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

[rcombs]

> On Feb 1, 2020, at 13:43, Derek Buitenhuis <derek.buitenhuis at gmail.com> wrote:
> 
> On 01/02/2020 19:34, 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.
> 
> If "I don't agree with the spec" or "this spec does something stupid" were valid
> arguments, we may as well not have specs at all, and FFmpeg would have a lot more
> garbage in it. You can write at length why you think it's bad, but that does not
> negeate the fact that it's the spec,

I'm not sure why the spec should have any bearing on the defaults of configurable options, particularly when those defaults result in unexpected behavior that's unlike that used for any other codec.

> and the defacto way to handle decoding AC-3.

Is it, though? My understanding is that AV receivers usually only enable AC3 DRC in "night mode" or the like, and few even provide a fully-tunable setting for it. Apple's decoder in AudioToolbox (implemented on top of Dolby's) doesn't provide the feature at all (I haven't checked what Win32 Media Foundation does yet). So where does this "de facto" situation come from, other than lavc itself?

Meanwhile, Dolby's own decoder applies digital dialog normalization by default (contrary to how the spec says it should be applied downstream), and I haven't seen anyone arguing that lavc should do that.

Additionally, TrueHD has a very similar DRC feature that lavc doesn't support whatsoever, and nobody seems to mind.

One other thing I neglected to mention in the commit message: the actual audio encoded in an (E)AC3 track often has levels similar to those of a track in another codec (PCM, AAC, etc) from the same or other sources, but sounds very different when both are decoded with lavc. For instance, Netflix's EAC3 sounds about the same as BD audio of the same content when decoded without DRC, but in lavc-based players by default, it sounds, well... compressed. This had led to an impression in the anime community that Netflix applies excessive DRC to their EAC3 streams at encode-time, and thus that those streams were low-quality and should be avoided.

> 
> And I'd rather not give Dolby more ammunition for their anti-FOSS FUD campaign.
> 
> For whatever my opinion is worth nowadays on this list, it's a very, very hard
> NAK from me.
> 
> - Derek
> _______________________________________________
> ffmpeg-devel mailing list
> ffmpeg-devel at ffmpeg.org
> https://ffmpeg.org/mailman/listinfo/ffmpeg-devel
> 
> To unsubscribe, visit link above, or email
> ffmpeg-devel-request at ffmpeg.org with subject "unsubscribe".