[FFmpeg-trac] #6330(undetermined:new): Strange behavior with native FFmpeg AAC encoder, when converting audio to mono

FFmpeg trac at avcodec.org
Tue Apr 18 20:15:04 EEST 2017

#6330: Strange behavior with native FFmpeg AAC encoder, when converting audio to
             Reporter:  edumj        |                     Type:  defect
               Status:  new          |                 Priority:  normal
            Component:               |                  Version:
  undetermined                       |  unspecified
             Keywords:               |               Blocked By:
             Blocking:               |  Reproduced by developer:  0
Analyzed by developer:  0            |
 Summary of the bug:

 When converting stereo to mono, even it seems to really convert to mono,
 it's been reported as stereo and with a strange bitrate (not CBR??) by
 MediaInfo and VLC.

 How to reproduce:
 % ffmpeg -i input -c:v libx264 -b:v 750k -profile:v baseline -level 3.0
 -pix_fmt yuv420p -c:a aac -b:a 64k -ac 1 output

 ffmpeg version FFmpeg version: 20170330-ad7aff0

 MediaInfo and '''VLC''' report '''stereo''' audio.

 MediaInfo also reports a strange '''64.5''' kbps bitrate (when I wrote
 64k), with '''2 channels''', '''1 original channel''' and '''Front C'''


 ID                                       : 2
 Format                                   : AAC
 Format/Info                              : Advanced Audio Codec
 Format profile                           : LC
 Codec ID                                 : 40
 Duration                                 : 1min.
 Duration_LastFrame                       : -11ms
 Bit rate mode                            : Constante
 Bit rate                                 : 64,5Kbps
 Channel(s)                               : 2canales
 Channel(s)_Original                      : 1canal
 Channel positions                        : Front: C
 Sampling rate                            : 48,0KHz
 Stream size                              : 472Kib (8%)
 Language                                 : Español

 Testing other bitrates:
 48k -> 48Kbps (Constant)
 64k -> '''64,5Kbps''' (Constant)
 96k -> 96,0Kbps (Constant)
 112k -> '''114Kbps''' (Constant)
 128k -> '''129Kbps''' (Constant)
 192k -> '''159Kbps''' and '''Variable''' (with a max bitrate of 192Kbps)

 So, it makes me think maybe it's not really CBR?

 And althought "-b:a" is supossed to make CBR, when compressing only audio,
 MediaInfo always reports '''Variable Bitrate''', so no bitrate is visible
 with it.

 ffmpeg -i "1min stereo.wav" -c:a aac -b:a 64k -ac 1 "1min 64kb mono.aac"


 Formato                                  : AAC
 Formato/Info                             : Advanced Audio Codec
 Formato de la versión                    : Version 4
 Formato del perfil                       : LC
 Tipo de tasa de bits                     : Variable
 Canal(es)                                : 1canal
 Posiciones del canal                     : Front: C
 Velocidad de muestreo                    : 48,0KHz
 Tamaño de pista                          : 491Kib (100%)

Ticket URL: <https://trac.ffmpeg.org/ticket/6330>
FFmpeg <https://ffmpeg.org>
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