Does Encoding AAC Files with VBR on Improve Audio Quality ?

Question

Does Encoding AAC Files with VBR on Improve Audio Quality ?
Are there Disadvantages ?
Bonus Question 1:
Usually, an 'auto' type encoding tool will screw up the natural flow of the music and miss subtle changes in the energy, volume, etc.. -stuff that is just to subtle for it to catch -Generally I wouldn't trust an 'auto' type setting to pick up this subtle stuff. Yet people say using VBR improves sound quality (and NOT file size). Why and how ?
Bonus Question 2:
What is the max bit rate VBR uses ? If I set my AAC encoder to 320 kbps and turned VBR off, wouldn't the sound quality be superior to encoding with VBR on (simply because the kbps are set at 320 the whole time...) ? Sure, the files encoded with VBR off would be larger, but wouldn't the quality be better ?
Bonus Question 3:
Lastly, I did a little test and encoded one song with VBR off and one with VBR on. The VBR song was 1.5 MBs bigger - Huh, I thought, is that increased file size the result of improved resolution throughout the whole song, or just one little section (haven't had time to listen to them)?
Message was edited by: temptemp9

Answer 1

These are some pretty tough questions. I like VBR encoding in general, as it does allow for more complex passages to take advantage of higher bit rates while less complex passages fall back to a lower bit rate, while keeping within fairly consistent file size parameters. Whether VBR yields better results at different target/average bit rates is another matter, and really requires that you encode and properly test output files before deciding on anything but "recommended" settings.
At maximum lossy bit rates of 320 kbps, it doesn't make sense to use VBR. If I recall, the LAME MP3 encoder presets actually preclude VBR encoding for the highest quality files, and the same may be true for iTunes AAC.
I have to admit that to speak with any certainty or confidence about the issues surrounding your various questions, one would really need to encode a bunch of files with iTunes from lossless sources and test them in software such as foobar2000. I could investigate this easily on my own PC, but I don't encode music at 320 kbps AAC, since I have enough disk space to just listen to lossless files. iTunes doesn't display dynamic bit rate changes as foobar2000 does, and I haven't come across any Mac software that does as good a job of allowing for proper testing as foobar2000 for Windows does.
If you really need definitive answers to these three questions, then I think your best bet would be to do as I've hinted and use iTunes and foobar2000 if you've got access to a Windows computer to do this testing and evaluation for yourself, or sign up for a user account at Hydrogenaudio and post the same message there.