AAC using VBR

I think you need to upgrade your QuickTime to version 7. I just checked and that check box is selectable for me and I am running QT 7 with iTunes 5.

Maybe QT 7.02 is required for
b all
features.
The VBR checkbox is selectable on my system (10.3.9; QT 7.02)

AAC VBR requires QuickTime 7

Mike, could you arrange to get that note into an iTunes FAQ or the KB? If it's anywhere on the Apple web site, I couldn't find it, though I presumed that to be the case.

Thanks.

Folks,

Perhaps it's too early to get excited.

I just did a test with AAC VBR at 128/192/256....the file sizes were the same as the CBR rips!

Perhaps this is another 5 feature that didn't get any QA from Apple....

😟

And I thought I was the only one with this problem ... glad to know that I'm not alone. It would be nice if this feature gets fixed. I really would like to try the VBR settings used for the AAC encoder. Other than that, iTunes 5 feels a bit more snappier!

Yeah, it doesn't look like VBR actually works. If you look at VBR MP3's, you'll see under the bitrate that iTunes lists the average bitrate and then "(VBR)" after it. If you encode something in AAC, it doesn't give any VBR indication. And, as Thomas Affinito says, if the files sizes are EXACTLY the same, then it sounds like it's not doing VBR at all. Did anyone at Apple do ANY QA on iTunes 5?

I knew I shouldn't have commented without doing tests myself.

I encoded tracks with VBR for AAC both turned on and turned off. The VBR AAC's were substantially different than the CBR AAC's. For me, though, it appears that it's just a display bug in iTunes 5. In the bitrate section of the file info, it gives the same bitrate as the CBR, and gives no indication that it is VBR.

If you open the VBR AAC in QuickTime Player, however, the Info window shows you the bitrate correctly, and you can see that it is not a standard CBR number, but some in-between number that variable bitrates always average out to.

Thanks for sharing that info, TheBang.

I've also done some tests, too but I've noticed that the bitrate for some of the songs I ripped (one orchestral CD and one pop-rock CD) seemed to be somewhat constraining. It's true that it's not at 256 kbps on the dot but at the same time, their bitrate doesn't seem to deviate pretty much ... I'd say it's a ±5 kbps difference instead of the considerably higher differences when I encode an LAME-MP3 using the highest VBR settings.

The thing about the Quicktime display though...it displays a different bitrate for m4a files encoded before Quicktime 7. I'm opening up music that I ripped (@ 128kbps) in Quicktime 6/iTunes 4 and the bitrates show up as 123k or 132k...a bit higher, a bit lower, just like supposed VBR AAC encodings.

Isn't AAC by it's very nature VBR anyway? What does the VBR switch really do?

AAC by its nature is ABR (average bit rate), not VBR.

ABR is when you select a target bitrate, and the encoder is allowed to fluctuate below or above that so long as the average bitrate remains close to the target. This results in files that are higher quality than CBR (constant bit rate), but still have a fairly predictable file size.

VBR (variable bit rate) is when you select a quality setting and the encoder attempts to maintain that quality throughout the song, using whatever bitrates it feels that it needs throughout the file. This results in predictable audio quality, but unpredictable file sizes.

Because the AAC "VBR" encoder in iTunes has the user select what appears to be a target bitrate, it would be better defined as ABR. However, if you encode a lot of songs, you should notice that the average bitrate tends to stray a bit more from the target than in the default mode. Most likely, toggling the "VBR" switch for AAC encoding in iTunes just widens the range of bitrates that the ABR system is allowed to select from when encoding the song.

Thanks for the input SpaceMonkey, but where do you see that AAC is ABR?

This page ( http://www.apple.com/quicktime/technologies/aac/) says this:
"The AAC codec in QuickTime builds upon new, state-of-the art signal processing technology from Dolby Laboratories and brings true variable bit rate (VBR) audio encoding to QuickTime."

And from what i can remember, that was up there before Quicktime gave us the option of a VBR switch with AAC encoding. As I check now the link at for aac-audio.com, linked from that page, isn't working for me, so I can't check that for further information.

I don't have Pro, so I don't know if there are more options when encoding with it. All I know is that with that switch on whole encoded albums tend to be larger than when the switch is off, usually around 3-5 megs larger.

I remember on Windows that Winamp would list all the bitrates that were being decoded right while an MP3 was playing. Does anyone know of anything on OSX that will do this?

Powmac G5 Mac OS X (10.4.3) Dual 2ghz, 2.5 gigs RAM