[FFmpeg-user] Transcoding with different codec (libfdk_aac) alters duration, how to fix?

Soeren Balko soeren at zfaas.com
Sun Dec 29 13:19:02 CET 2013

I figured that FFmpeg prefixes and suffixes transcoded audio streams with
"mute" intervals for other codecs too. In the same audio, I found that the
prefix/suffix lengths are libfdk_aac: 0.01024/0.01179 sec; libfaac:
0.0/0.01182 sec; mp2: 0.0044/0.00148 sec; ac3: 0.00257/0.00414 sec.

Even though the mute prefixes/suffixes are very different, this seems to be
a generic behavior, which obviously causes problems when concatenating
multiple transcoded audio streams. I wonder if there is a way to stop the
encoders from introducing these mute intervals? Will at the least look into
the libfdk_aac source.

On Fri, Dec 27, 2013 at 11:42 PM, <frafart at free.fr> wrote:

> Hi,
> > This works in principle but causes problems in the resulting output file
> at
> > the boundaries of the joined segments. Specifically, there is an
> > audible "gap" in the audio stream.
> You have the same "problem" than me : AAC protocol induced "Audio priming".
> See :
> https://developer.apple.com/library/mac/documentation/quicktime/qtff/QTFFAppenG/QTFF
> The 0.046s added in the file after AAC encoding must be a bunch of silence
> at the begining.
> You can see my post on this list ("Deal with AAC priming when join file").
> If you find a solution, thank to think to me ! :)
> Fabrice.
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