[Jeff Guidry]

No, if iTunes is actually making a 320 kbps VBR file, the LEAST bitrate it is working at is 320 kbps, and can go higher if the music is demanding. With a 320 kbps CBR file, 320 kbps is the only rate at which it will sample the data, so a CBR file in this case ought to be smaller than a VBR file.

 

[superbu]

I know this is an old thread, but I just ran across it.
 
320 is not the maximum bitrate for AAC, only for MP3.  Nero's AAC encoder can go up to 512kb, and I think that may be the max for AAC.
 
If you play an AAC with Windows Media Player (using the AAC plugin for WMA), it will display the peak bitrate.  I have many songs I've ripped with iTunes at 320kb VBR (which is Quicktime VBR Constrained at the "Maximum Quality" setting) that have a peak bitrate of well over 400kb, some peaking out at over 460kb at some points in the song.  (Several iTunes Plus songs that I've purchased, as well as ripped, also have a peak of over 400kb, and they're only 256kb VBR.)

 

[chinesekiwi]

Quote:

No, if iTunes is actually making a 320 kbps VBR file, the LEAST bitrate it is working at is 320 kbps, and can go higher if the music is demanding. With a 320 kbps CBR file, 320 kbps is the only rate at which it will sample the data, so a CBR file in this case ought to be smaller than a VBR file.

Wrong. With VBR implementations, it's usually fixed quality VBR encoding, Wikipedia describes it well:
 
 
 
Quote:

Fixed quality

One means of VBR encoding is fixed quantizer or fixed quality encoding. It is usually single-pass encoding. The user specifies a given subjective quality value, and the encoder allocates bits as needed to achieve the given level of quality. This ensures the output stream will have consistent quality throughout. A quality level usually has an associated bitrate range. The disadvantage of this encoding method is that the average bitrate (and hence file size) will not be known ahead of time, and achieving a certain average bitrate requires trial and error. This is typically more of a concern for video than for audio, since file sizes are much larger and encoding can take much longer.

 
It goes lower if needed, higher if needed. RMS level plays a key factor in this in audio. Louder = more bits = more data.
Anyway, there's little point of 320kbps VBR / CBR AAC when ~192 kbps AAC is better than 320kbps CBR / VBR -V0 mp3.
 

 

[superbu]

Quote:

Anyway, there's little point of 320kbps VBR / CBR AAC when ~192 kbps AAC is better than 320kbps CBR / VBR -V0 mp3.
 

Unless you want better sound than 320kb MP3.