iTunes Plus Files: VBR or CBR?
Reply #20 – 2009-03-12 13:08:49
A mode with a higher bitrate than q127 would be purely cosmetic. From the developer documentation:kAudioCodecBitRateControlMode_VariableConstrained The encoder dynamically allocates the bit resources according to the characteristics of the underlying signal; however, some constraints are applied in order to limit the variation of the bit rate. kAudioCodecBitRateControlMode_Variable Similar to the VBR constrained mode, however the packet size is virtually unconstrained. The coding process targets constant sound quality. This mode usually provides the best tradeoff between quality and bit rate. (1st is available in iTunes VBR, 2nd is only accessible through Quicktime) The encoder takes as much bits as it needs for every frame in unconstrained mode. If it doesn't get higher than 192kbit/s (or even 100kbit/s with some music) there isn't just any more information in the signal from the encoder's point of view. Going higher would require stuffing the file with zeroes. This bitrate fetish must end, quality/size is all that matters. So many false, bold statements in this thread again... We can thank people like Bodhi for not having Quicktime's True VBR accessible within iTunes. Apple just doesn't want to waste money on answering support calls from people like him. Today the only thing that's left for encoders to choke on are problem samples. Those often show the behavior of not getting better above a certain bitrate. So there is no need to push true VBR encoders higher with arbitrary "safety margins". Let q127 do it's job and be happy. You are very probably not smarter at bitrate allocation decisions.