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

Simon Brown simon.k.brown at gmail.com
Tue May 18 01:34:45 EEST 2021

> On Fri, May 14, 2021 at 14:31:42 +0100, Simon Brown wrote:
> > Hi,
> > I have a mpeg2 transport stream with video as H264 and audio as AAC LC.
> If
> > I use the following command to convert it to fragmented MP4 by just
> copying
> > the encoded data, then the result is now AAC, and not AAC LC.  If
> instead I
> > re-encode with AAC asking for profile:a aac_low then I get AAC LC.  But
> if
> > the input source is AAC LC why would it change the output type to AAC?
> >
> > ffmpeg.exe -f mpegts -fflags +nobuffer+nofillin -probesize 5000000 -i
> > soc_udp_rx_02.ts -c:a copy -bsf:a aac_adtstoasc -c:v copy -f mp4
> > -frag_duration 80000 -movflags +empty_moov+default_base_moof -metadata
> > title="media source exentions" testaudio.mp4
> This shouldn't change anything in the AAC stream, unless the bitstream
> filter is capable of ruining it.
> What does it say about the original? (Nothing, presumably, because it's
> MPEG-TS.) What does ffmpeg say about the output file? Do you have any
> other tool which can use to check?
> Cheers,
> Moritz
> Hi Moritz,
Thank you for your reply.  If it is the bitstream filter could you point me
to the relevant source file that is responsible for this filter?

mp4info doesn't deal with TS, as you surmise.  FFMpeg reports the output
file as AAC (not LC).  I'm not sure VLC gives that detail.  But given that
the ffmpeg processor and the mp4info program both seem to concur when there
is or isn't AAC-LC I'm not sure there is much value in trying a third

I will download the latest prebuilt binaries for Windows and give it a try
with those.  I'd still be interested in the source file responsible as the
version on the embedded system I'm using is not the most recent build, and
rebuilding for that isn't something I want to do at this stage.


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