[FFmpeg-user] forcing bitrate?

[Langdon]

My apologies, the command with output is below.

>*ffmpeg -i source/e0b.mov -vn -b:a 1536k e0b.wav*
ffmpeg version N-38622-g1eabd71 Copyright (c) 2000-2012 the FFmpeg
developers
  built on Mar  7 2012 00:21:47 with gcc 4.6.2
  configuration: --enable-gpl --enable-version3 --disable-w32threads
--enable-runtime-cpudetect --enable-avisynth --enable-bzlib --enable-frei0r
--enable
-libopencore-amrnb --enable-libopencore-amrwb --enable-libfreetype
--enable-libgsm --enable-libmp3lame --enable-libopenjpeg --enable-librtmp
--enable-lib
schroedinger --enable-libspeex --enable-libtheora --enable-libvo-aacenc
--enable-libvo-amrwbenc --enable-libvorbis --enable-libvpx --enable-libx264
--ena
ble-libxavs --enable-libxvid --enable-zlib
  libavutil      51. 42.100 / 51. 42.100
  libavcodec     54. 10.100 / 54. 10.100
  libavformat    54.  2.100 / 54.  2.100
  libavdevice    53.  4.100 / 53.  4.100
  libavfilter     2. 63.100 /  2. 63.100
  libswscale      2.  1.100 /  2.  1.100
  libswresample   0.  7.100 /  0.  7.100
  libpostproc    52.  0.100 / 52.  0.100
Input #0, mov,mp4,m4a,3gp,3g2,mj2, from 'source/e0b.mov':
  Metadata:
    creation_time   : 2012-03-15 16:22:30
  Duration: 00:00:01.66, start: 0.000000, bitrate: 85949 kb/s
*    Stream #0:0(eng): Audio: pcm_s16be (twos / 0x736F7774), 48000 Hz, 2
channels, s16, 1536 kb/s*
    Metadata:
      creation_time   : 2012-03-15 16:22:30
      handler_name    : ?Apple Alias Data Handler
    Stream #0:1(eng): Video: prores (apch / 0x68637061), yuv422p10le,
1280x720, 83140 kb/s, 23.98 fps, 23.98 tbr, 23976 tbn, 23976 tbc
    Metadata:
      creation_time   : 2012-03-15 16:22:30
      handler_name    : ?Apple Alias Data Handler
    Stream #0:2(eng): Data: none (tmcd / 0x64636D74)
    Metadata:
      creation_time   : 2012-03-15 16:22:32
      handler_name    : ?Apple Alias Data Handler
      timecode        : 01:00:18:01
Output #0, wav, to 'e0b.wav':
  Metadata:
    creation_time   : 2012-03-15 16:22:30
    encoder         : Lavf54.2.100
*    Stream #0:0(eng): Audio: pcm_s16le ([1][0][0][0] / 0x0001), 48000 Hz,
2 channels, s16, 1536 kb/s*
    Metadata:
      creation_time   : 2012-03-15 16:22:30
      handler_name    : ?Apple Alias Data Handler
Stream mapping:
  Stream #0:0 -> #0:0 (pcm_s16be -> pcm_s16le)
Press [q] to stop, [?] for help
*size=     313kB time=00:00:01.66 bitrate=1536.2kbits/s*
video:0kB audio:313kB global headers:0kB muxing overhead 0.014361%

Is the "muxing overhead" bit causing it?  I noticed when I do MP3, the
bitrate changes to 320kbps (which I believe is the maximum for MP3), and
the length of the clip noticeably changes to 1.07s (with a higher muxing
overhead).

Output #0, mp3, to 'e0b.mp3':
  Metadata:
    TDEN            : 2012-03-15 16:22:30
    TSSE            : Lavf54.2.100
    Stream #0:0(eng): Audio: mp3, 48000 Hz, 2 channels, s16, 1536 kb/s
    Metadata:
      creation_time   : 2012-03-15 16:22:30
      handler_name    : ?Apple Alias Data Handler
Stream mapping:
  Stream #0:0 -> #0:0 (pcm_s16be -> libmp3lame)
Press [q] to stop, [?] for help
*size=      68kB time=00:00:01.70 bitrate= 324.8kbits/s*
video:0kB audio:67kB global headers:0kB muxing overhead 1.503815%

Again, I'm wondering if this is because the video clip was made, that it
wasn't cut on a GOP boundary (based on
http://ffmpeg-users.933282.n4.nabble.com/inaccurate-splitting-of-PCM-audio-td935314.html
).

I'm ultimately trying to take a sequence of clips (that will vary based on
user input), and compile the variation into a single video.  This is why I
need the seams to be non-existent.

Thanks!




On Tue, Mar 20, 2012 at 8:36 AM, Carl Eugen Hoyos <cehoyos at ag.or.at> wrote:

> Langdon <langdon <at> gmail.com> writes:
>
> > is there a way to force the bitrate when extracting audio from a video?
> >
> > I tried -b:a 1536k, but ffmpeg will force it to 1536.2kbps, which seems
> to
> > affect the duration of the sound clip I get back (seems to add silence to
> > the beginning).
>
> Command line and complete, uncut console output missing.
>
> Carl Eugen
>
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