[FFmpeg-user] stereo downmix error aac HEv2

[Sean Darcy]

On 03/14/2014 07:58 PM, Christian Ebert wrote:
> * Sean Darcy on Friday, March 14, 2014 at 19:10:30 -0400
>> More messing around, and it seems the HEv2 won't handle mono at all!
>
> Have a look at:
> https://trac.ffmpeg.org/ticket/3361
> and:
> http://sourceforge.net/p/opencore-amr/mailman/message/31874638/
>

Thanks for the quick response. Very interesting.

So as I understand it HEv2 is HE + PS. And PS stands for Parametric 
_Stereo_.  I found: 
https://tech.ebu.ch/docs/techreview/trev_305-moser.pdf  which taught me 
more than I ever wanted to know about AAC HE encoding.

So therefore HEv2 only makes sense with stereo.

We should put a note in the wiki not to use AAC-HEv2 with mono.

FWIW, here's the command line I'm now using to downmix to mono at a low 
bitrate:

ffmpeg -i <input> -vn -c:a libfdk_aac \
   -profile:a aac_he -b:a 48k -ac 1 -signaling implicit \
    <output.m4a>

The output is shown as mono:

Output #0, ipod, to 'test-48kmono.m4a':
   Metadata:
     encoder         : Lavf55.34.101
     Stream #0:0: Audio: aac (libfdk_aac) (HE-AAC) (mp4a / 0x6134706D), 
48000 Hz, mono, s16, 48 kb/s

but ffprobe shows the output file as stereo. mediainfo shows mono.

I'm assuming/hoping it's mono.

sean