[FFmpeg-trac] #6549(ffmpeg:new): low bitrate mp3 encoding result in vbr instead of cbr

[FFmpeg]

#6549: low bitrate mp3 encoding result in vbr instead of cbr
-------------------------------------+-------------------------------------
             Reporter:  mboe78       |                     Type:  defect
               Status:  new          |                 Priority:  normal
            Component:  ffmpeg       |                  Version:  git-
             Keywords:  CBR mp3      |  master
  libmp3lame                         |               Blocked By:
             Blocking:               |  Reproduced by developer:  0
Analyzed by developer:  0            |
-------------------------------------+-------------------------------------
 Summary of the bug:
 Regarding to Bug #431 encoding of low bitrate mp3 using ffmpeg results in
 VBR and not in CBR. Everything encoded lower than 64kbit is affected.
 Everything higher 64kbit will result in CBR.

 How to reproduce:

 {{{
 ffmpeg -i "$infile" -vn -acodec libmp3lame -ac 1 -b:a 32k recode.mp3

 }}}


 {{{

 mediainfo recode.mp3
 General
 Complete name                            : recode.mp3
 Format                                   : MPEG Audio
 File size                                : 114 KiB
 Overall bit rate mode                    : Variable
 Album                                    : XXXXX
 Part/Position                            : 01
 Part/Total                               : 01
 Track name                               : XXXX
 Track name/Position                      : 01
 Track name/Total                         : 01
 Performer                                : XXXX
 Recorded date                            : UTC 2017-07-24 00:00:00.0
 Writing library                          : Lavf56.40.101
 ISRC                                     : GBUM71605388

 Audio
 Format                                   : MPEG Audio
 Format version                           : Version 1
 Format profile                           : Layer 3
 Bit rate mode                            : Variable
 Channel(s)                               : 1 channel
 Sampling rate                            : 32.0 kHz
 Compression mode                         : Lossy
 Stream size                              : 113 KiB (100%)

 }}}

 versions affected:
 3.1.4
 3.3.2
 git-snapshot (2017/07/21)

--
Ticket URL: <https://trac.ffmpeg.org/ticket/6549>
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