iTunes 4 and VBR AAC encoding
Reply #1 – 2003-04-30 08:13:23
AAC codec builds upon state-of-the art signal processing technology from Dolby Laboratories and brings true variable bit rate (VBR) audio encoding to QuickTime. The encoding settings in iTunes doesn't list VBR options. QuickTime is not the same as iTunes... Now if all AAC encoding rates are VBR (are they?) what do the bitrates mean? Guaranteed minimums? Rough middle ground? Never quite understood previous iTunes mp3 VBR settings either. Unlike LAME encoding which made much more sense (and was documented). Anyone have any ideas about the mp4 settings? It's a bit difficult to clear up what Apple means and knows or wants to tell about CBR and VBR and/or ABR in their applications. To start from the top: the MP4 container that is used in QuickTime 6 Pro understands VBR. This is documented in a comprehensive paper about the new features of QuickTime 6. This may be the reason why you often find statements that QuickTime uses VBR for AAC encodings. As far as I know, the Dolby AAC codec inside QT does not use VBR, but rather CBR with its usual bit reservoir, so there is a small headroom for variations of the bitrate. Maybe you should test it yourself with two different WAVs/AIFFs that you know are completely opposite in sound and resulting bitrates. If you get almost the same file size afterwards, QT 6 uses CBR. What they've decided to do in iTunes4 now isn't clear yet, but probably they use the same Dolby codec. The resulting *.m4a file is a simple MPEG-4 AAC LC file, by the way (thanks, Menno), so you can just rename them to *.mp4 and look for the header infos with an appropriate player (Winamp or foobar2000). Keep in mind that these might not be the truth, though...