low bitrate mp3 encoding result in vbr instead of cbr

[mboe78]

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)

[mboe78]

Workaround found
Adding "-write_xing 0" solved the problem.