[FFmpeg-user] Use ffmpeg to convert stereo audio to mono

[Christian Ebert]

* Moritz Barsnick on Friday, December 05, 2014 at 10:37:14 +0100
>> Could we know how ffmpeg determine audio channel number from MP4 file? MP4
>> container indicates it's channel count is 2, but ffprobe shows mono as
>> expected.
> 
> ffprobe probably looks at the actual AAC stream?
> 
> Here's a hint:
> http://sourceforge.net/p/opencore-amr/mailman/message/31876733/
> 
> "The cause for this is that libavformat always writes 2 as number of 
> channels in the m4a header regardless of the actual stream."
> (This was about HE-AAC, but may be valid here as well.)
> 
> Is this causing any issues except your trust in the nature of the file?
> 
> I must say I'm not sure whether that is actually true, whether it's
> okay for ffmpeg to do so, and whether this is still valid with recent
> ffmpeg. I can confirm the behavior though.
> 
> Moritz
> 
> P.S.: I like adding "other tools" to the confusion, without knowing
> what they look at:
> 
> $ mediainfo audio_48k_mono.mp4 | grep -Fi chann
> Channel(s)                               : 2 channels
> Channel(s)_Original                      : 1 channel
> Channel positions                        : Front: C
> $ mplayer -vo null -ao null audio_48k_mono.mp4 2>/dev/null | grep -F AUDIO
> AUDIO: 44100 Hz, 1 ch, s16le, 48.3 kbit/6.85% (ratio: 6040->88200)
> $ # vlc says: Codec -> Stream 0 -> Channels: Stereo

There's more confusion:
https://trac.ffmpeg.org/ticket/4106

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