On 1/29/20, rcombs <rco...@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 > _______________________________________________ > ffmpeg-devel mailing list > ffmpeg-devel@ffmpeg.org > https://ffmpeg.org/mailman/listinfo/ffmpeg-devel > > To unsubscribe, visit link above, or email > ffmpeg-devel-requ...@ffmpeg.org with subject "unsubscribe".
Sure that fate does not need update after this? Rest LGTM. _______________________________________________ ffmpeg-devel mailing list ffmpeg-devel@ffmpeg.org https://ffmpeg.org/mailman/listinfo/ffmpeg-devel To unsubscribe, visit link above, or email ffmpeg-devel-requ...@ffmpeg.org with subject "unsubscribe".
