[FFmpeg-devel] [PATCH] lavc/ac3dec: disable DRC by default

[Paul B Mahol]

On 1/29/20, rcombs <rcombs at rcombs.me> wrote:
> This issue has been argued before, with the status quo being preserved under
> the logic that the spec says this parameter is supposed to default to 1,
> and that we should follow the spec. The spec was misguided, and thus so was
> blindly following it.
>
> The (E-)AC3 DRC architecture is a clever optimization: the (relatively)
> heavy
> lifting of analyzing the audio data and calculating the per-block gain
> needed
> is offloaded to the encoder, so that the player only needs to apply the gain
> coefficients (at the desired scale) to get the DRC effect. In theory, this
> seems like an efficient way to reduce the computational complexity of
> playback.
> In practice, this isn't actually the case.
>
> Actual (E-)AC3 is not guaranteed to have useful DRC metadata at all. For
> instance, ffmpeg's encoder has never supported generating it. This means
> that
> a user who wants DRC can't merely use the internal filter for (E-)AC3 and an
> external one for other codecs; they have to use an external filter at al
> times! They could also apply the internal filter, but it's hard to see what
> benefit this would give.
>
> I've also seen it argued that (E-)AC3 DRC does a better job of reproducing
> the
> intent of the audio mastering engineer than playback-time DRC can. I don't
> believe this argument is credible either. The official encoder does support
> passing in custom gain values, but it doesn't appear that this capability is
> frequently used. I'm not aware of any tooling that actually passes custom
> data
> into the encoder's loudness parameters, and Dolby's first-party GUI tools
> don't
> appear to support doing so. Dolby's own tools expose the parameters
> available in
> the encoder itself, which comprise a set of 5 presets with no
> customizability:
> "Film Standard", "Film Light", "Music Standard", "Music Light", and
> "Speech",
> along with the null profile. These profiles are documented (minus their time
> constants) in this PDF on the Dolby website:
> https://www.dolby.com/us/en/technologies/a-guide-to-dolby-metadata.pdf
> These are not advanced filters; they don't provide any complex control to
> the
> content producer, who can't even tune the details or _know_ the time
> constants.
> Even if the mastering engineer did want to customize the DRC with some
> custom
> tool, in most cases I don't think they'd have the ability to do so because
> they
> don't have access to the point in the pipeline where audio encoding occurs.
> In TV, audio is often encoded live rather than streaming a pre-compressed
> bitstream. In web streaming, masters are usually delivered with uncompressed
> PCM audio and no loudness metadata, and encoded using stock configurations.
> Film for DVD or Blu-Ray is the only plausible case, but again, I'm not aware
> of any widely-used tooling for this with any more configurability than
> Dolby's.
>
> Additional context: the AudioToolbox (E-)AC3 decoder (which is Dolby's)
> seems
> not to apply DRC; you can see this by comparing the output of ffmpeg's
> decoder
> with drc_scale set to 0 and 1 with the output of the (e)ac3_at decoder.
>
> So to summarize:
> - (E-)AC3 DRC is designed as an optimization for the decoder
> - There's no inherent reason (E-)AC3 _requires_ DRC for playback more than
> any
>   other codec
> - Due to inconsistent encoding, it can't be relied upon
> - There's no reason to believe it's better than decode-side DRC when it
> works
> - It's not universally applied by other software
>
> I think this makes a compelling case to stop applying (E-)AC3 DRC by
> default.
> The user can enable it on their own if they prefer, but I believe having it
> off
> by default will improve the apparent quality of (E-)AC3 playback for the
> vast
> majority of ffmpeg users, and improve the consistency between (E-)AC3
> playback
> and playback of any other codec. Thus, this patch.
> ---
>  libavcodec/ac3dec_fixed.c | 2 +-
>  libavcodec/ac3dec_float.c | 2 +-
>  2 files changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/libavcodec/ac3dec_fixed.c b/libavcodec/ac3dec_fixed.c
> index bd66175d50..312616edfd 100644
> --- a/libavcodec/ac3dec_fixed.c
> +++ b/libavcodec/ac3dec_fixed.c
> @@ -169,7 +169,7 @@ static void ac3_downmix_c_fixed16(int16_t **samples,
> int16_t **matrix,
>
>  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 },
> +    { "drc_scale", "percentage of dynamic range compression to apply",
> OFFSET(drc_scale), AV_OPT_TYPE_FLOAT, {.dbl = 0.0}, 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 b85a4ce336..69bd47aa6a 100644
> --- a/libavcodec/ac3dec_float.c
> +++ b/libavcodec/ac3dec_float.c
> @@ -33,7 +33,7 @@
>
>  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 },
> +    { "drc_scale", "percentage of dynamic range compression to apply",
> OFFSET(drc_scale), AV_OPT_TYPE_FLOAT, {.dbl = 0.0}, 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 },
>
> --
> 2.24.1
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Sure that fate does not need update after this?

Rest LGTM.