[FFmpeg-user] Preserving AAC LC status when converting to fragmented MP4

[Tobias Rapp]

On 14.05.2021 15:31, Simon Brown wrote:
> [...] 
> 
> For although there it says aac (LC) if I run mp4info on the resultant file
> I get this:
> 
> [...]
> 
> Track 2:
>    flags:        3 ENABLED IN-MOVIE
>    id:           2
>    type:         Audio
>    duration: 0 ms
>    language: und
>    media:
>      sample count: 0
>      timescale:    48000
>      duration:     0 (media timescale units)
>      duration:     0 (ms)
>      bitrate (computed): 125.373 Kbps
>      sample count with fragments: 848
>      duration with fragments:     868354
>      duration with fragments:     18091 (ms)
>    Sample Description 0
>      Coding:      mp4a (MPEG-4 Audio)
>      Stream Type: Audio
>      Object Type: MPEG-4 Audio
>      Max Bitrate: 128250
>      Avg Bitrate: 0
>      Buffer Size: 0
>      Codecs String: mp4a.40.0
>      MPEG-4 Audio Object Type: 0 (UNKNOWN)
>      Sample Rate: 48000
>      Sample Size: 16
>      Channels:    2
> 
> whereas if I used -c:a aac -profile:a aac_low instead of -c:a copy then I
> get this from mp4info:
> Track 2:
>    flags:        3 ENABLED IN-MOVIE
>    id:           2
>    type:         Audio
>    duration: 0 ms
>    language: und
>    media:
>      sample count: 0
>      timescale:    48000
>      duration:     0 (media timescale units)
>      duration:     0 (ms)
>      bitrate (computed): 49.589 Kbps
>      sample count with fragments: 849
>      duration with fragments:     869378
>      duration with fragments:     18112 (ms)
>    Sample Description 0
>      Coding:      mp4a (MPEG-4 Audio)
>      Stream Type: Audio
>      Object Type: MPEG-4 Audio
>      Max Bitrate: 128000
>      Avg Bitrate: 0
>      Buffer Size: 0
>      Codecs String: mp4a.40.2
>      MPEG-4 Audio Object Type: 2 (AAC Low Complexity)
>      MPEG-4 Audio Decoder Config:
>        Sampling Frequency: 48000
>        Channels: 2
>        Extension:
>          Object Type: Spectral Band Replication
>          SBR Present: no
>          PS Present:  no
>          Sampling Frequency: 0
>      Sample Rate: 48000
>      Sample Size: 16
>      Channels:    2
> 
> where it clearly states it as Mpeg-4 Audio AAC Low complexity.

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

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.

Regards,
Tobias