AAC audio not Constant

Samus · 2021-11-29, 03:03 PM

When transcoding audio streams as AAC constant bitrate, TotalCode doesn't listen and only outputs variable.

**Frank Schoenberger** · 2021-11-30, 07:37 AM

Hello Samus,

Can you please tell me what preset you have used?

Moreover, can you share some details about the source file (video codec, audio codec, container, framerate, resolution, etc.)

Thanks.

Samus · 2021-11-30, 08:21 PM

Completely unnecessary. Please just do your own encode of a minute or two, and inspect with MediaInfo...doesn't matter which preset. All AAC transcodes are wrong.

**Frank Schoenberger** · 2021-12-02, 11:53 AM

Hello Samus,

Thanks for the additional note. I can indeed reproduce it. I had a chat with the team and it seems that this is basically not an issue in TotalCode Studio.

There’s no such thing as Constant bitrate for AAC, just approximation. Mediainfo sets an arbitrary limit for bitrate variance and makes a decision on the type based on this. So Mediainfo finds the average size of all the AAC frames, and if any vary in size by more than about 3%, it flags it as Variable. However, they will all vary in size though because that’s how AAC works. You can see this because regardless whether you use the MainConcept or the Fraunhofer AAC Encoder in TotalCode Studio, Mediainfo always display "Variable".

From the experience in the past we know that there is a bug in Mediainfo. CBR is constant bit rate, but not constant frame size. But MediaInfo treats CBR as constant frame size.