[FFmpeg-user] ffmpeg create wrong time information

[Cecil Westerhof]

I have the recording of a zoom meeting where I need to cut out a
portion from and the first little bit has to be blacked out.

I use the following command:
    ffmpeg -y                                    \
        -ss 00:26:05                             \
        -to 00:44:04                             \
        -i input.mp4                             \
        -vf "
             drawbox=enable='between(t, 0, 1.6)' :
                color     = black                :
                w         = in_w                 :
                h         = in_h                 :
                thickness = fill
        "                                        \
        -acodec copy                             \
        -vcodec libx264                          \
        -crf 23                                  \
        -preset veryfast                         \
        output.mkv

This creates a seemingly correct file, with one little problem: the
time starts at 00:00:27 and ends at 00:18:26. (As displayed by the
player mpv.)

When I play it with vlc I find out that the 27 seconds of audio are
before the video. Mpv starts with the video, but vlc starts with the
audio.
What could be the problem?


I am using ffmpeg version 4.3.3-0+deb11u.


The metadata of the input file:
Input #0, mov,mp4,m4a,3gp,3g2,mj2, from 'GMT20211216-184321_Recording_640x360.mp4':
  Metadata:
    major_brand     : mp42
    minor_version   : 0
    compatible_brands: isommp42
    creation_time   : 2021-12-16T18:43:21.000000Z
  Duration: 01:21:57.47, start: 0.000000, bitrate: 646 kb/s
    Chapter #0:0: start 0.000000, end 4917.440000
    Metadata:
      title           : Recording Started
    Stream #0:0(und): Audio: aac (LC) (mp4a / 0x6134706D), 32000 Hz, mono, fltp, 126 kb/s (default)
    Metadata:
      creation_time   : 2021-12-16T18:43:21.000000Z
      handler_name    : AAC audio
    Stream #0:1(und): Video: h264 (High) (avc1 / 0x31637661), yuv420p, 640x360, 518 kb/s, 25 fps, 25 tbr, 30k tbn, 60k tbc (default)
    Metadata:
      creation_time   : 2021-12-16T18:43:21.000000Z
      handler_name    : H.264/AVC video
      encoder         : AVC Coding
    Stream #0:2(und): Data: bin_data (text / 0x74786574)
    Metadata:
      creation_time   : 2021-12-16T18:43:21.000000Z
      handler_name    : Text

The metadata of the output file:
Input #0, matroska,webm, from 'howToReachYourNewYearsResolutionsIn2022.mkv':
  Metadata:
    COMPATIBLE_BRANDS: isommp42
    MAJOR_BRAND     : mp42
    MINOR_VERSION   : 0
    ENCODER         : Lavf58.45.100
  Duration: 00:18:26.34, start: 0.000000, bitrate: 346 kb/s
    Chapter #0:0: start 0.000000, end 3352.440000
    Metadata:
      title           : Recording Started
    Stream #0:0: Video: h264, none, 640x360, 25 fps, 25 tbr, 1k tbn, 2k tbc (default)
    Metadata:
      HANDLER_NAME    : H.264/AVC video
      ENCODER         : Lavc58.91.100 libx264
      DURATION        : 00:18:26.336000000
    Stream #0:1: Audio: aac (LC), 32000 Hz, mono, fltp (default)
    Metadata:
      HANDLER_NAME    : AAC audio
      DURATION        : 00:18:26.336000000


By the way when I create a mp4 file (which is not really an option,
because I need to create chapters), the video seems to be correct,
with one strange thing: both mpv and vlc think it is 00:55:52 long.
Which it is not: it stops where it should 00:17:59.

-- 
Cecil Westerhof
Senior Software Engineer
LinkedIn: http://www.linkedin.com/in/cecilwesterhof