Change in iTunes AAC Encoding
Jul 21, 2008 at 3:16 AM Thread Starter Post #1 of 29

WyldRage

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I just updated to iTunes 7.7, and I noticed a major change when converting wav files to 256 VBR: the files are much smaller than they used to.

I was used to seeing the file take rougly 2 MB per minute, now it's reduced to 1.2 MB per minute. I'm trying to find out if it's better compression, or if it's a reduction in the sound file quality.

Trying to find the change list but it's surprisingly difficult. If anyone has any information, it would be appreciated.

On another note, they now note when the file is VBR.
 
Jul 21, 2008 at 3:39 AM Post #3 of 29

WyldRage

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I'm comparing whole albums. I know they can have slightly different sizes, but on 40+ minutes they should be minor: this is a 40% reduction in size and it's consistent for all albums I have ripped since 7.7.
 
Jul 21, 2008 at 7:20 AM Post #4 of 29

krmathis

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Thats strange!
2MB per minute imply a bitrate of 276kbps. While 1.2MB per minute imply a bitrate of 164kbps.
So you set iTunes to import using "Higher Quality (256kbps)", but end up with files at around 164kbps? That certainly seems like a bug to me, since its way off the targeted bitrate...
 
Jul 21, 2008 at 5:42 PM Post #8 of 29

krmathis

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Quote:

Originally Posted by scompton /img/forum/go_quote.gif
iTunes 7.7 broke itunesencoder so I'm not happy about the upgrade. I haven't been able to rip since upgrading.


Sounds like Apple changed some parts of iTunes, which itunesencoder access. Meaning that it may be permanent, and not just a bug.
Guess its time for itunesencoder's developer to update his software...
 
Jul 21, 2008 at 7:33 PM Post #9 of 29

scompton

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Unfortunately, he's lost the source code. I'll eventually have to break down and write some vbscript to do what itunesencoder does. I don't really have good tools to do Com programming. I end up using the macro editor in Word which is a PIA
 
Jul 21, 2008 at 7:54 PM Post #10 of 29

WyldRage

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I've done some test. It doesn't matter if you use 96, 128 or 256 kbps with VBR, iTunes always gives me the same file (average bit rate of 152 kbps with the file I tested). It seems iTunes 7.7 doesn't recognize your Bit Rate selection when you use VBR.

I've posted this info on Apple support, and I hope we'll get a patch soon.

In the mean time, I won't be using VBR.
 
Jul 22, 2008 at 1:21 AM Post #11 of 29

mrarroyo

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Quote:

Originally Posted by WyldRage /img/forum/go_quote.gif
I've done some test. It doesn't matter if you use 96, 128 or 256 kbps with VBR, iTunes always gives me the same file (average bit rate of 152 kbps with the file I tested). It seems iTunes 7.7 doesn't recognize your Bit Rate selection when you use VBR.

I've posted this info on Apple support, and I hope we'll get a patch soon.

In the mean time, I won't be using VBR.



I apple lossless afected? or only the mp3 files. How about AAC? Thanks.
 
Jul 22, 2008 at 7:41 AM Post #13 of 29

krmathis

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Quote:

Originally Posted by WyldRage /img/forum/go_quote.gif
and you can't use VBR with a lossless format.


What? A lossless format is VBR by nature...
wink.gif
 
Jul 22, 2008 at 7:47 AM Post #14 of 29

badmonkey

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Not really. Lossless formats are just container formats for the raw WAV, which is obviously CBR.
Of course in reality the number of bits going into the decoder per second varies, but you might as well say that a ZIP'ed Word doc has a VBR.
You might notice that Foobar displays the current but varying bitrate of an MP3 or AAC file when playing. But it displays the average for a FLAC.
Depends how you want to think about it. But the encoding philosophy that is implied by the term "VBR" is unique to lossy formats.
 
Jul 22, 2008 at 7:57 AM Post #15 of 29

krmathis

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Quote:

Originally Posted by badmonkey /img/forum/go_quote.gif
Not really. Lossless formats are just container formats for the raw WAV, which is obviously CBR.
Of course in reality the number of bits going into the decoder per second varies, but you might as well say that a ZIP'ed Word doc has a VBR.
You might notice that Foobar displays the current but varying bitrate of an MP3 or AAC file when playing. But it displays the average for a FLAC.
Depends how you want to think about it. But the encoding philosophy that is implied by the term "VBR" is unique to lossy formats.



Well, when I say "lossless" I mean a lossless codec (like Apple Lossless, FLAC, WavPack, ..). Not uncompressed PCM in a WAV or AIFF container.
I am quite sure a lossless codec like FLAC encodes each bit at a variable bitrate (VBR). Where silence get close to 0kbps, and more complex parts get much higher bitrate. Then the average bitrate (file size / playing time) is calculated and displayed to the user.

For me variable bitrate (VBR) mean that the the codec vary the allocated bits for each sample through the file, depending on its complexity. Resulting in a smaller file than using CBR, which have to identical to the most complex sample.


Edit: Or am I totally wrong?

Is its...
CBR (Constant bitrate)? Obviously not, since there are no constant bitrate shared between several files.
ABR (Average bitrate)? I fail to see why, as it target minimum sice, and not a specific targeted one (like ex. MP3 and its 128kbps, 192kbps, which are ABR).
Quote:

Average bit rate can also refer to a form of variable bitrate encoding where the encoder will try to reach a target average bitrate or file size while allowing the bitrate to vary between different parts of the audio or video


Source: http://en.wikipedia.org/wiki/Average_bitrate
VBR (Variable bitrate)? Most likely, since thats the most effective way to encode a certain audio data stream to the smallest possible size. Varying allocated bits depending audio sample complexity.
 

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