[FFmpeg-user] amix with atempo: Inconsistent behaviour creating m4a with aac

[Pavel Koshevoy]

On Sep 13, 2017 4:32 AM, "Jonathan Girven" <jonathan.girven at trickbook.com>
wrote:

I am trying to take the audio from two mp4 files, merge them together one
after the other (perhaps with some other effects such as a fade transition:
not shown), and apply atempo to the result.

I am working with the current FFmpeg git master branch. I have attached an
example file to reproduce the issue with:  BigBuckBunny_320x180_t10.mp4
<http://www.ffmpeg-archive.org/file/t377720/BigBuckBunny_320x180_t10.mp4>  .
And here is the command and output:

$ ffmpeg -y \
  -i BigBuckBunny_320x180_t10.mp4 \
  -i BigBuckBunny_320x180_t10.mp4 \
  -filter_complex "
aevalsrc=0:d=10[na1];[na1][1:a]concat=n=2:v=0:a=1[a1];
[0:a][a1]amix=inputs=2[mix_audio];
[mix_audio]asplit=3[mix_audio0][mix_audio1][mix_audio2];
[mix_audio0]atrim=0:5,asetpts=expr=PTS-STARTPTS[a_trim0];
[mix_audio1]atrim=5:10,asetpts=expr=PTS-STARTPTS[a_trim1];
[mix_audio2]atrim=10:20,asetpts=expr=PTS-STARTPTS[a_trim2];
[a_trim1]atempo=0.5[a_slomo1];
[a_trim0][a_slomo1][a_trim2]concat=n=3:v=0:a=1[com_a_slomo]
" \
  -map [com_a_slomo] \
  -c:a aac \
  output.m4a



I haven't tested such use case, but I can tell you atempo also changes
output pts (computes it based on the number of samples output so far).  So
shouldn't mix_audio2 asetpts expression reference the output of atempo for
STARTPTS?

    Pavel