[FFmpeg-user] non-zero start after transcoding

[egoeimi at mail.ru]

 hello,

i have a file I want to transcode:

Input #0, mov,mp4,m4a,3gp,3g2,mj2, from 'input/BM_1_2014-06-28_1345_C0003.mov':
  Metadata:
    creation_time   : 2014-06-28 08:45:30
    timecode        : 13:45:30:00
  Duration: 00:28:38.60, start: 0.000000, bitrate: 185819 kb/s
    Stream #0:0(eng): Data: none (tmcd / 0x64636D74) (default)
    Metadata:
      creation_time   : 2014-06-28 08:45:30
      handler_name    : Apple Alias Data Handler
      timecode        : 13:45:30:00
    Stream #0:1(eng): Video: prores (apch / 0x68637061), yuv422p10le, 1920x1080, 183460 kb/s, SAR 1:1 DAR 16:9, 25 fps, 25 tbr, 2500 tbn, 2500 tbc (default)
    Metadata:
      creation_time   : 2014-06-28 08:45:30
      handler_name    : Apple Alias Data Handler
      timecode        : 13:45:30:00
    Stream #0:2(eng): Audio: pcm_s24le (in24 / 0x34326E69), 48000 Hz, stereo, s32, 2304 kb/s (default)
    Metadata:
      creation_time   : 2014-06-28 08:45:30
      handler_name    : Apple Alias Data Handler
      timecode        : 13:45:30:00

i'm trying to make a sample:

ffmpeg -t 00:00:15 -i input/BM_1_2014-06-28_1345_C0003.mov -vcodec h264 -preset slow -pix_fmt yuv420p -acodec libvorbis -ac 1 -ar 44100 yuv420p.mkv

and getting this:

Input #0, matroska,webm, from 'yuv420p.mkv':
  Metadata:
    TIMECODE        : 13:45:30:00
    ENCODER         : Lavf55.33.101
  Duration: 00:00:15.08, start: 0.077000, bitrate: 1626 kb/s
    Stream #0:0(eng): Video: h264 (High), yuv420p, 1920x1080 [SAR 1:1 DAR 16:9], 25 fps, 25 tbr, 1k tbn, 50 tbc (default)
    Metadata:
      CREATION_TIME   : 2014-06-28 08:45:30
      LANGUAGE        : eng
      HANDLER_NAME    : Apple Alias Data Handler
      TIMECODE        : 13:45:30:00
    Stream #0:1(eng): Audio: vorbis, 44100 Hz, mono, fltp (default)
    Metadata:
      CREATION_TIME   : 2014-06-28 08:45:30
      LANGUAGE        : eng
      HANDLER_NAME    : Apple Alias Data Handler
      TIMECODE        : 13:45:30:00

why does the start become non-zero? how to keep it at zero?

thank you.