MP4 muxer: muxing with empty moov fails to signal audio encoder delay

[John Regan]

When generating an mp4 file using -movflags empty_moov - the edit box is missing, which is required to signal the audio encoder delay. As far as I can tell there's no movflag or other muxer option that will add the edit box.

How to reproduce:

% ffmpeg -f lavfi -i anullsrc=r=48000:d=10 -c:a aac -movflags empty_moov fragmented.mp4

If you inspect fragmented.mp4, the Edit Box is missing entirely.

I believe the file should have an Edit Box, containing an Edit List Box with an entry showing the number of samples to discard from the beginning of the audio (in this case for AAC, 1024). Otherwise, decoders have no way of knowing how many priming samples should be discarded.

Version info:

ffmpeg -version
ffmpeg version n6.0 Copyright (c) 2000-2023 the FFmpeg developers
built with gcc 13.1.1 (GCC) 20230429
configuration: --prefix=/usr --disable-debug --disable-static --disable-stripping --enable-amf --enable-avisynth --enable-cuda-llvm --enable-lto --enable-fontconfig --enable-gmp --enable-gnutls --enable-gpl --enable-ladspa --enable-libaom --enable-libass --enable-libbluray --enable-libbs2b --enable-libdav1d --enable-libdrm --enable-libfreetype --enable-libfribidi --enable-libgsm --enable-libiec61883 --enable-libjack --enable-libjxl --enable-libmfx --enable-libmodplug --enable-libmp3lame --enable-libopencore_amrnb --enable-libopencore_amrwb --enable-libopenjpeg --enable-libopenmpt --enable-libopus --enable-libpulse --enable-librav1e --enable-librsvg --enable-libsoxr --enable-libspeex --enable-libsrt --enable-libssh --enable-libsvtav1 --enable-libtheora --enable-libv4l2 --enable-libvidstab --enable-libvmaf --enable-libvorbis --enable-libvpx --enable-libwebp --enable-libx264 --enable-libx265 --enable-libxcb --enable-libxml2 --enable-libxvid --enable-libzimg --enable-nvdec --enable-nvenc --enable-opencl --enable-opengl --enable-shared --enable-version3 --enable-vulkan
libavutil      58.  2.100 / 58.  2.100
libavcodec     60.  3.100 / 60.  3.100
libavformat    60.  3.100 / 60.  3.100
libavdevice    60.  1.100 / 60.  1.100
libavfilter     9.  3.100 /  9.  3.100
libswscale      7.  1.100 /  7.  1.100
libswresample   4. 10.100 /  4. 10.100
libpostproc    57.  1.100 / 57.  1.100