Max and VBR AAC

Discuss Max, an open source CD audio extractor and audio converter.
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Lateralus
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Joined: Fri May 29, 2009 5:01 am

Max and VBR AAC

Post by Lateralus » Fri May 29, 2009 5:17 am

So I decided to play around with Max to see what it could achieve when creating a VBR AAC file, either directly from a CD or from a converted Apple Lossless file. And the end result is pretty odd...

Image

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Bit rates are off the chart. It's especially odd when you consider that, as far as I understand, when setting an encoder to rip at a certain bit rate, that bit rate is the peak bit rate with most of the frequencies falling below that mark. Which is why when ripping with 320k VBR MP3, the bit rate is often reported in the 200 range. And I was under the impression that the same would be the case with VBR AAC. But Max is reporting bit rates beyond what should be possible with the settings I've chosen.

Is this a bug? If not, how is Max achieving this quality when set to 320k? And are the bit rates I'm seeing peak frequencies, or the median?

Thanks.

Lateralus
Posts: 6
Joined: Fri May 29, 2009 5:01 am

Re: Max and VBR AAC

Post by Lateralus » Sun May 31, 2009 11:02 pm

Ah, okay. Apparently my understanding of lossly encoding was just completely incorrect.

I'd believed that when ripping with a lossy codec, the selected bit rate (say... 320) functioned as the peak bit rate with frequencies above it being filtered. Now I understand that the selected bit rate serves as the baseline frequency. I had also been under the impression that VBR existed only to save on file size, as it does in lossless codecs like FLAC and ALAC, but I now understand it is actually higher quality as it doesn't constrain the encoder as much and allows a broader frequency range to be conveyed.

Well, there goes 8 years of preconceptions out the window. :p

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sbooth
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Re: Max and VBR AAC

Post by sbooth » Sun May 31, 2009 11:15 pm

I'm glad you figured it out, because I'd been putting off my response :)

Anyway, iTunes doesn't show the bitrate you've selected for encoding because Max doesn't write a special iTunes atom that contains it. In any case, the display is for info only.

Lateralus
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Joined: Fri May 29, 2009 5:01 am

Re: Max and VBR AAC

Post by Lateralus » Sun May 31, 2009 11:31 pm

sbooth wrote:I'm glad you figured it out, because I'd been putting off my response :)
Mean... :D

I'm apparently still learning about lossy encoding, so when I see an album encoded with VBR showing different rates for each track, what am I looking at; base, median or peak? I assume median.

Regardless, I prefer the way Max writes the information because it's an easy way to gauge how 'busy' a track is before playing it.

Anyways, I think part of the reason I was so confused is because I'm used to seeing MP3 VBR albums displaying bit rates below the selected encode rate. Like, it seems that 320k VBR MP3 albums display bit rates in the 200-300 range. So seeing Max output VBR AAC 320 tracks with rates beyond the selected encode rate just broke my brain.

Is AAC just that much better of an encoder than MP3 when it comes to VBR?

Lateralus
Posts: 6
Joined: Fri May 29, 2009 5:01 am

Re: Max and VBR AAC

Post by Lateralus » Tue Jun 02, 2009 8:06 pm

Lateralus wrote:And are the bit rates I'm seeing peak frequencies, or the median?
Lateralus wrote:...when I see an album encoded with VBR showing different rates for each track, what am I looking at; median or peak? I assume median.
Anybody? :(

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