On Fri, Mar 8, 2024 at 12:48 PM Joanna White <joanna.wh...@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.wh...@bfi.org.uk > +44(0)7900 228888 > > > > _______________________________________________ > ffmpeg-user mailing list > ffmpeg-user@ffmpeg.org > https://ffmpeg.org/mailman/listinfo/ffmpeg-user > > To unsubscribe, visit link above, or email > ffmpeg-user-requ...@ffmpeg.org with subject "unsubscribe". > _______________________________________________ ffmpeg-user mailing list ffmpeg-user@ffmpeg.org https://ffmpeg.org/mailman/listinfo/ffmpeg-user To unsubscribe, visit link above, or email ffmpeg-user-requ...@ffmpeg.org with subject "unsubscribe".
