Ideally VBR means constant quality, varying bitrate. Where as CBR means variable quality, constant bitrate.
It seems to me that the VBR implimentation added to iTunes 5 is currently very limited in the amount of fluctuation allowable in a file. In theory VBR should allow variation between the full range of bitrate options. However when using my nominal 160K VBR setting, I am yet to encode a file that has finished larger than 170K. Also, very few of the files are lower than 160K. Suggesting that the current VBR routines are biased in favour of increasing, rather than decreasing the file size. Yet as I stated, the increases are only very slight.
I have a feeling that Apple is concerned that the iPod shuffle, and possibly the mini will be unable to correctly playback VBR files with large fluctuations. The mini has shown to have trouble with certain VBR mp3 files, and the shuffle has been shown to have problems with VBR AAC files made using other encoders. So it seems that Apple has constrained the operation of the VBR encoder so that problems are not caused on those models. I consider this unfortunate, because VBR is the most efficient encoding option for people that want constant quality, instead of a constant bitrate.
