HLS with fragmented mp4 muxing: audio-only stream not signaling encoder delay

[John Regan]

When producing an audio-only HLS stream with fragmented mp4, the initialization segment has an Edit List box, but the media_time is set to 0. So stream consumers are unaware of any priming samples that need to be discarded.

How to reproduce:

Generate an audio-only hls stream

% ffmpeg -f lavfi -i anullsrc=r=48000:d=10 -c:a aac -f hls -hls_segment_type fmp4 -hls_time 1 hls/stream.m3u8

Then inspect the generated init.mp4 file. Using boxdumper from l-smash, here's the edit list box:

[edts: Edit Box]
    position = 244
    size = 36   
    [elst: Edit List Box]
        position = 252
        size = 28
        version = 0 
        flags = 0x000000
        entry_count = 1
        entry[0]    
            segment_duration = 0
            media_time = 0
            media_rate = 1.000000

In this case, the media_time should be 1024, since that's how many priming samples the native aac encoder adds.

ffmpeg version:

% 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