[FFmpeg-devel] CNG (consistent noise generation) patch for AC-3 decoder

[Jonathan Campbell]

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 <jonathan at 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 },