Re: AAC Constant Bit Rate coding

[Neil Waterman <neil.waterman@xxxxxxxxxxxx>]

Even when AAC is listed as CBR, the bitrate will still change. I like to think of it as quasi-ABR in that you pick a 128kbps CBR bitrate but the bitrate in the file can change +-3-4kbps while the overall average bitrate will remain at about 128kbps.

I have used the Nero CODEC > http://www.nero.com/eng/technologies-aac-codec.html

I don't have time to look at the options for VBR versus CBR for Nero right now...

In general, with any lossy encoder, VBR is always a better choice than CBR. AAC is no different here. Nero's AAC encoder performs much better in a VBR environment. The iTunes AAC encoder dosn't benefit too much from VBR implementations but that is because Apple restricted it. With iTunes AAC, you pick a minimum bitrate and the encoder can change the bitrate +10-15kbps throughout the song. The overall bitrate will still remain near the target you selected. Nero AAC works with different quality levels (like Lame mp3) while iTunes AAC works with a known bitrate. Either way, enabling VBR will only help with the quality of the file.               

What is the reason for wanting CBR (which is pseudo-CBR!).

- Neil
On Nov 15, 2011, at 8:07 AM, fabien gouyon wrote:

> Dear list,
> 
> Could somebody recommend a free software, or even better some
> open-source code, for converting audio from WAV to AAC with *constant*
> bit rate (and not variable bit rate).
> 
> Any help would be much appreciated.
> Best,
> 
> -- 
> Fabien Gouyon, PhD
> Senior Research Scientist, INESC Porto
> Invited Assistant Professor, Engineering Faculty, University of Porto
> http://www.inescporto.pt/~fgouyon
> http://smc.inescporto.pt/
>