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Topic: iTunes Plus Files: VBR or CBR? (Read 20615 times) previous topic - next topic
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iTunes Plus Files: VBR or CBR?

Does anyone know whether the iTunes Plus files on the iTunes Music Store are encoded at 256kbps VBR (constrained) or CBR?


iTunes Plus Files: VBR or CBR?

Reply #2
I believe the setting is 256kbps CBR but you can analyze the bitrate itself (or just look at the properties under foobar2000).  Most of the songs I have purchased come out with an overall average bitrate of around 260kbps.  So the CBR setting is used but, as usual with the iTunes AAC encoder, the results aren't true CBR.

The reason why I say this is because the 256kbps VBR setting normally produces files that have average bitrates at around 264-270kbps (I have some iTunes Plus songs on a lossless CD that I ripped at both 256kbps setting, the 256kbps "cbr" setting matched the iTunes Plus download more so than the VBR file).

iTunes Plus Files: VBR or CBR?

Reply #3
So the CBR setting is used but, as usual with the iTunes AAC encoder, the results aren't true CBR.

In other words, ABR.
They may call it CBR, but it's actually ABR.

iTunes Plus Files: VBR or CBR?

Reply #4
ABR is correct.

iTunes Plus Files: VBR or CBR?

Reply #5
Does the QuickTime AAC encoder use an ABR code for its "CBR" setting or is that just its normal "CBR" code?

iTunes Plus Files: VBR or CBR?

Reply #6
Does the QuickTime AAC encoder use an ABR code for its "CBR" setting or is that just its normal "CBR" code?


There's a separate ABR setting in the Apple AAC encoder. The setting used on the store is the regular CBR mode.

I guess it might be the bit reservoir (that the bitrate isn't constant on a frame by frame basis) that might account for some of the files not coming out at exactly 256kbit. The encoder also doesn't use much bits on silence even in the CBR mode.

iTunes Plus Files: VBR or CBR?

Reply #7
It's important to know that CBR in AAC is not as strict as CBR in MP3. Mp3 uses bit reservoir in frame boundaries. AAC changes frame size.

so
CBR AAC = CBR, but without fixed size frames.
ABR AAC = CBR AAC, but with a bigger window (i.e more frames can vary from the target bitrate)
Constrained VBR AAC = VBR, but with a sort of slow change, and in iTunes implementation, a minimum frame size (bitrate).
True VBR AAC = Different implementation depending on QuickTime or Nero, but more like the usual VBR that we're used to with LAME.

iTunes Plus Files: VBR or CBR?

Reply #8
Related question:

How do iTunes files sound in comparison to say, Lame -v5?  I don't buy from iTunes, but that doesn't mean that i never will.  I don't want to be hearing artifacts now that HA has taught me what they sound like.

iTunes Plus Files: VBR or CBR?

Reply #9
The chance of perceptual transparency is higher with an iTunes Plus file than it is with a LAME V5 file -- I think there would be a pretty strong consensus on that. That being said, the chance that both will be perceptually transparent to you is probably similarly high.

iTunes Plus Files: VBR or CBR?

Reply #10
AAC's VBR is a joke anyway!
Pick"VBR 128 Kbps" file and you can get a 170 Kbps one!

iTunes Plus Files: VBR or CBR?

Reply #11
That's how VBR works.
If you want an average bitrate that's always close to a specific bitrate, use ABR.

Moderation: Removed useless quotation of previous post.  When will you ever learn?  Nevermind, you've been banned.

iTunes Plus Files: VBR or CBR?

Reply #12
Bodhi, how is that a joke?  I can pick -V 5 with Lame and encode some Nine Inch Nails songs, they come out having an overall average bitrate at around 200kbps.  As ameyer said, pick ABR if you want a predictable bitrate.

iTunes Plus Files: VBR or CBR?

Reply #13
I agree in one way but in another (we were talking about that in a pre-listening test session) it's not right to compare files that don't have same size while you were aiming for the same bitrate, 128 VBR for example.
If I choose 128vbr I don't mind getting 140 kbps files but not 170. aac used to do that!

iTunes Plus Files: VBR or CBR?

Reply #14
The chance of perceptual transparency is higher with an iTunes Plus file than it is with a LAME V5 file -- I think there would be a pretty strong consensus on that. That being said, the chance that both will be perceptually transparent to you is probably similarly high.

Thanks, exactly the information I was looking for.  Lame v5 is transparent to me, so this is good news if i ever decide to buy music from apple.

iTunes Plus Files: VBR or CBR?

Reply #15
I agree in one way but in another (we were talking about that in a pre-listening test session) it's not right to compare files that don't have same size while you were aiming for the same bitrate, 128 VBR for example.
If I choose 128vbr I don't mind getting 140 kbps files but not 170. aac used to do that!


There was a discussion about this not too long ago comparing iTunes AAC and Nero AAC at 96kbps VBR.  Some iTunes files resulted in having overall bitrates at around 110kbps while Nero was stuck down at 90 some odd kbps (it was around a 10-20kbps difference in bitrate).  Someone said that this type of comparison was alright as the iTunes AAC encoder was smart enough to increase the bitrate when needed while the Nero AAC encoder didn't.  So it is perfectly acceptable if 128kbps VBR files come out having much higher bitrates.  As I said, I have encountered this with Lame quite a bit.  I have many songs that I can encode at -V 0 that come out having bitrates over 300kbps.  So it isn't really fair to say that an encoder is bad when you get a 170kbps file by using a 128kbps VBR setting.  The encoder is doing its job.

iTunes Plus Files: VBR or CBR?

Reply #16
So the new iTunes 8.1 encodes with 'iTunes Plus' as default, and surprisingly this seems to give different files than the regular 256kbps VBR mode. My guess is that Plus uses the best encoder quality setting whereas the custom iTunes AAC settings use 'normal'.

Now if Apple could just enable the true VBR mode with the best setting in iTunes too, I'd be a happy camper.

iTunes Plus Files: VBR or CBR?

Reply #17
Well, I guess that is OK.  Too bad though as now there will be a flood of people thinking they need to encode at the iTunes Plus setting even though they haven't conducted any blind ABX tests or understand the very basics behind perceptual transparency.  The same thing happened whenever Apple added the "Higher Quality" setting which used 256kbps "CBR."  A whole bunch of people suddenly thought that they needed to use that setting and claimed that it was much better than the standard "High Quality" 128kbps CBR setting.

Offering true VBR may throw a wrench into Apple's iTunes machine.  I would welcome true VBR but the last thing I want to see is yet another flood of people (here and on iLounge) asking if true VBR would be better to use, thinking that they need to use an ultra high bitrate setting, etc.  I don't know.  In my opinion it is kind of like a win-no win situation where I would welcome the addition of true VBR but would hate to see the genera public's reaction.

iTunes Plus Files: VBR or CBR?

Reply #18
Well, I guess that is OK.  Too bad though as now there will be a flood of people thinking they need to encode at the iTunes Plus setting even though they haven't conducted any blind ABX tests or understand the very basics behind perceptual transparency.  The same thing happened whenever Apple added the "Higher Quality" setting which used 256kbps "CBR."  A whole bunch of people suddenly thought that they needed to use that setting and claimed that it was much better than the standard "High Quality" 128kbps CBR setting.

Offering true VBR may throw a wrench into Apple's iTunes machine.  I would welcome true VBR but the last thing I want to see is yet another flood of people (here and on iLounge) asking if true VBR would be better to use, thinking that they need to use an ultra high bitrate setting, etc.  I don't know.  In my opinion it is kind of like a win-no win situation where I would welcome the addition of true VBR but would hate to see the genera public's reaction.


Hmm I doubt there'd be a "public outcry" of any sort by just adding the true VBR mode as an option in the custom settings menu.. Most people want the best quality they can get using as little space as possible after all, so a brief explanation of what VBR is in the encoder window would probably be enough to avoid any confusion.

The only problem I can see with the current implementation of the true VBR mode is that the very top setting produces files that are ~192kbps, but I'm sure there'd be no problem for Apple to make it go higher so that q127 produces files that are ~320kbps like the other modes.

iTunes Plus Files: VBR or CBR?

Reply #19
So it isn't really fair to say that an encoder is bad when you get a 170kbps file by using a 128kbps VBR setting.  The encoder is doing its job.


Well, yes.. sort of..  Is IS the job of vbr to vary the bit rate as needed, so some songs could come out much higher. 

Just like you  can flip a coin and get 5 heads in a row.  Over a set of thousands of flips you would even EXPECT to get runs of at least 5 heads in a row.  But the overall average should still be 50%. 

If the overall average bit rate of many songs of various styles is higher than target, then the settings don't work right and comparison to another encoder whose overall average *is* on target isn't fair.  Otherwise the best encoder will be the one that stuffs a file with lossless compression even if you specify 32 kb/s vbr.



iTunes Plus Files: VBR or CBR?

Reply #20
A mode with a higher bitrate than q127 would be purely cosmetic. From the developer documentation:

Code: [Select]
kAudioCodecBitRateControlMode_VariableConstrained
The encoder dynamically allocates the bit resources according to the characteristics of the underlying signal; however, some constraints are applied in order to limit the variation of the bit rate.

kAudioCodecBitRateControlMode_Variable
Similar to the VBR constrained mode, however the packet size is virtually unconstrained.

The coding process targets constant sound quality. This mode usually provides the best tradeoff between quality and bit rate.


(1st is available in iTunes VBR, 2nd is only accessible through Quicktime)

The encoder takes as much bits as it needs for every frame in unconstrained mode. If it doesn't get higher than 192kbit/s (or even 100kbit/s with some music) there isn't just any more information in the signal from the encoder's point of view. Going higher would require stuffing the file with zeroes.

This bitrate fetish must end, quality/size is all that matters. So many false, bold statements in this thread again... We can thank people like Bodhi for not having Quicktime's True VBR accessible within iTunes. Apple just doesn't want to waste money on answering support calls from people like him.

Today the only thing that's left for encoders to choke on are problem samples. Those often show the behavior of not getting better above a certain bitrate. So there is no need to push true VBR encoders higher with arbitrary "safety margins". Let q127 do it's job and be happy. You are very probably not smarter at bitrate allocation decisions.

iTunes Plus Files: VBR or CBR?

Reply #21
Yeah for me it's certainly not an important feature to let it use more bits but it's nice to have the option if you want to reduce the amount of support calls, if thats the main concern (especially as there's still the occasional killer sample here and there). I'd continue to use the ~192 mode (q127) myself, but since the VBR Constrained mode currently allows 320kbps encoding, which often results in files over ~360kbps(!), it makes no sense to limit the true VBR mode from doing the same.

iTunes Plus Files: VBR or CBR?

Reply #22
I'd continue to use the ~192 mode (q127) myself, but since the VBR Constrained mode currently allows 320kbps encoding, which often results in files over ~360kbps(!), it makes no sense to limit the true VBR mode from doing the same.


That logic is flawed. In the first case you push the bitrate up because the encoder is constrained (limited bitrate variation). You enlarge the reservoir until constraints practically aren't constraints anymore as you always have enough bytes left. In the second case (true VBR) there are no constraints (at q127) so there is no need to artificially raise bitrates.

iTunes Plus Files: VBR or CBR?

Reply #23
Hmm I doubt there'd be a "public outcry" of any sort by just adding the true VBR mode as an option in the custom settings menu.. Most people want the best quality they can get using as little space as possible after all, so a brief explanation of what VBR is in the encoder window would probably be enough to avoid any confusion.


Not public outcry but rather a flood of people who are confused.  They don't know what VBR setting to use and there would be people who complain about the "highest quality" VBR setting resulting in files at around 190kbps.  People would think that the highest quality true VBR setting should be at around 320kbps.  There would also be a bunch of people who use the highest quality setting "just because."  I saw the same thing whenever Apple added the "Higher Quality 256kbps" setting in iTunes.  All of the sudden people were confused about what bitrate to use (when they had been using the "High Quality 128kbps" setting) and other people were adamant about using the 256kbps setting even though they didn't conduct any blind ABX tests.  They came to the conclusion by simply switching tracks back and forth in iTunes.  These people further advise others that they too should use the 256kbps setting because it is "higher quality."  Adding the "Higher Quality" setting took the simplicity out of iTunes by making people think about what bitrate to encode at.  They were no longer popping in their CDs and ripping as iTunes was offering a "higher quality" setting.  I have a feeling that the same thing would happen if Apple were to add a true VBR mode unless, on the main import settings page (without going into the custom options), they offered q127 and labeled it as "Highest Quality" or something along the lines.  People would still be confused at to how a "Highest Quality" setting could produce files at 190kbps as they have their minds set that 320kbps (or 400kbps in the case of AAC) is the highest quality.

I also don't think that you can compare 320kbps constrained VBR encoding to true VBR encoding.  That is like complaining that --abr310 (with Lame) produces overall higher bitrates than -V 2.  You are forcing the encoder to use higher bitrates with one setting (ABR, constrained VBR) and you are using quality with another setting (letting the encoder decide which bitrates to use, -V 2 and q127).

iTunes Plus Files: VBR or CBR?

Reply #24
So it isn't really fair to say that an encoder is bad when you get a 170kbps file by using a 128kbps VBR setting.  The encoder is doing its job.


Well, yes.. sort of..  Is IS the job of vbr to vary the bit rate as needed, so some songs could come out much higher. 

Just like you  can flip a coin and get 5 heads in a row.  Over a set of thousands of flips you would even EXPECT to get runs of at least 5 heads in a row.  But the overall average should still be 50%. 

If the overall average bit rate of many songs of various styles is higher than target, then the settings don't work right and comparison to another encoder whose overall average *is* on target isn't fair.  Otherwise the best encoder will be the one that stuffs a file with lossless compression even if you specify 32 kb/s vbr.

The problem here is that you (and/or perhaps Apple) are confusing VBR and ABR. With Average BitRate, the target bitrate is the most important factor, and bitrates should be within a few kbps of the target. With Variable BitRate, the point is to maintain consistent perceptual quality, and some music requires higher bitrate than other music, to hit a certain level of perceptual quality. That's why, with Lame V2, I'll get some entire albums that average around 120 kbps, and others that average around 250. This IS "the encoder doing its job."

Now, Apple's terminology of "128 VBR" may be misleading as people (e.g., DonP) may interpret it that 128 kbps is a target bitrate. But I think what Apple means is that 128 VBR will average 128 kbps over a wide variety of music for a given song or album, it may be much higher or lower.
God kills a kitten every time you encode with CBR 320

 
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