Bitrates off when including encoder settings

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Rebel
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Joined: Tue Jun 20, 2006 4:31 am

Bitrates off when including encoder settings

Post by Rebel » Mon Dec 10, 2007 11:58 am

I just discovered a bug, at least on my system. :o

On Nov. 8th, I began converting my new CD's and FLAC files to AAC, instead of Mp3. Ever since then, almost all of the bitrates for my converted songs are not what they should be. I have Max set to Quality=Maximum and Bitrate=320, but most of the songs have bitrates of 319kbps. This never happened before, and I've been using Max for more than a year.
I just performed a test, and found that this only happens when converting to AAC with the encoder settings included in comments. If I convert to Mp3, or convert to AAC without including the encoder settings, the songs are fine.

I knew before that the bitrates were off, but I didn't know why until I began writing this post, and discovered that that the problem coincided precisely with the time I began including the settings, and that was also when I began converting to AAC instead of Mp3.

Most are AAC files that were converted from FLAC, but some were ripped and converted to AAC from brand new CD's I bought.

Now I have to go back and re-convert 16 CD's. :evil:

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sbooth
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Re: Bitrates off when including encoder settings

Post by sbooth » Mon Dec 10, 2007 2:29 pm

Rebel wrote:On Nov. 8th, I began converting my new CD's and FLAC files to AAC, instead of Mp3. Ever since then, almost all of the bitrates for my converted songs are not what they should be. I have Max set to Quality=Maximum and Bitrate=320, but most of the songs have bitrates of 319kbps. This never happened before, and I've been using Max for more than a year.
I just performed a test, and found that this only happens when converting to AAC with the encoder settings included in comments. If I convert to Mp3, or convert to AAC without including the encoder settings, the songs are fine.
What are you using to determine the bitrate? The iTunes display is not a reliable indicator of the actual audio bitrate of the file. Do you seem to have issues with the audio quality of the file? I suppose it is possible that since including the settings in the comment will slightly increase the file size, the calculated bitrate (for a naïve implementation) cause go down. Also, are you on Tiger or Leopard? There is a new AAC encoder in Leopard which could affect this as well (I haven't run any tests, though).

MP3 truly supports CBR, whereas AAC is a natively VBR format, so any CBR mode with AAC is more of a constraint on the VBR than true CBR. That is why the MP3 bitrate will always match exactly what you have set the encoder to do.

Rebel
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Joined: Tue Jun 20, 2006 4:31 am

Re: Bitrates off when including encoder settings

Post by Rebel » Sun Dec 16, 2007 1:01 pm

Yes, I am going by what iTunes says. This must be an AAC issue: After searching iTunes for AAC files, I found many with odd bitrates from as far back as 2 years ago, many of which Max had nothing to do with.

I'm currently running OSX 10.4.11.

I re-converted several CD's worth of the files, without encoder settings included, and they show the same bitrates - most at 319, some at 320.

Seems like someone else would have noted this before. Before posting this thread, I searched for 319 and didn't get a single hit.


Thanks for your help! 8)

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