[FFmpeg-user] Help to understand -ac 6 behaviour encoding ProRes to MP4 EAC3

[Paul B Mahol]

On Fri, Mar 8, 2024 at 12:48 PM Joanna White <Joanna.White at bfi.org.uk>
wrote:

> Good morning,
>
> Here at the BFI National Archive I have MP4 transcode scripts which handle
> many different AV source files, converting them into derivative access
> copies, generally mapping two streams of audio to the MP4. Many source
> files with 5.1 and higher are failing naturally, as we don't have
> consistent metadata in the file to aid automation, eg mapping mixed down Lt
> Rt to the MP4 for example.
>
> I'm using -ac 6 with a loudness setting to stop those files with the Left
> total/Right total stereo mix down included from exceeding safe audio levels
> for the MP4 derivative. I've not tested this loudness max addition yet,
> still to research (grateful for advice!).
>
> What I'm finding though is that when using the -ac 6 flag to compress the
> source file into MP4 with eac3 audio codec, that each audio stream shifts
> from a single channel to multiple channels per stream. There is no
> additional audio in these extra channels so the file sounds okay (though
> loud due to inclusion of Lt Rt).
>
> Source file (which does have metadata in this case):
> ProRes mov with 8 audio streams encoded PCM LE signed, 1152kb/s bit rate:
> #1 single channel L
> #2 single channel R
> #3 single channel C
> #4 single channel LFE
> #5 single channel Ls
> #6 single channel Rs
> #7 single channel Lt
> #8 single channel Rr
>
> Generated MP4 file with Enhanced AC-3, 448 kb/s bit rate:
> #1 six channels  L R C LFE Ls Rs
> #2 six channel L R C LFE Ls Rs
> #3 six channel L R C LFE Ls Rs
> #4 six channel L R C LFE Ls Rs
> #5 six channel L R C LFE Ls Rs
> #6 six channel L R C LFE Ls Rs
> #7 six channel L R C LFE Ls Rs
> #8 six channel L R C LFE Ls Rs
>
> It appears the original audio from the ProRes maps to the second channel
> (shown as C, the channels ordered L C R Ls Rs LFE respectively) in each of
> the audio streams in the MP4.
>
> I wondered if I was missing a flag that prevents creation of these
> additional channels though, or is this caused by something like bit-rate
> management in FFmpeg? Possibly it's anticipated behaviour but thought I
> should check so I can feed back to colleagues.
>
> Thanks so much for this excellent email list assistance, it's highly
> appreciated.
>
>
>
MP4 muxer in FFmpeg is of questionable quality.
For custom channel layouts mixing and encoding, one needs to specify custom
mixing, usually with pan filter (or join/split/merge) explicitly.
I really do not know what is default rematrix for 8ch -> 6channels, but it
is usually given in verbose ffmpeg tool output when transcoding.


> Kind regards,
> Joanna
>
>
> Joanna White (she/her)
> Knowledge & Collections Developer
> British Film Institute
>
> BFI National Archive
> Kingshill Way, Berkhamsted HP4 3TP
> joanna.white at bfi.org.uk
> +44(0)7900 228888
>
>
>
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