[bsf filter] Cannot copy eac3_core

[microchip]

Summary of the bug: Using -c:a copy -bsf:a eac3_core on a DD+ + Atmos audio or video file does not strip the Atmos part

How to reproduce:

% ffmpeg -i input -vn -c:a copy -bsf:a eac3_core output.eac3

ffmpeg version N-117812-gbbb0fdedb7
built on 17 november 2024

% ffprobe -i output.eac3 -v quiet -select_streams -show_entries stream=profile

"Dolby Digital Plus + Dolby Atmos"

I can confirm that mediainfo also reports it as DD+ + Atmos

[Hendrik]

That is not the function of the eac3_core filter. Its supposed to be used on Blu-ray style EAC3 streams that contain two streams, either ac3+eac3 or eac3+eac3, one core, and one extension stream.

It will extract only the core stream from such a setup. It does not interact with Atmos.

[microchip]

OK, so I have a remux of a bluray movie (speak no evil, 2024) that has a stream reported as "DD+ with Atmos". You're sying that I can't strip the Atmos part and just get the E-AC-3 with the bsf filter?

[Balling]

Blu-ray style EAC3 streams that contain two streams

Erm, Blu-ray officially only support AC3+EAC3, though there is no issue to use EAC3 1 layer stream for any modern Blu-fay player, like what Netflix uses.

I do not think I ever saw EAC3+EAC3, though it does exist: ​https://vlc-commits.videolan.narkive.com/YQDcpmGt/converter-tospdif-fix-eac3-with-dependent-streams

[Jérôme Martinez]

You're sying that I can't strip the Atmos part and just get the E-AC-3 with the bsf filter?

It depends.
1/ If you have 1 core stream + 1 additional stream with Atmos, the filter strips the additional stream and provides you core (so without Atmos).
2/ If you have 1 core stream with Atmos, the filter does nothing (no additionnal stream to discard) provides core (so with Atmos).

Your case is 2/ and the filter is useful for 1/. i.e. this is not a bug, just not the filter you are looking for.

In practice, if you want to remove Atmos, you have to edit deeply the stream. Technically doable but (I guess) not implemented, and in practice not so useful (you have to keep the same bitrate so you fill Atmos part with padding bits) without complete reencoding (you can still remove "-c:a copy" from your command line if the goal is to reduce the bitrate and if you don't care much about the loss of quality due to re-encoding vs the gain in size).

I do not think I ever saw EAC3+EAC3, though it does exist

I confirm that I have such file... (And MediaInfo also supports them).

[microchip]

OK, then I guess this ticket can be closed :) Thanks to all for making it clear.