We've been able to adjust our AAC encoders for CBR (constant bitrate) encoding. Almost no padding is added to compensate for the peak bitrate. See this
topic for a previous discussion on the subject.
The PS and SBR parameters are automatically adjusted from the target mode and bitrate described below.
(HE-AAC v1) SBR is used for mono target from 16 to 48 kbps.
(AAC) None is used for mono target 56 kbps and over.
(HE-AAC v2) SBR and PS is used for stereo target from 24 to 40 kbps.
(HE-AAC v1) SBR is used for stereo target from 48 to 56 kbps.
(AAC) None is used for stereo target 64 kbps and over.