- What is the difference between the AAC in the two "MPEG4 Audio/Core Audio" encoders? Is the only difference the filename extension that is used?
Yes, that's it. And in both cases the container is MP4; it's just Apple likes to distinguish MP4 files that are audio only by using that different file extension.
What do the AAC settings ("Quality", "Bitrate", and "Use VBR") do?
Quality works with VBR. If you select VBR, you set a quality level to go with it.
If I have a quality of "Medium" and a bitrate of "320", how will that compare with a quality of "Maximum" and a bitrate of "128"?
If VBR isn't ticked, the quality field isn't used. It's Core Audio's way of offering different ways to optimize the variation in the bitrate.
And I thought that the AAC always used VBR? So what is the difference between ticking the VBR box or not.[/list]
It doesn't by default (and hasn't even always had it). Some people like VBR, because, as someone put it, you're getting the "best bang for the byte":
But in practice, I suppose, the Core Audio/Tunes encoder that Max uses is fine for most purposes at medium bitrates using CBR.
I think it's safe to say there's probably not a lot of point encoding in a lossy encoder at a fixed bitrate at 320kbps: anyone who's going to do that might as well go the whole hog and encode to lossless (FLAC or ALAC).