When I encode a musicsample with Nero in 128kb/s CBR then the average bitrate is not exactly 128kb/s (which is normal as it uses ABR).

When I encode the same sample with Quicktime 128kb/s CBR then the bitrate is exact 128kb/s although Quicktime 6.5 also uses ABR (as you can see while playing in Foobar2000).

Does quicktime use a 2-pass mode (like WMA)? Or does it just have a very flexible bit reservoir?

I'm pretty sure its just plain CBR which is why you get a predicatable file size.

I don´t think so, when I play the file with Foobar2000, the bitrate can go down to 100kb/s and go up to 180kb/s (VBR bitrate display = 1 frame), when I choose 128kb/s in iTunes.

Foobar2000 doesn't show the bitrate very presicely if the music sample is relatively short.
Also IIRC Nero's CBR mode is CBR according to the specs, not ABR -> the bit reservoir is something like 1-1.5 kB (don't remember exactly) which means it's "CBR" according to specs although it can fluctuate few kbps.

I´ve encode the song "Wim Mertens - Struggle for pleasure" with a length of 4:01. Foobar shows that the average bitrate for 128kb/s Nero CBR = 124kb/s

When I encode with quicktime, the bitrate is shown as 128kb/s but looks rather ABR.

Better look a the file size. foobar2000 shows the bitrate of the aac stream, not the average bitrate of the file (stream and overhead, something like that). Take a look at the following discussion :

http://www.hydrogenaudio.org/forums/index....showtopic=16418

Or extract aac stream for you 128 kbps m4a files, and reconvert it in mp4. The same file is now 124 kbps, and not 128 kbps anymore

I think this has already been answered CBR+ bit resevoir can look like ABR if the resevoir is reasonabley large.

It's CBR.

Fairy

Is it accurate to say that CBR->ABR->VBR is really just a continuum, and the distinction is the size of the bit resevoir?

You can watch the bitrate changing, while playing your AAC-encoded files (iTunes) with WinAmp 5 (you need the plugin).

Ciao