AAc encoding

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Mike
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AAc encoding

Post by Mike » Mon Jan 23, 2006 10:34 pm

I have noticed one rather strange thing when encoding into ACC (m4a) at 192. If I look at the file once I have imported it into itunes it says the bit rate is 191. Any reason for this?
Also how are the quality settings used for AAC? Do they only apply if you chose the VBR option. What about Apple Lossless - what are the quality settings used for here?

Mike
Last edited by Mike on Mon Jan 23, 2006 11:39 pm, edited 1 time in total.

Mike
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Follow up

Post by Mike » Mon Jan 23, 2006 11:05 pm

I've just encoded an album from FLAC to AAC at bit rate of 192. However when looking at the bit rates in itunes the tracks seem to vary from around 188 to 192.

Mike

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sbooth
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AAC encoding bitrates

Post by sbooth » Tue Jan 24, 2006 1:14 am

I am by no means an expert but what I gather by reading is that "true" CBR only applies to codecs with with a fixed frame size (e.g. old-style MP3). For encoders that have a minimum frame size or use a "bit pool" for CBR encoding (like AAC) you will never get an exact bitrate.

Mike
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Post by Mike » Tue Jan 24, 2006 8:16 am

I'm no expert either. However, if I use itunes to encode at 192 AAC all the files show up with a bit rate of 192 - there is no variation - so I'm wondering why it is different using MAX

Also could you explain what the quality drop-down box is for when encoding CBR AAC and Apple Lossless.

Thanks

Mike

cluffydog
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Post by cluffydog » Tue Jan 24, 2006 10:59 am

The reason I have started to use Max is because it shows the proper VBR bit rate & iTunes doesn't. If you encode in iTunes at 192 CBR & compare it to a file encoded in iTunes at 192 VBR, some of the file sizes will differ. This to me means the bit rates should be different but iTunes doesn't appear to show this.

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sbooth
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Post by sbooth » Tue Jan 24, 2006 2:16 pm

I wonder if this is similar to the Apple Lossless issue reported elsewhere.

(To recap, Apple Lossless files generated with Max do not show up as having a bitrate in iTunes. It turns out the bitrate displayed in iTunes is read from a magic cookie that isn't set by the Core Audio lossless encoder. So not a bug really, but different behavior.)

I wonder if iTunes sets some sort of magic cookie in AAC that says "this is the bitrate" versus Core Audio which does not?

This is purely speculation...

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sbooth
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Post by sbooth » Tue Jan 24, 2006 2:25 pm

Mike wrote:Also could you explain what the quality drop-down box is for when encoding CBR AAC and Apple Lossless.
The quality settings have no effect for Apple Lossless. The ability to adjust quality at all is probably an artifact of my inexperience with Core Audio and trying to treat all the codecs the same way. (Versus a specialized interface for each one.)

Here is some background info I gathered while trying to answer your question.

The header file "AudioCodec.h" has the following:

Code: Select all

	kAudioCodecPropertyQualitySetting						= 'srcq',
		//	"Some Relative Codec Quality"
		//
		//	A UInt32 that specifies the relative quality of a codec.
		//	(see enum constants below)
Googling for that I came across the following at http://developer.apple.com/documentatio ... ion_3.html:
Quality Settings
Specify the relative quality of a codec. The values are arbitrary and are provided as a suggestion. You can add more quality setting constants, or ignore them.

kAudioCodecQuality_Max = 0x7F
The maximum value allowed for the codec.

kAudioCodecQuality_High = 0x60
A high quality setting.

kAudioCodecQuality_Medium = 0x40
A medium quality setting.

kAudioCodecQuality_Low = 0x20
A low quality setting.

kAudioCodecQuality_Min = 0
The minimum quality setting allowed for the codec.

These constants are used by the kAudioCodecPropertyQualitySetting property. They are arbitrary and are provided as a suggestion. You can add more quality setting constants or ignore them altogether.
Hopefully this helps a little bit...

Mike
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Post by Mike » Tue Jan 24, 2006 8:23 pm

Thanks for this info...

anamika
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Re: AAc encoding

Post by anamika » Tue Sep 09, 2014 4:42 am

So, i've tried it now and the result is that my mini can't do anywhere close to realtime encoding to mp3, and it can't do it to flac quite fast as my drive can rip - i have 4 songs left in the encode queue when i get to the end of a 20-song CD.
anamika

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