Is AAC CBR?

[sderenzi]

I've noticed that while some applications report CBR for AAC files QuickTime (OS X) and Fission (OS X) report something much different. Even foobar tells me the bitrates are not balancing out, for example a 256 AAC I am told through QuickTime is really 234, and in iTunes it shows up as 256. Upon further investigation I found almost all my audio is this way, then I ran it in foobar and discovered that each frame of audio is VBR not CBR?! I was utterly lost, so wtf is this about?

If AAC isn't CBR then it's VBR, but what the hell does encoding it as VBR do then?

Another question I have is what sort of system AAC uses that causes VBR for each frame, to me is makes no sense.

Anyways do you guys know?

[nurbs]

Itunes can't tell cbr from vbr. I assume the same goes for quicktime.

cbr in aac doesn't mean that each frame is the same size, only that bitrate is constant over a short period of time (fraction of a second IIRC). You can find a better explaination somewhere here or at hydrogenaudio if you search.