[FFmpeg-user] FDK_AAC OUTPUTS CBR AUDIO STREAM ONLY?!!!

[Moritz Barsnick]

Hi bigshot,
why are you screaming?

On Mon, May 11, 2015 at 16:53:32 -0700, bigshot wrote:
> Hello,
> 
> I have noticed that /libfdk_aac/ in VBR mode spits out CBR audio encodes,
> ONLY, or at least is flagged as CBR in MediaInfo. While encoding there is a
> warning as you can see in the log below.

The warning you see does not say that this option isn't being applied.
It is no longer relevant for newer versions of libfdk_aac, because VBR
encoding modes are now meant to work properly.

> [libfdk_aac @ 00d37440] Note, the VBR setting is unsupported and only works with some parameter combinations

> It seems /-vbr/ switch in /fdk-aac/ will NOT output VBR sound track... Also,
> any other options I select will still produce CBR audio, ONLY.   
> Is this a bug or I am doing smth. wrong?

Is there anything else than mediainfo we can use to confirm this
behavior?

I can indeed see a difference whether I:

a) create a VBR AAC track in M4A:
   $ ffmpeg -f lavfi -i sine -c:a libfdk_aac -vbr 2 -t 5 sine1.m4a -y

b) create a raw VBR AAC, and then remux that to M4A:
   $ ffmpeg -f lavfi -i sine -c:a libfdk_aac -vbr 2 -t 5 sine.aac -y
   $ ffmpeg -i sine.aac -c copy sine2.m4a

sine1.m4a reports
  Overall bit rate mode                    : Constant
  Bit rate mode                            : Constant
sine2.m4a reports
  Overall bit rate mode                    : Variable
  Bit rate mode                            : Variable

If I unmux sine1.m4a to sine1.aac, mediainfo reports:
  Overall bit rate mode                    : Variable
  Bit rate mode                            : Variable
even though the same raw content was previously detected as CBR.

So, I believe either ffmpeg of mediainfo is doing something wrong with
the headers. I would like to have another tool to confirm, though. I
have very little knowledge of MOV/MP4 though. Can AtomicParsley do
this, or does it only see the MPEG headers?

Moritz