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

Sat Feb 1 21:50:17 EET 2020

On 2/1/2020 4:34 PM, rcombs 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 +-
>  tests/fate/ac3.mak        | 32 ++++++++++++++++----------------
>  3 files changed, 18 insertions(+), 18 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 },
>  
> diff --git a/tests/fate/ac3.mak b/tests/fate/ac3.mak
> index bb02d3829d..c810b53ad2 100644
> --- a/tests/fate/ac3.mak
> +++ b/tests/fate/ac3.mak
> @@ -1,67 +1,67 @@
>  FATE_AC3 += fate-ac3-2.0
> -fate-ac3-2.0: CMD = pcm -i $(TARGET_SAMPLES)/ac3/monsters_inc_2.0_192_small.ac3
> +fate-ac3-2.0: CMD = pcm -drc_scale 1 -i $(TARGET_SAMPLES)/ac3/monsters_inc_2.0_192_small.ac3
>  fate-ac3-2.0: REF = $(SAMPLES)/ac3/monsters_inc_2.0_192_small_v2.pcm
>  
>  FATE_AC3 += fate-ac3-4.0
> -fate-ac3-4.0: CMD = pcm -i $(TARGET_SAMPLES)/ac3/millers_crossing_4.0.ac3
> +fate-ac3-4.0: CMD = pcm -drc_scale 1 -i $(TARGET_SAMPLES)/ac3/millers_crossing_4.0.ac3
>  fate-ac3-4.0: REF = $(SAMPLES)/ac3/millers_crossing_4.0_v2.pcm
>  
>  #request_channel_layout 4 -> front channel
>  FATE_AC3 += fate-ac3-4.0-downmix-mono
> -fate-ac3-4.0-downmix-mono: CMD = pcm -request_channel_layout 4 -i $(TARGET_SAMPLES)/ac3/millers_crossing_4.0.ac3
> +fate-ac3-4.0-downmix-mono: CMD = pcm -drc_scale 1 -request_channel_layout 4 -i $(TARGET_SAMPLES)/ac3/millers_crossing_4.0.ac3
>  fate-ac3-4.0-downmix-mono: REF = $(SAMPLES)/ac3/millers_crossing_4.0_mono_v2.pcm
>  
>  #request_channel_layout 3 -> left channel + right channel
>  FATE_AC3 += fate-ac3-4.0-downmix-stereo
> -fate-ac3-4.0-downmix-stereo: CMD = pcm -request_channel_layout 3 -i $(TARGET_SAMPLES)/ac3/millers_crossing_4.0.ac3
> +fate-ac3-4.0-downmix-stereo: CMD = pcm -drc_scale 1 -request_channel_layout 3 -i $(TARGET_SAMPLES)/ac3/millers_crossing_4.0.ac3
>  fate-ac3-4.0-downmix-stereo: REF = $(SAMPLES)/ac3/millers_crossing_4.0_stereo_v2.pcm
>  
>  FATE_AC3 += fate-ac3-5.1
> -fate-ac3-5.1: CMD = pcm -i $(TARGET_SAMPLES)/ac3/monsters_inc_5.1_448_small.ac3
> +fate-ac3-5.1: CMD = pcm -drc_scale 1 -i $(TARGET_SAMPLES)/ac3/monsters_inc_5.1_448_small.ac3
>  fate-ac3-5.1: REF = $(SAMPLES)/ac3/monsters_inc_5.1_448_small_v2.pcm
>  
>  FATE_AC3 += fate-ac3-5.1-downmix-mono
> -fate-ac3-5.1-downmix-mono: CMD = pcm -request_channel_layout 4 -i $(TARGET_SAMPLES)/ac3/monsters_inc_5.1_448_small.ac3
> +fate-ac3-5.1-downmix-mono: CMD = pcm -drc_scale 1 -request_channel_layout 4 -i $(TARGET_SAMPLES)/ac3/monsters_inc_5.1_448_small.ac3
>  fate-ac3-5.1-downmix-mono: REF = $(SAMPLES)/ac3/monsters_inc_5.1_448_small_mono_v2.pcm
>  
>  FATE_AC3 += fate-ac3-5.1-downmix-stereo
> -fate-ac3-5.1-downmix-stereo: CMD = pcm -request_channel_layout 3 -i $(TARGET_SAMPLES)/ac3/monsters_inc_5.1_448_small.ac3
> +fate-ac3-5.1-downmix-stereo: CMD = pcm -drc_scale 1 -request_channel_layout 3 -i $(TARGET_SAMPLES)/ac3/monsters_inc_5.1_448_small.ac3
>  fate-ac3-5.1-downmix-stereo: REF = $(SAMPLES)/ac3/monsters_inc_5.1_448_small_stereo_v2.pcm
>  
>  FATE_AC3 += fate-ac3-fixed-2.0
> -fate-ac3-fixed-2.0: CMD = pcm -c ac3_fixed -i $(TARGET_SAMPLES)/ac3/monsters_inc_2.0_192_small.ac3
> +fate-ac3-fixed-2.0: CMD = pcm -drc_scale 1 -c ac3_fixed -i $(TARGET_SAMPLES)/ac3/monsters_inc_2.0_192_small.ac3
>  fate-ac3-fixed-2.0: REF = $(SAMPLES)/ac3/monsters_inc_2.0_192_small_v2.pcm
>  
>  FATE_AC3 += fate-ac3-fixed-4.0-downmix-mono
> -fate-ac3-fixed-4.0-downmix-mono: CMD = pcm -c ac3_fixed -request_channel_layout 4 -i $(TARGET_SAMPLES)/ac3/millers_crossing_4.0.ac3
> +fate-ac3-fixed-4.0-downmix-mono: CMD = pcm -drc_scale 1 -c ac3_fixed -request_channel_layout 4 -i $(TARGET_SAMPLES)/ac3/millers_crossing_4.0.ac3
>  fate-ac3-fixed-4.0-downmix-mono: REF = $(SAMPLES)/ac3/millers_crossing_4.0_mono_v2.pcm
>  
>  FATE_AC3 += fate-ac3-fixed-5.1-downmix-mono
> -fate-ac3-fixed-5.1-downmix-mono: CMD = pcm -c ac3_fixed -request_channel_layout 4 -i $(TARGET_SAMPLES)/ac3/monsters_inc_5.1_448_small.ac3
> +fate-ac3-fixed-5.1-downmix-mono: CMD = pcm -drc_scale 1 -c ac3_fixed -request_channel_layout 4 -i $(TARGET_SAMPLES)/ac3/monsters_inc_5.1_448_small.ac3
>  fate-ac3-fixed-5.1-downmix-mono: REF = $(SAMPLES)/ac3/monsters_inc_5.1_448_small_mono_v2.pcm
>  
>  FATE_AC3 += fate-ac3-fixed-5.1-downmix-stereo
> -fate-ac3-fixed-5.1-downmix-stereo: CMD = pcm -c ac3_fixed -request_channel_layout 3 -i $(TARGET_SAMPLES)/ac3/monsters_inc_5.1_448_small.ac3
> +fate-ac3-fixed-5.1-downmix-stereo: CMD = pcm -drc_scale 1 -c ac3_fixed -request_channel_layout 3 -i $(TARGET_SAMPLES)/ac3/monsters_inc_5.1_448_small.ac3
>  fate-ac3-fixed-5.1-downmix-stereo: REF = $(SAMPLES)/ac3/monsters_inc_5.1_448_small_stereo_v2.pcm
>  
>  FATE_EAC3 += fate-eac3-1
> -fate-eac3-1: CMD = pcm -i $(TARGET_SAMPLES)/eac3/csi_miami_5.1_256_spx_small.eac3
> +fate-eac3-1: CMD = pcm -drc_scale 1 -i $(TARGET_SAMPLES)/eac3/csi_miami_5.1_256_spx_small.eac3
>  fate-eac3-1: REF = $(SAMPLES)/eac3/csi_miami_5.1_256_spx_small_v2.pcm
>  
>  FATE_EAC3 += fate-eac3-2
> -fate-eac3-2: CMD = pcm -i $(TARGET_SAMPLES)/eac3/csi_miami_stereo_128_spx_small.eac3
> +fate-eac3-2: CMD = pcm -drc_scale 1 -i $(TARGET_SAMPLES)/eac3/csi_miami_stereo_128_spx_small.eac3
>  fate-eac3-2: REF = $(SAMPLES)/eac3/csi_miami_stereo_128_spx_small_v2.pcm
>  
>  FATE_EAC3 += fate-eac3-3
> -fate-eac3-3: CMD = pcm -i $(TARGET_SAMPLES)/eac3/matrix2_commentary1_stereo_192_small.eac3
> +fate-eac3-3: CMD = pcm -drc_scale 1 -i $(TARGET_SAMPLES)/eac3/matrix2_commentary1_stereo_192_small.eac3
>  fate-eac3-3: REF = $(SAMPLES)/eac3/matrix2_commentary1_stereo_192_small_v2.pcm
>  
>  FATE_EAC3 += fate-eac3-4
> -fate-eac3-4: CMD = pcm -i $(TARGET_SAMPLES)/eac3/serenity_english_5.1_1536_small.eac3
> +fate-eac3-4: CMD = pcm -drc_scale 1 -i $(TARGET_SAMPLES)/eac3/serenity_english_5.1_1536_small.eac3
>  fate-eac3-4: REF = $(SAMPLES)/eac3/serenity_english_5.1_1536_small_v2.pcm
>  
>  FATE_EAC3 += fate-eac3-5
> -fate-eac3-5: CMD = pcm -i $(TARGET_SAMPLES)/eac3/the_great_wall_7.1.eac3
> +fate-eac3-5: CMD = pcm -drc_scale 1 -i $(TARGET_SAMPLES)/eac3/the_great_wall_7.1.eac3
>  fate-eac3-5: REF = $(SAMPLES)/eac3/the_great_wall_7.1.pcm
>  
>  $(FATE_AC3) $(FATE_EAC3): CMP = oneoff

May want to rename these to fate-*-drc-1 and add new tests for drc 0
while at it.
I can create the reference pcm files for them and add them to the fate
sample suit, if you don't have access.