All M4A files are considered to be MP4 video files

[pddshk]

Simple as that, FileIO has no explicitly declared m4a extension, but from my small research it infers MP4 from some bytes. However m4a files usually store only audio.

And due to this pure audio m4a file is decoded as video file

[stemann]

Which Julia package would you like to have open the M4A-files other than VideoIO?

If there is another package, then stemann:feature/m4a might help.

[pddshk]

Actually m4a (should be) a container with audio data only, which is encoded in ALAC or AAC.

ALAC can be supported in LibSndFile.jl and I didn't find anything regarding reading binary AAC on the internet (maybe decode via ffmpeg on-the-go as a fallback)

We can check out if the container has only audio box (magic bytes are located at a distance of +8 from hdlr and should be soun) and then locate alac or mp4a at distance of +16 from stsd

[stemann]

Right.

I looked at LibSndFile.jl, but I got the impression that upstream https://github.com/libsndfile/libsndfile explicitly left out some MPEG(-3)-support ... , but I probably hit an ancient URL somewhere... as I can see that there is both MP3 - and ALAC at this point.

AAC seems to not be supported at this point: libsndfile/libsndfile#389

In (the hack) stemann:feature/m4a, I did explicitly experiment with m4a files containing AAC (and they had b"M4A ", where a video file would have b"M4V ").

But it sounds right to also check for b"soun", and codec.

Test file: /usr/local/Homebrew/Library/Homebrew/test/support/fixtures/test.m4a