Two MPEG4 Audio encoders & AAC settings

[jolinwarren]

Hello,

These might be elementary questions, but I just want to be sure I understand what I'm doing when setting up Max's encoders.

1. 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?
2. What do the AAC settings ("Quality", "Bitrate", and "Use VBR") do? 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"? And I thought that the AAC always used VBR? So what is the difference between ticking the VBR box or not.

Sorry if these questions are basic, but I couldn't find definite answers on the forums (one post mentions the quality settings, but doesn't really explain what the practical effect is). I thought that maybe someone with experience using the encoders might be able to help. Thanks!

[Mike1]

1. 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":

http://www.codinghorror.com/blog/archives/000470.html

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).

HTH

[jolinwarren]

Hi Mike,

Thanks so much for your replies -- they were very helpful.

Quality works with VBR. If you select VBR, you set a quality level to go with it.

As I've already settled on "160kbps/VBR" AAC as my encoding setttings in iTunes, I decided to try both "160kbps/Max quality/VBR" and "160kbps/Med quality/VBR" in Max. The resulting file sizes were almost identical -- different by only a few hundred bytes (not kilobytes, ordinary bytes). I haven't actually had a chance to do a comparison listening test, but don't feel it's necessary. Using the maximum quality setting in Max doesn't change the file size, so that's what I'm going to use.

And I thought that the AAC always used VBR? So what is the difference between ticking the VBR box or not.

It doesn't by default (and hasn't even always had it).

From discussions elsewhere, I believe that the AAC format is inherently VBR. The bitrate varies within a certain amount (+/- about 25bps?) depending on audio complexity. My understanding is that a VBR setting essentially instructs the encoder to allow much greater variation in bitrate so that, for example, it can drop to near 0bps for periods of silence. While this is my understanding, I've never had it definitively confirmed. Can anyone here confirm this is the case?

Thanks again for your help.

Cheers,
Jolin