Hi Folks,
Just wanted to try shed some light on this situation from what I've found by looking around the net after reading this post. The good news, if you want to call it that, is that this is not a bug. Accoding to a thread from a different site,
http://www.hydrogenaudio.org/forums/index.php?showtopic=58273, Apple has updated its AAC encoder, which by unfortunate coincedence is also the bad news. Apparently the new encoder first appeared in Leopard, which I am not personally running, but has been subsequently implemented in iTunes 7.5 / Quicktime 7.3. The relevant post from this thread describing the changes to the encoder is quoted below,
"The newly released Apple AAC encoder offers four encoding modes:
Constant Bit Rate (CBR)
-- Recommended for live streaming --
This mode achieves a constant target bit rate and is completely compliant to the CBR mode specified in the MPEG-4 standard. This mode is suitable for constant-bit-rate network transmission when decoding in real-time with a fixed end-to-end audio delay.
*Average Bit Rate (ABR) - Default Mode*
-- Recommended for controlling file size --
A target bit rate is achieved over a long term average (typically after the first few seconds of encoding). Unlike CBR mode, this mode does not provide constant delay when using constant bit rate transmission, but this mode provides almost best global quality while still being able to strictly control the resulting file size and with less complexity than the CBR mode.
Variable Bit Rate (VBR)
-- Recommended for controlling the audio quality --
The audio signal is encoded with constant (and settable) quality and virtually no bit rate constraints. This is the best mode to achieve consistent audio quality across many files with the smallest file size to achieve that quality. It also has the lowest complexity of all the encoding modes.
*Variable Bit Rate But Constrained (VBR_Constrained)*
-- Recommended as a compromise between VBR and ABR --
This mode is similar to VBR but limits the average bit rate variation. The lower limit is the user-selected bit rate. Higher bit rate is adapted for difficult tracks and can generate up to 10% larger files than the ABR mode.
*Note that QuickTime and the commandline tools on Mac can access all these four encoding modes. However, iTunes can only access ABR and VBR_Constrained.*"
The take away from this, as far as I can interpret, is that checking VBR in iTunes use the *Variable Bit Rate Constrained* mode and leaving the VBR checkbox unchecked in iTunes uses the *Average Bit Rate* mode (the default according to the above quoted post), which would explain why bitrates are not consistent despite leaving the VBR checkbox in iTunes unchecked.
If this is indeed true (and it seems like it is), then importing through iTunes (using the most recent versions of iTunes / Quicktime) will not yeild constant bitrates. The only suggestions that I can think of (1) downgrade your version of iTunes and Quicktime; and/or (2) write to apple feedback and tell them you want to be able to have an option to convert in Constant Bit Rate mode, which I did prior to posting this message. For the people using Windows, option (1) seems somewhat viable as you can uninstall both iTunes and Quicktime somewhat easily, but for the Mac users here uninstalling Quicktime is very difficult and the only safe way to downgrade Quicktime is to do an "archive and install".
I hope this has been useful.
