ffmpeg - duração diferente de entrada e saída

Eu tenho um problema de duração estranho ao converter vídeos de mp4 para flv. Também há um "start_time" diferente, o que é isso? O que significa start_time?

input duration - 0:00:29.577000
input start_time - 0:00:00.000000

output duration - 0:00:51.518000
output start_time - 0:00:21.840000

Comando ffmpeg:

ffmpeg -y -i 18579-1386758783533.mp4 -c:a libfdk_aac -b:a 128k -ar 22050 -c:v libx264 -pix_fmt yuv420p -s 1920x1080 -framerate 30 v18579-1386758783533.flv

FFprobe - arquivo de entrada:

dead@dead-work-desktop:~/36369/wowzaoutput$ ffprobe 18579-1386758783533.mp4 
ffprobe version git-2013-11-29-0d640c0 Copyright (c) 2007-2013 the FFmpeg developers
  built on Nov 29 2013 11:54:34 with gcc 4.6 (Ubuntu/Linaro 4.6.3-1ubuntu5)
  configuration: --extra-libs=-ldl --enable-gpl --enable-libass --enable-libfdk-aac --enable-libmp3lame --enable-libopencore-amrnb --enable-libopencore-amrwb --enable-libspeex --enable-libtheora --enable-libvorbis --enable-libopus --enable-libvpx --enable-libx264 --enable-nonfree --enable-version3
  libavutil      52. 55.100 / 52. 55.100
  libavcodec     55. 44.100 / 55. 44.100
  libavformat    55. 21.102 / 55. 21.102
  libavdevice    55.  5.102 / 55.  5.102
  libavfilter     3. 91.100 /  3. 91.100
  libswscale      2.  5.101 /  2.  5.101
  libswresample   0. 17.104 /  0. 17.104
  libpostproc    52.  3.100 / 52.  3.100
[mov,mp4,m4a,3gp,3g2,mj2 @ 0x39cb280] decoding for stream 0 failed
Input #0, mov,mp4,m4a,3gp,3g2,mj2, from '18579-1386758783533.mp4':
  Metadata:
    major_brand     : f4v 
    minor_version   : 0
    compatible_brands: isommp42m4v 
    creation_time   : 2013-12-11 10:46:22
  Duration: 00:00:29.58, start: 0.000000, bitrate: 134 kb/s
    Stream #0:0(eng): Video: h264 (Baseline) (avc1 / 0x31637661), yuv420p(tv), 640x360 [SAR 1:1 DAR 16:9], 121 kb/s, 6.53 fps, 30 tbr, 90k tbn, 60 tbc (default)
    Metadata:
      creation_time   : 2013-12-11 10:46:22
      handler_name    : WowzaMediaServerPro
    Stream #0:1(eng): Audio: speex (spex / 0x78657073), 16000 Hz, mono, s16, 11 kb/s (default)
    Metadata:
      creation_time   : 2013-12-11 10:46:22
      handler_name    : WowzaMediaServerPro

FFprobe - arquivo de saída:

dead@dead-work-desktop:~/36369/wowzaoutput$ ffprobe v18579-1386758783533.flv
ffprobe version git-2013-11-29-0d640c0 Copyright (c) 2007-2013 the FFmpeg developers
  built on Nov 29 2013 11:54:34 with gcc 4.6 (Ubuntu/Linaro 4.6.3-1ubuntu5)
  configuration: --extra-libs=-ldl --enable-gpl --enable-libass --enable-libfdk-aac --enable-libmp3lame --enable-libopencore-amrnb --enable-libopencore-amrwb --enable-libspeex --enable-libtheora --enable-libvorbis --enable-libopus --enable-libvpx --enable-libx264 --enable-nonfree --enable-version3
  libavutil      52. 55.100 / 52. 55.100
  libavcodec     55. 44.100 / 55. 44.100
  libavformat    55. 21.102 / 55. 21.102
  libavdevice    55.  5.102 / 55.  5.102
  libavfilter     3. 91.100 /  3. 91.100
  libswscale      2.  5.101 /  2.  5.101
  libswresample   0. 17.104 /  0. 17.104
  libpostproc    52.  3.100 / 52.  3.100
Input #0, flv, from 'v18579-1386758783533.flv':
  Metadata:
    major_brand     : f4v 
    minor_version   : 0
    compatible_brands: isommp42m4v 
    encoder         : Lavf55.21.100
  Duration: 00:00:51.52, start: 21.840000, bitrate: 366 kb/s
    Stream #0:0: Video: h264 (High), yuv420p, 1920x1080 [SAR 1:1 DAR 16:9], 30.33 tbr, 1k tbn, 60 tbc
    Stream #0:1: Audio: aac, 22050 Hz, mono, fltp, 128 kb/s

Saída completa da linha de comando ffmpeg:

dead@dead-work-desktop:~/36369/wowzaoutput$ ffmpeg -y -i 18579-1386758783533.mp4 -c:a libfdk_aac -b:a 128k -ar 22050 -c:v libx264 -pix_fmt yuv420p -s 1920x1080 -framerate 30 1v18579-1386758783533.flv
ffmpeg version git-2013-11-29-0d640c0 Copyright (c) 2000-2013 the FFmpeg developers
  built on Nov 29 2013 11:54:34 with gcc 4.6 (Ubuntu/Linaro 4.6.3-1ubuntu5)
  configuration: --extra-libs=-ldl --enable-gpl --enable-libass --enable-libfdk-aac --enable-libmp3lame --enable-libopencore-amrnb --enable-libopencore-amrwb --enable-libspeex --enable-libtheora --enable-libvorbis --enable-libopus --enable-libvpx --enable-libx264 --enable-nonfree --enable-version3
  libavutil      52. 55.100 / 52. 55.100
  libavcodec     55. 44.100 / 55. 44.100
  libavformat    55. 21.102 / 55. 21.102
  libavdevice    55.  5.102 / 55.  5.102
  libavfilter     3. 91.100 /  3. 91.100
  libswscale      2.  5.101 /  2.  5.101
  libswresample   0. 17.104 /  0. 17.104
  libpostproc    52.  3.100 / 52.  3.100
[mov,mp4,m4a,3gp,3g2,mj2 @ 0x29f3b00] decoding for stream 0 failed
Input #0, mov,mp4,m4a,3gp,3g2,mj2, from '18579-1386758783533.mp4':
  Metadata:
    major_brand     : f4v 
    minor_version   : 0
    compatible_brands: isommp42m4v 
    creation_time   : 2013-12-11 10:46:22
  Duration: 00:00:29.58, start: 0.000000, bitrate: 134 kb/s
    Stream #0:0(eng): Video: h264 (Baseline) (avc1 / 0x31637661), yuv420p(tv), 640x360 [SAR 1:1 DAR 16:9], 121 kb/s, 6.53 fps, 30 tbr, 90k tbn, 60 tbc (default)
    Metadata:
      creation_time   : 2013-12-11 10:46:22
      handler_name    : WowzaMediaServerPro
    Stream #0:1(eng): Audio: speex (spex / 0x78657073), 16000 Hz, mono, s16, 11 kb/s (default)
    Metadata:
      creation_time   : 2013-12-11 10:46:22
      handler_name    : WowzaMediaServerPro
[libx264 @ 0x29f8f60] using SAR=1/1
[libx264 @ 0x29f8f60] using cpu capabilities: MMX2 SSE2Fast SSSE3 SSE4.2 AVX
[libx264 @ 0x29f8f60] profile High, level 4.0
[libx264 @ 0x29f8f60] 264 - core 140 r2 1ca7bb9 - H.264/MPEG-4 AVC codec - Copyleft 2003-2013 - http://www.videolan.org/x264.html - options: cabac=1 ref=3 deblock=1:0:0 analyse=0x3:0x113 me=hex subme=7 psy=1 psy_rd=1.00:0.00 mixed_ref=1 me_range=16 chroma_me=1 trellis=1 8x8dct=1 cqm=0 deadzone=21,11 fast_pskip=1 chroma_qp_offset=-2 threads=6 lookahead_threads=1 sliced_threads=0 nr=0 decimate=1 interlaced=0 bluray_compat=0 constrained_intra=0 bframes=3 b_pyramid=2 b_adapt=1 b_bias=0 direct=1 weightb=1 open_gop=0 weightp=2 keyint=250 keyint_min=25 scenecut=40 intra_refresh=0 rc_lookahead=40 rc=crf mbtree=1 crf=23.0 qcomp=0.60 qpmin=0 qpmax=69 qpstep=4 ip_ratio=1.40 aq=1:1.00
Output #0, flv, to '1v18579-1386758783533.flv':
  Metadata:
    major_brand     : f4v 
    minor_version   : 0
    compatible_brands: isommp42m4v 
    encoder         : Lavf55.21.102
    Stream #0:0(eng): Video: h264 (libx264) ([7][0][0][0] / 0x0007), yuv420p, 1920x1080 [SAR 1:1 DAR 16:9], q=-1--1, 1k tbn, 30 tbc (default)
    Metadata:
      creation_time   : 2013-12-11 10:46:22
      handler_name    : WowzaMediaServerPro
    Stream #0:1(eng): Audio: aac (libfdk_aac) ([10][0][0][0] / 0x000A), 22050 Hz, mono, s16, 128 kb/s (default)
    Metadata:
      creation_time   : 2013-12-11 10:46:22
      handler_name    : WowzaMediaServerPro
Stream mapping:
  Stream #0:0 -> #0:0 (h264 -> libx264)
  Stream #0:1 -> #0:1 (libspeex -> libfdk_aac)
Press [q] to stop, [?] for help
frame=  193 fps= 16 q=-1.0 Lsize=    2300kB time=00:00:29.60 bitrate= 636.4kbits/s    
video:1821kB audio:464kB subtitle:0 global headers:0kB muxing overhead 0.648628%
[libx264 @ 0x29f8f60] frame I:3     Avg QP:15.65  size: 31001
[libx264 @ 0x29f8f60] frame P:110   Avg QP:22.60  size: 12905
[libx264 @ 0x29f8f60] frame B:80    Avg QP:25.85  size:  4399
[libx264 @ 0x29f8f60] consecutive B-frames: 24.9% 56.0% 10.9%  8.3%
[libx264 @ 0x29f8f60] mb I  I16..4: 17.6% 76.5%  5.9%
[libx264 @ 0x29f8f60] mb P  I16..4:  6.9% 17.2%  0.2%  P16..4: 22.4%  3.4%  1.2%  0.0%  0.0%    skip:48.7%
[libx264 @ 0x29f8f60] mb B  I16..4:  0.4%  0.5%  0.0%  B16..8: 22.2%  1.0%  0.1%  direct: 1.4%  skip:74.3%  L0:45.4% L1:52.8% BI: 1.8%
[libx264 @ 0x29f8f60] 8x8 transform intra:71.0% inter:91.3%
[libx264 @ 0x29f8f60] coded y,uvDC,uvAC intra: 16.3% 19.8% 0.9% inter: 4.7% 7.2% 0.0%
[libx264 @ 0x29f8f60] i16 v,h,dc,p: 23% 17%  5% 54%
[libx264 @ 0x29f8f60] i8 v,h,dc,ddl,ddr,vr,hd,vl,hu: 51% 14% 22%  2%  2%  2%  2%  3%  1%
[libx264 @ 0x29f8f60] i4 v,h,dc,ddl,ddr,vr,hd,vl,hu: 48% 18% 12%  3%  7%  4%  3%  3%  1%
[libx264 @ 0x29f8f60] i8c dc,h,v,p: 69% 13% 17%  2%
[libx264 @ 0x29f8f60] Weighted P-Frames: Y:0.0% UV:0.0%
[libx264 @ 0x29f8f60] ref P L0: 75.4%  4.7% 14.1%  5.8%
[libx264 @ 0x29f8f60] ref B L0: 88.6% 10.9%  0.5%
[libx264 @ 0x29f8f60] ref B L1: 98.2%  1.8%
[libx264 @ 0x29f8f60] kb/s:504.48

Link do arquivo de entrada

Fiz o download do seu exemplo de vídeo .mp4 e ele é muito ruim, a menos que eu mude o tipo de arquivo para .f4v. O vídeo permanece congelado por algum tempo enquanto o som é reproduzido. Após cerca de 21 segundos, o vídeo começa a se mover e sincroniza com o som.

Parece-me que a má conversão obtida reflete apenas o estado real do seu vídeo.

Observo que a diferença de output duration - output start_timeé a mesma antes e depois da conversão, pelo que parece que o ffmpeg apenas refletiu na sua conversão o problema de apresentação existente encontrado no início deste vídeo.

Eu sugiro que você encontre outro método de gravação na sua webcam, porque evidentemente o seu método e software atuais geram arquivos de vídeo inválidos. Também acho estranho que o arquivo .mp4 seja codificado usando o codec Flash de f4v.