[FFmpeg-trac] #6288(undetermined:new): Wrong bitrate information on produced .mp3 when using "-acodec copy" on low bitrate input

FFmpeg trac at avcodec.org
Mon Apr 3 14:47:37 EEST 2017


#6288: Wrong bitrate information on produced .mp3 when using "-acodec copy" on low
bitrate input
-------------------------------------+-------------------------------------
             Reporter:  kohlhaas     |                     Type:  defect
               Status:  new          |                 Priority:  normal
            Component:               |                  Version:
  undetermined                       |  unspecified
             Keywords:  mp3 bitrate  |               Blocked By:
             Blocking:               |  Reproduced by developer:  0
Analyzed by developer:  0            |
-------------------------------------+-------------------------------------
 What i try to do: I am trying to get the audio stream from a .flv and save
 it as .mp3 with "-acodec copy". The input file's audio stream is mp3 with
 bitrate less than 56kB/s.

 Problem: When inspecting the output file with VLC media player, it shows a
 bitrate of 56 kB/s. For a input file with audio stream bitrate of 16kB/s,
 the output file showed a bitrate of 32kB/s.
 The actual file size fits the lower bitrate of the input stream.
 I did not get this problem when the input stream had a bitrate of 56kB/s
 or more.

 Consequence: When using the file in a HTML <audio>-Tag, some browsers
 (Safari, maybe others too) estimate a shorter duration than the file
 actually has.

 Command line:

 {{{
 ffmpeg -i input.flv -acodec copy output.mp3
 }}}

 I tried this with several .flv files, also some from the web
 (‚Äčhttp://www.mediacollege.com/adobe/flash/video/tutorial/example-flv.html)
 and also with a .mp3 file.

 verbose -report output:

 {{{
 ffmpeg started on 2017-04-03 at 13:43:55
 Report written to "ffmpeg-20170403-134355.log"
 Command line:
 ffmpeg -v 9 -loglevel 99 -i 20051210-w50s.flv -acodec copy output.mp3
 -report
 ffmpeg version N-84348-gdb7a05d Copyright (c) 2000-2017 the FFmpeg
 developers
   built with gcc 6.3.0 (GCC)
   configuration: --enable-gpl --enable-version3 --enable-cuda --enable-
 cuvid --enable-d3d11va --enable-dxva2 --enable-libmfx --enable-nvenc
 --enable-avisynth --enable-bzlib --enable-fontconfig --enable-frei0r
 --enable-gnutls --enable-iconv --enable-libass --enable-libbluray
 --enable-libbs2b --enable-libcaca --enable-libfreetype --enable-libgme
 --enable-libgsm --enable-libilbc --enable-libmodplug --enable-libmp3lame
 --enable-libopencore-amrnb --enable-libopencore-amrwb --enable-libopenh264
 --enable-libopenjpeg --enable-libopus --enable-librtmp --enable-libsnappy
 --enable-libsoxr --enable-libspeex --enable-libtheora --enable-libtwolame
 --enable-libvidstab --enable-libvo-amrwbenc --enable-libvorbis --enable-
 libvpx --enable-libwavpack --enable-libwebp --enable-libx264 --enable-
 libx265 --enable-libxavs --enable-libxvid --enable-libzimg --enable-lzma
 --enable-zlib
   libavutil      55. 50.100 / 55. 50.100
   libavcodec     57. 83.101 / 57. 83.101
   libavformat    57. 66.105 / 57. 66.105
   libavdevice    57.  3.100 / 57.  3.100
   libavfilter     6. 78.100 /  6. 78.100
   libswscale      4.  3.101 /  4.  3.101
   libswresample   2.  4.100 /  2.  4.100
   libpostproc    54.  2.100 / 54.  2.100
 Splitting the commandline.
 Reading option '-v' ... matched as option 'v' (set logging level) with
 argument '9'.
 Reading option '-loglevel' ... matched as option 'loglevel' (set logging
 level) with argument '99'.
 Reading option '-i' ... matched as input url with argument
 '20051210-w50s.flv'.
 Reading option '-acodec' ... matched as option 'acodec' (force audio codec
 ('copy' to copy stream)) with argument 'copy'.
 Reading option 'output.mp3' ... matched as output url.
 Reading option '-report' ... matched as option 'report' (generate a
 report) with argument '1'.
 Finished splitting the commandline.
 Parsing a group of options: global .
 Applying option v (set logging level) with argument 9.
 Applying option loglevel (set logging level) with argument 99.
 Applying option report (generate a report) with argument 1.
 Successfully parsed a group of options.
 Parsing a group of options: input url 20051210-w50s.flv.
 Successfully parsed a group of options.
 Opening an input file: 20051210-w50s.flv.
 [file @ 00000000004eac40] Setting default whitelist 'file,crypto'
 [flv @ 00000000003285a0] Format flv probed with size=2048 and score=100
 [flv @ 00000000003285a0] Before avformat_find_stream_info() pos: 13 bytes
 read:32768 seeks:0 nb_streams:0
 [flv @ 00000000003285a0] All info found
 [flv @ 00000000003285a0] After avformat_find_stream_info() pos: 55624
 bytes read:65536 seeks:0 frames:103
 Input #0, flv, from '20051210-w50s.flv':
   Metadata:
     audiodelay      : 0
     canSeekToEnd    : 1
     creationdate    : Fri Feb 03 11:52:46 2006
                     :
   Duration: 00:00:16.92, start: 0.000000, bitrate: 316 kb/s
     Stream #0:0, 62, 1/1000: Audio: mp3, 22050 Hz, stereo, s16p, 40 kb/s
     Stream #0:1, 41, 1/1000: Video: vp6f, 1 reference frame, yuv420p,
 360x288 (368x288), 0/1, 266 kb/s, 25 fps, 25 tbr, 1k tbn
 Successfully opened the file.
 Parsing a group of options: output url output.mp3.
 Applying option acodec (force audio codec ('copy' to copy stream)) with
 argument copy.
 Successfully parsed a group of options.
 Opening an output file: output.mp3.
 [file @ 0000000000546f20] Setting default whitelist 'file,crypto'
 Successfully opened the file.
 Output #0, mp3, to 'output.mp3':
   Metadata:
     audiodelay      : 0
     canSeekToEnd    : 1
     creationdate    : Fri Feb 03 11:52:46 2006
                     :
     TSSE            : Lavf57.66.105
     Stream #0:0, 0, 1/1000: Audio: mp3, 22050 Hz, stereo, s16p, 40 kb/s
 Stream mapping:
   Stream #0:0 -> #0:0 (copy)
 Press [q] to stop, [?] for help
 cur_dts is invalid (this is harmless if it occurs once at the start per
 stream)
 size=      60kB time=00:00:12.14 bitrate=  40.3kbits/s speed=24.2x
 No more output streams to write to, finishing.
 size=      83kB time=00:00:16.92 bitrate=  40.2kbits/s speed=24.8x
 video:0kB audio:83kB subtitle:0kB other streams:0kB global headers:0kB
 muxing overhead: 0.385763%
 Input file #0 (20051210-w50s.flv):
   Input stream #0:0 (audio): 649 packets read (84767 bytes);
   Input stream #0:1 (video): 41 packets read (45536 bytes);
   Total: 690 packets (130303 bytes) demuxed
 Output file #0 (output.mp3):
   Output stream #0:0 (audio): 649 packets muxed (84767 bytes);
   Total: 649 packets (84767 bytes) muxed
 0 frames successfully decoded, 0 decoding errors
 [AVIOContext @ 000000000053ffe0] Statistics: 1 seeks, 651 writeouts
 [AVIOContext @ 00000000004f2e00] Statistics: 649512 bytes read, 2 seeks
 }}}

--
Ticket URL: <https://trac.ffmpeg.org/ticket/6288>
FFmpeg <https://ffmpeg.org>
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