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

[Jonathan Girven]

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
ffmpeg version N-87286-g6ce4a63 Copyright (c) 2000-2017 the FFmpeg
developers
  built with gcc 4.9.2 (Debian 4.9.2-10)
  configuration: --prefix=/usr/local --extra-cflags=-I/usr/local/include
--extra-ldflags=-L/usr/local/lib --bindir=/usr/local/bin --disable-doc
--disable-static --enable-shared --disable-ffplay --extra-libs=-ldl
--enable-version3 --enable-libfreetype --enable-libx264 --enable-gpl
--enable-openssl --enable-nonfree --disable-debug
  libavutil      55. 74.100 / 55. 74.100
  libavcodec     57.105.100 / 57.105.100
  libavformat    57. 82.100 / 57. 82.100
  libavdevice    57.  8.100 / 57.  8.100
  libavfilter     6.105.100 /  6.105.100
  libswscale      4.  7.103 /  4.  7.103
  libswresample   2.  8.100 /  2.  8.100
  libpostproc    54.  6.100 / 54.  6.100
Input #0, mov,mp4,m4a,3gp,3g2,mj2, from 'BigBuckBunny_320x180_t10.mp4':
  Metadata:
    major_brand     : isom
    minor_version   : 512
    compatible_brands: isomiso2avc1mp41
    title           : Big Buck Bunny
    artist          : Blender Foundation
    composer        : Blender Foundation
    date            : 2008
    encoder         : Lavf57.68.100
  Duration: 00:00:10.01, start: 0.000000, bitrate: 277 kb/s
    Stream #0:0(und): Video: h264 (High) (avc1 / 0x31637661), yuv420p,
320x180 [SAR 1:1 DAR 16:9], 143 kb/s, 24 fps, 24 tbr, 12288 tbn, 48 tbc
(default)
    Metadata:
      handler_name    : VideoHandler
    Stream #0:1(und): Audio: aac (LC) (mp4a / 0x6134706D), 48000 Hz, stereo,
fltp, 127 kb/s (default)
    Metadata:
      handler_name    : SoundHandler
Input #1, mov,mp4,m4a,3gp,3g2,mj2, from 'BigBuckBunny_320x180_t10.mp4':
  Metadata:
    major_brand     : isom
    minor_version   : 512
    compatible_brands: isomiso2avc1mp41
    title           : Big Buck Bunny
    artist          : Blender Foundation
    composer        : Blender Foundation
    date            : 2008
    encoder         : Lavf57.68.100
  Duration: 00:00:10.01, start: 0.000000, bitrate: 277 kb/s
    Stream #1:0(und): Video: h264 (High) (avc1 / 0x31637661), yuv420p,
320x180 [SAR 1:1 DAR 16:9], 143 kb/s, 24 fps, 24 tbr, 12288 tbn, 48 tbc
(default)
    Metadata:
      handler_name    : VideoHandler
    Stream #1:1(und): Audio: aac (LC) (mp4a / 0x6134706D), 48000 Hz, stereo,
fltp, 127 kb/s (default)
    Metadata:
      handler_name    : SoundHandler
Stream mapping:
  Stream #0:1 (aac) -> amix:input0
  Stream #1:1 (aac) -> concat:in1:a0
  concat -> Stream #0:0 (aac)
Press [q] to stop, [?] for help
Output #0, ipod, to 'output.m4a':
  Metadata:
    major_brand     : isom
    minor_version   : 512
    compatible_brands: isomiso2avc1mp41
    title           : Big Buck Bunny
    artist          : Blender Foundation
    composer        : Blender Foundation
    date            : 2008
    encoder         : Lavf57.82.100
    Stream #0:0: Audio: aac (LC) (mp4a / 0x6134706D), 48000 Hz, stereo,
fltp, 128 kb/s (default)
    Metadata:
      encoder         : Lavc57.105.100 aac
[Parsed_aevalsrc_0 @ 0x244b680] EOF timestamp not reliable
[aac @ 0x2473880] Queue input is backward in time
    Last message repeated 1 times
[ipod @ 0x2474920] Non-monotonous DTS in output stream 0:0; previous:
717824, current: -442721857768310647; changing to 717825. This may result in
incorrect timestamps in the output file.
[ipod @ 0x2474920] Non-monotonous DTS in output stream 0:0; previous:
717825, current: -442721857768309623; changing to 717826. This may result in
incorrect timestamps in the output file.
[aac @ 0x2473880] Queue input is backward in time
... <repeated combination of above two lines many times> ...
size=     399kB time=00:00:14.98 bitrate= 218.0kbits/s speed=22.4x    
video:0kB audio:393kB subtitle:0kB other streams:0kB global headers:0kB
muxing overhead: 1.401464%
[aac @ 0x2473880] Qavg: 1475.414

I would expect output.m4a to be 25 seconds long: 5 seconds normal playback,
10 seconds atempo=0.5, 10 seconds normal. Instead it is 15 seconds long and
doesn't contain the last 10 seconds of normal playback.




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