[TMT]

this is strange...
I played around with import options ad different datarates, and now iTunes won't code VBR files. I´m using AAC, 160 kBit/s, VBR checked, but it simply ignores that.
Any idea?
delete some preference files?

[Brad]

How are you sure they're not VBR? Have you compared a checksum of a file that supposedly is against one that isn't?

[TMT]

I imported the same track twice and compared the files. The VBR-file was about 8 kByte bigger, because the files had not exact the same datarate (159,5 and 159,95). The datarate is shown more precise in quicktime.

hmm... I relized that all VBR files I have are MP3s.
Should't files coded with aac look like MP3 files, where the VBR data rate is the minimal datarete, so they would have something like 171 kB/s ?

[Dorian Gray]

iTunes doesn't report the true bit rate of AAC VBR files, or even the fact that they are VBR files at all. Instead it reports the nominal bit rate (such as 160 kbps) and simply refers to the file as an "AAC audio file", the same tag it uses for standard AAC files. QuickTime Player reports the average bit rate of the file if you need to check the actual bit rate.

Although I haven't looked into it in detail, I understand that AAC is inherently VBR in that it selects a [potentially different] bit rate for each frame of audio as it encodes, according to the complexity of the sound wave in that frame. When you encode "CBR" files (notice that iTunes doesn't use the term CBR), the bit rate you specify becomes the ABR (Average Bit Rate). When iTunes is encoding VBR files it increases the bit rate for complex frames, but maintains a minimum quality level for frames of low complexity (probably the nominal bit rate specified though I'm not sure about this).

However, AAC is a very efficient codec, so if you encode sound of low complexity (most baroque music, for instance) as an AAC VBR file, the iTunes encoder (QuickTime) will not increase the bit rate substantially as there would be little point to do so. So if you encode at a nominal bit rate of 128 kbps you might well end up with a file of between 128 and 130 kbps. On the other hand if you encode acoustically complex music such as punk the file might reach 140 kbps or more. I don't have access to a CD at the moment but I have a 266 kbps VBR MP3 of art rock/post-punk band the Kaiser Chiefs in my iTunes library. Converting this to "128 kbps" AAC VBR results in a file of 140.82 kbps (according to QuickTime Player).

In short, I suspect AAC VBR encoding is working fine, but at a nominal bit rate of 160 kbps your music doesn't have enough complexity to warrant the significantly higher bit rate that iTunes might employ if it were needed.

[TMT]

I see... so it's my fault, it seems that I used the MP3 codec before...
Thanks for the explanation!

[SpecMode]

A side comment/question: the iTunes MP3 encoder preference page states that the VBR option is used to provide a guaranteed minimum bitrate; does this mean that iTunes will not use a bitrate less than that selected at any point in the file, or does it mean that the average bitrate for the entire file will not be less than that specified (as with most MP3 encoders)?