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

[Rostislav Pehlivanov]

On 2 September 2016 at 02:59, Jonathan Campbell <
jonathan at impactstudiopro.com> wrote:

> 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 },
>
> _______________________________________________
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> ffmpeg-devel at ffmpeg.org
> http://ffmpeg.org/mailman/listinfo/ffmpeg-devel
>

Any random noise, hardcoded constant or not sill sounds like white noise.
What is the point of this patch?