[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
mono
-------------------------------------+-------------------------------------
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'''
position:
{{{
Audio
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"
}}}
{{{
Audio
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>
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