_BUFFER_SIZE)));
+ else
+ av_log(avctx, AV_LOG_ERROR, "CNG unable to seed from frame");
+ }
+
buf = s->input_buffer;
/* initialize the GetBitContext with the start of valid AC-3 Frame */
if ((ret = init_get_bits8(&s->gbc, buf, buf_size)) < 0)
diff --git a/libavcodec/ac3dec.h b/libavcodec/ac3dec.h
index c2b867e..98184e9 100644
--- a/libavcodec/ac3dec.h
+++ b/libavcodec/ac3dec.h
@@ -177,6 +177,10 @@ typedef struct AC3DecodeContext {
int end_freq[AC3_MAX_CHANNELS]; ///< end frequency
bin (endmant)
///@}
+///@name Consistent noise generation
+ int consistent_noise_generation; ///< seed noise generation
with AC-3 frame on decode
+///@}
+
///@name Rematrixing
int num_rematrixing_bands; ///< number of rematrixing
bands (nrematbnd)
int rematrixing_flags[4]; ///< rematrixing
flags (rematflg)
diff --git a/libavcodec/ac3dec_fixed.c b/libavcodec/ac3dec_fixed.c
index 6416da4..1f79ade 100644
--- a/libavcodec/ac3dec_fixed.c
+++ b/libavcodec/ac3dec_fixed.c
@@ -168,6 +168,7 @@ static void ac3_downmix_c_fixed16(int16_t **samples,
int16_t (*matrix)[2],
#include "ac3dec.c"
static const AVOption options[] = {
+ { "cons_noisegen", "enable consistent noise generation",
OFFSET(consistent_noise_generation), AV_OPT_TYPE_BOOL, {.i64 = 0 }, 0,
1, PAR },
{ "drc_scale", "percentage of dynamic range compression to apply",
OFFSET(drc_scale), AV_OPT_TYPE_FLOAT, {.dbl = 1.0}, 0.0, 6.0, PAR },
{ "heavy_compr", "enable heavy dynamic range compression",
OFFSET(heavy_compression), AV_OPT_TYPE_BOOL, {.i64 = 0 }, 0, 1, PAR },
{ NULL},
diff --git a/libavcodec/ac3dec_float.c b/libavcodec/ac3dec_float.c
index 0a5319a..b85a4ce 100644
--- a/libavcodec/ac3dec_float.c
+++ b/libavcodec/ac3dec_float.c
@@ -32,6 +32,7 @@
#include "ac3dec.c"
static const AVOption options[] = {
+ { "cons_noisegen", "enable consistent noise generation",
OFFSET(consistent_noise_generation), AV_OPT_TYPE_BOOL, {.i64 = 0 }, 0,
1, PAR },
{ "drc_scale", "percentage of dynamic range compression to apply",
OFFSET(drc_scale), AV_OPT_TYPE_FLOAT, {.dbl = 1.0}, 0.0, 6.0, PAR },
{ "heavy_compr", "enable heavy dynamic range compression",
OFFSET(heavy_compression), AV_OPT_TYPE_BOOL, {.i64 = 0 }, 0, 1, PAR },
{ "target_level", "target level in -dBFS (0 not applied)",
OFFSET(target_level), AV_OPT_TYPE_INT, {.i64 = 0 }, -31, 0, PAR },
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