Opened 7 days ago

#6330 new defect

Strange behavior with native FFmpeg AAC encoder, when converting audio to mono

Reported by: edumj Owned by:
Priority: normal Component: undetermined
Version: unspecified Keywords:
Cc: Blocked By:
Blocking: Reproduced by developer: no
Analyzed by developer: no


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 position:

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%)

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