BIG UPDATEiTunes 18.104.22.168
Here is my latest update, all major encoder settings used in the newest iTunes. I've encoded the Main Wave several times with iTunes's AAC and MP3 encoder, with both CBR and VBR.
At first Glance the VBR versions of iTunes' AAC looks the same, this is not true, the files are marginally different. iTunes' encoder searches for 'low-bitrate spots' when in VBR-mode, this is makes the test sample very uneffective since the tone sweeps are all over the file, without silent gaps. iTunes give every second of the file the maximum bitrate, this is the reason why the filesizes, wich i posted a hour or so back, are very close.Original Wave
You can get the file HERE
AAC at 16kbps, this is the lowest bitrate iTunes can encode, i did it just to make the comparison more complete as this bandwith is way to low to sound decently.
AAC at 32kbps, this is quite usefull for speech only, but i would seriously consider HE-AAC or AAC+ instead
AAC at 64kbps
AAC at 96kbps, from this point music is actually listenable (IMHO) on speakers
AAC 96kbps VBR, as you can see, the difference between CBR and VBR is very minor
AAC at 128kbps, a lot of improvement since my first post
AAC at 128kbps VBR
AAC at 160kbps, not so much difference with the 128kbps sample
AAC at 160kbps VBR
AAC at 192kbps, if they would bump up the low-pass filter it would sound alot better, it is acousticly indisguisable from the 160kbps version when i did a quick listening test.
AAC at 192kbps VBR, same as above, but it has a 'bitrate storage' for very dynamic music (like classical music), probably none shall hear the difference between this and the 224 CBR file.
AAC at 224kbps, not woth the extra space, use the 192kbps VBR instead
AAC at 256kbps, here the AAC file's lowpass is at his maximum, close listerers with very refined equipment can hear minor improvement over the sub 256kbps files, others should not bother and go for VBR 192kbps or VBR 256kbps instead.
AAC at 256kbps, the best AAC file encode-able with iTunes
AAC at 320kbps, useless because of the 20k lowpass filter, nothing better then a VBR 256kbps file....picture limit.. MP3 coming up in a few minutes