On 24.05.2021 22:26, Moritz Barsnick wrote:
On Wed, May 19, 2021 at 12:17:38 +0200, Tobias Rapp wrote:
Looks like the data that should be found in the "esds" atom is missing or
incomplete when using -c:a copy. See this link for details on how AAC
profile information is stored in MP4:
https://stackoverflow.com/questions/3987850/mp4-atom-how-to-discriminate-the-audio-codec-is-it-aac-or-mp3
Interesting stuff.
Don't know enough about TS (de)muxing, though, to find out where this
profile data should be parsed out from the input by FFmpeg.
MPEG-TS and demuxing isn't the problem. If you look at the ffmpeg
output in the original e-mail from decoding the file, it correctly
identifies the codec as "aac (LC)". The issue is the encoding to MP4.
The reason is that the AAC variants have only one codec tag
AV_CODEC_ID_AAC, and that needs to map to various tags, e.g. 0x40 for
AAC and 0x67 for AAC LC.
https://github.com/FFmpeg/FFmpeg/blob/3749eede66c3774799766b1f246afae8a6ffc9bb/libavformat/isom.c#L34
When writing the tag in the esds atom, there's no additional info
available allowing mapping to 0x67, therefore 0x40 is written:
https://github.com/FFmpeg/FFmpeg/blob/3749eede66c3774799766b1f246afae8a6ffc9bb/libavformat/movenc.c#L711
I have no idea how this info could be carried forward.
From the StackOverflow answers the AAC profile seems to be part of the
AudioSpecificConfig (ASC) part which is written as "TAG 5" within the
Elementary Stream Descriptor MP4 atom.
Looking at the FFmpeg movenc.c link you posted it is written some lines
below the codec id from track->vos_data. This pointer is filled from
AVCodecParameters->extradata in ff_mov_write_packet:
https://github.com/FFmpeg/FFmpeg/blob/3749eede66c3774799766b1f246afae8a6ffc9bb/libavformat/movenc.c#L5586
So it seems to be a matter of TS demuxing passing along the codec
extradata correctly.
Regards,
Tobias
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