[FFmpeg-user] Create an AAC stream matching the Core Media Audio packet format / priming etc?

Sat Jul 1 22:24:16 EEST 2017

There is already a flag called -use_editlist in ffmpeg MOV muxer
http://git.videolan.org/?p=ffmpeg.git;a=blob;f=libavformat/movenc.c;h=ca389e3a97f92e05b1abfda29aa7bde25e1089a0;hb=HEAD#l88
. I think you can use that along with the audiotoolbox encoder to achieve
what you want.

On Sat, Jul 1, 2017 at 6:56 AM, Mark Burton [via FFmpeg-users] <
ml+s933282n4680447h21 at n4.nabble.com> wrote:

> How about this for a partial solution - ffmpeg supports the audiotoolbox
> aac encoder on macOS. For the sake of being able to create .mov which are
> compatible with the Quicktime format, there is a single change which I
> think could work well, albeit the ‘fix' would be limited to macOS system
> encoders.
>
> Currently a .mov created by ffmpeg when using the aac_at audiotoolbox
> encoder creates a sound track edit list with a media time of 2112. This
> will not work with Quicktime decoders, the file with decode out of sync. If
> this value was 0 however, then the file will decode properly in sync.
>
> For now, would it be possible to change this value to 0 when using the
> aac_at encoder in ffmpeg?
>
> Doing the same for the ffmpeg native aac encoder does help quite a bit,
> about 1/2 frame out of sync as opposed to a full frame.
>
>
>
>
>
> > On 29 Jun 2017, at 18:52, Sasi Inguva <[hidden email]
> <http:///user/SendEmail.jtp?type=node&node=4680447&i=0>> wrote:
> > I am not knowledgeable enough about audio codec implementations, but I
> > don't think we can control an audio codec implementation to follow a
> > certain delay.  For AAC codec for example, I believe libfhg-aac sets
> 1600
> > samples of dealy, while libfdk-aac would set 1024 etc. Some one with
> more
> > knowledge can tag in here.
> >
> > On Thu, Jun 29, 2017 at 10:46 AM, Mark Burton <[hidden email]
> <http:///user/SendEmail.jtp?type=node&node=4680447&i=1>> wrote:
> >
> >>
> >>> On 28 Jun 2017, at 01:12, Sasi Inguva <[hidden email]
> <http:///user/SendEmail.jtp?type=node&node=4680447&i=2>>
> >> wrote:
> >>> I have been helping Mark test Marton's patch. I looked at the test
> file
> >> Mark
> >>> was using to test the sync. There are multiple reasons for audio being
> >>> off-sync.
> >>>
> >>> i) That file doesn't contain a non-zero edit list or 'sgpd' atom, as
> >>> https://developer.apple.com/library/content/documentation/
> >> QuickTime/QTFF/QTFFAppenG/QTFFAppenG.html
> >>> suggests to put. For this kind of file the spec says that, use the
> >>> historical solution of assuming the delay as 2112 samples. And this is
> >> what
> >>> the QuickTime player and iMovie on my MAC seem to be doing. However in
> >>> Ffmpeg we don't assume the delay as 2112 samples. If there is no edit
> >> list,
> >>> we assume it as zero.
> >>> Hence, when we transcode the video using ffmpeg, we are adding 2112
> >> samples
> >>> of silence in the transcoded file ( as actual audio data).
> >>> ii) On top of that ffmpeg AAC encoder itself introduces 1024 samples
> of
> >>> silence, and ffmpeg then uses edit list to denote that as the encoder
> >> delay.
> >>> However the spec says that along with the edit list, we should also
> set
> >> the
> >>> "sgpd" atom, ( which is what Marton's patch does).
> >>>
> >>> So to fix this, I hacked ffmpeg MOV demuxer to assume 2112 delay for
> AAC,
> >>> and combined it with Marton's patch.  I hoped that the file transcoded
> >> from
> >>> ffmpeg built from these two patches, will correctly match the original
> >> test
> >>> file when decoded with Apple tools (iMovie) .
> >>>
> >>> However it was not to be. It seems like even Apple tools don't respect
> >> the
> >>> new way of setting the encoder delay. When I decode the file using
> >> iMovie, I
> >>> observe that 2112 samples from the beginning are gone, indicating that
> >> Apple
> >>> is still assuming 2112 samples of delay for AAC.
> >>>
> >>> I am attaching the original test file, and the file I generated.
> >>> SyncTest_2sec_24p_Compressor_enc.mp4
> >>> <http://www.ffmpeg-archive.org/file/n4680414/SyncTest_
> >> 2sec_24p_Compressor_enc.mp4>
> >>> SyncTestFfmpegAdEdit2112.mp4
> >>> <http://www.ffmpeg-archive.org/file/n4680414/
> >> SyncTestFfmpegAdEdit2112.mp4>
> >>
> >> Thanks Sasi. So the short of this is that its unclear whether it is in
> >> fact possible to create a mov file which Apple tools will decode using
> the
> >> ‘new’ method.
> >>
> >> If Apple tools will always reliably use the 2112 decode method, would
> it
> >> not make more sense to include a new option in ffmpeg to encode in this
> way
> >> (same as Quicktime encoders) and therefore ensure accurate decode of
> the
> >> file in the actual formats decoder? If the current methods are either
> not
> >> fully meeting the spec or are simply being ignored by Quicktime, it
> would
> >> seem that in order to maintain compatibility with the format itself,
> having
> >> a way to encode with 2112 delay would be the most reliable way to go.
> >>
> >> Thanks
> >> Mark
> >> _______________________________________________
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