[FFmpeg-user] Problem about duration value of converted mp3 file

myounggun jang jinmi80 at gmail.com
Thu Sep 3 05:23:58 EEST 2020 

Thank you for your interest

The contents requested for confirmation have been retested and confirmed.
This is the result of recording a wav file on an Android device and
converting it on Windows PC.

The length of the original file is 26:39, and the result of converting it
to the default option is 25:47, which is displayed in Windows Explorer and
the file size is 4,686KB.
If this is converted using the -b:a 96k option, it has the same length as
the original 26:39 and the file size is 18,740KB.

I checked and played both the original file and the converted file using
ocenaudio SW, it marked as 26:39 and played.
However, the converted file by default is displayed in the time of 25:47 in
Media Player and played.

Below is the console output I tested.
I don't find any difference.

=========================================================================================================
defalut converting test
=========================================================================================================

ffmpeg.exe -i 1.wav 1_default.mp3
ffmpeg version git-2020-08-28-ccc7120 Copyright (c) 2000-2020 the FFmpeg
developers                                       built with gcc 10.2.1
(GCC) 20200805
  configuration: --enable-gpl --enable-version3 --enable-sdl2
--enable-fontconfig --enable-gnutls --enable-iconv --enable-libass
--enable-libdav1d --enable-libbluray --enable-libfreetype
--enable-libmp3lame --enable-libopencore-amrnb --enable-libopencore-amrwb
--enable-libopenjpeg --enable-libopus --enable-libshine --enable-libsnappy
--enable-libsoxr --enable-libsrt --enable-libtheora --enable-libtwolame
--enable-libvpx --enable-libwavpack --enable-libwebp --enable-libx264
--enable-libx265 --enable-libxml2 --enable-libzimg --enable-lzma
--enable-zlib --enable-gmp --enable-libvidstab --enable-libvmaf
--enable-libvorbis --enable-libvo-amrwbenc --enable-libmysofa
--enable-libspeex --enable-libxvid --enable-libaom --enable-libgsm
--enable-librav1e --enable-libsvtav1 --disable-w32threads --enable-libmfx
--enable-ffnvcodec --enable-cuda-llvm --enable-cuvid --enable-d3d11va
--enable-nvenc --enable-nvdec --enable-dxva2 --enable-avisynth
--enable-libopenmpt --enable-amf
  libavutil      56. 58.100 / 56. 58.100
  libavcodec     58.101.100 / 58.101.100
  libavformat    58. 51.101 / 58. 51.101
  libavdevice    58. 11.101 / 58. 11.101
  libavfilter     7. 87.100 /  7. 87.100
  libswscale      5.  8.100 /  5.  8.100
  libswresample   3.  8.100 /  3.  8.100
  libpostproc    55.  8.100 / 55.  8.100
Guessed Channel Layout for Input Stream #0.0 : mono
Input #0, wav, from '1.wav':
  Duration: 00:26:39.00, bitrate: 256 kb/s
    Stream #0:0: Audio: pcm_s16le ([1][0][0][0] / 0x0001), 16000 Hz, mono,
s16, 256 kb/s
Stream mapping:
  Stream #0:0 -> #0:0 (pcm_s16le (native) -> mp3 (libmp3lame))
Press [q] to stop, [?] for help
Output #0, mp3, to '1_default.mp3':
  Metadata:
    TSSE            : Lavf58.51.101
    Stream #0:0: Audio: mp3 (libmp3lame), 16000 Hz, mono, s16p
    Metadata:
      encoder         : Lavc58.101.100 libmp3lame
size=    4685kB time=00:26:39.01 bitrate=  24.0kbits/s speed= 426x
video:0kB audio:4685kB subtitle:0kB other streams:0kB global headers:0kB
muxing overhead: 0.004690%

=========================================================================================================
96k converting test
=========================================================================================================

ffmpeg.exe -i 1.wav -b:a 96k 1_96k.mp3
ffmpeg version git-2020-08-28-ccc7120 Copyright (c) 2000-2020 the FFmpeg
developers
  built with gcc 10.2.1 (GCC) 20200805
  configuration: --enable-gpl --enable-version3 --enable-sdl2
--enable-fontconfig --enable-gnutls --enable-iconv --enable-libass
--enable-libdav1d --enable-libbluray --enable-libfreetype
--enable-libmp3lame --enable-libopencore-amrnb --enable-libopencore-amrwb
--enable-libopenjpeg --enable-libopus --enable-libshine --enable-libsnappy
--enable-libsoxr --enable-libsrt --enable-libtheora --enable-libtwolame
--enable-libvpx --enable-libwavpack --enable-libwebp --enable-libx264
--enable-libx265 --enable-libxml2 --enable-libzimg --enable-lzma
--enable-zlib --enable-gmp --enable-libvidstab --enable-libvmaf
--enable-libvorbis --enable-libvo-amrwbenc --enable-libmysofa
--enable-libspeex --enable-libxvid --enable-libaom --enable-libgsm
--enable-librav1e --enable-libsvtav1 --disable-w32threads --enable-libmfx
--enable-ffnvcodec --enable-cuda-llvm --enable-cuvid --enable-d3d11va
--enable-nvenc --enable-nvdec --enable-dxva2 --enable-avisynth
--enable-libopenmpt --enable-amf
  libavutil      56. 58.100 / 56. 58.100
  libavcodec     58.101.100 / 58.101.100
  libavformat    58. 51.101 / 58. 51.101
  libavdevice    58. 11.101 / 58. 11.101
  libavfilter     7. 87.100 /  7. 87.100
  libswscale      5.  8.100 /  5.  8.100
  libswresample   3.  8.100 /  3.  8.100
  libpostproc    55.  8.100 / 55.  8.100
Guessed Channel Layout for Input Stream #0.0 : mono
Input #0, wav, from '1.wav':
  Duration: 00:26:39.00, bitrate: 256 kb/s
    Stream #0:0: Audio: pcm_s16le ([1][0][0][0] / 0x0001), 16000 Hz, mono,
s16, 256 kb/s
Stream mapping:
  Stream #0:0 -> #0:0 (pcm_s16le (native) -> mp3 (libmp3lame))
Press [q] to stop, [?] for help
Output #0, mp3, to '1_96k.mp3':
  Metadata:
    TSSE            : Lavf58.51.101
    Stream #0:0: Audio: mp3 (libmp3lame), 16000 Hz, mono, s16p, 96 kb/s
    Metadata:
      encoder         : Lavc58.101.100 libmp3lame
size=   18740kB time=00:26:39.01 bitrate=  96.0kbits/s speed= 397x
video:0kB audio:18739kB subtitle:0kB other streams:0kB global headers:0kB
muxing overhead: 0.002486%


2020년 9월 2일 (수) 오전 8:08, Edward Park <kumowoon1025 at gmail.com>님이 작성:

> Hi,
>
> > When converting a wav file to MP3 using the default option, an error
> occurs
> > in the length.
> > Converted using the following command
> >
> > ffmpeg.exe -i 1.wav 1.mp3
> >
> > The duration of the original wav is 1:09:30, but the length of the
> > converted MP3 is 1:07:16.
> > The length of the file was checked through Windows Explorer and Windows
> > Media Player.
> > However, when checking with ocen audio and other software, it is normally
> > displayed as 1:09:30.
>
> That's more of a compromise than an error, encoder is most likely lame,
> and in ffmpeg would use vbr by default with a command like that. So to get
> accurate duration you pretty much need to decode, and it appears windows
> media player estimates instead. If there is a difference when you playback
> with a stopwatch in one hand that would be strange.
>
> > When I tested using the -b:a option, 64k and 96k are converted to the
> same
> > length, but there is a problem with 32k and 48k.
> >
> > ffmpeg.exe -i sample_2.wav -b:a 96k sample_2_96.mp3
> >
> > In addition, when converting m4a files to MP3, a problem occurs also in
> 96k.
> > Please help me on what to fix or give options.
>
> I think setting the bitrate makes it encode at cbr and that makes it
> possible to determine the duration more accurately but not sure why
> different bitrates gives different results.
>
> Actually how confident are you about the accuracy of the input file
> duration that you are making these comparisons to?
>
> Regards,
> Ted Park
>
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