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Topic: Encoding with Nero AAC CBR - Why isn't the bitrate constant? (Read 8963 times) previous topic - next topic
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Encoding with Nero AAC CBR - Why isn't the bitrate constant?

Hi
I'm using dBPowerAmp encoder with the Nero AAC plugin to convert WAV to AAC.

After I encode the files (CBR @ 96kbps) and play them using WinAmp, their bitrates change (98-101kbps) over time.

Why isn't the bitrate constant if my Nero AAC converter settings are CBR 96 kbps (Force AAC LC)?

Any help is appreciated!

Encoding with Nero AAC CBR - Why isn't the bitrate constant?

Reply #1 –
Quote from: Unit1 on
After I encode the files (CBR @ 96kbps) and play them using WinAmp, their bitrates change (98-101kbps) over time.

the AAC format grows like living things do. that's normal.


later

Encoding with Nero AAC CBR - Why isn't the bitrate constant?

Reply #2 –
That is the nature of the AAC format.  AAC will give a quasi-ABR bitrate instead of a CBR, this is due to the implementations of the format itself.  My only question is, why would you want to encode to CBR with Nero's AAC encoder.  Tests have shown that it is much better at VBR encoding than CBR.

Encoding with Nero AAC CBR - Why isn't the bitrate constant?

Reply #3 –
Thanks for the replies

I am testing an AAC decoder and would like to throw at it many different combination of files.  I will also be using FAAC-VBR and FAAC-CBR encoded files.

If Nero is quasi-ABR, why does it have a ABR setting? I guess the question would be: what's the difference between ABR and CBR in Nero AAC encoder?

Encoding with Nero AAC CBR - Why isn't the bitrate constant?

Reply #4 –
Not so much as it is quasi but that the different standard when it applies to AAC.  What is CBR for AAC is not CBR for mp3, vice versa.  Most people are used to the old CBR that they have seen with countless mp3 files but CBR as defined under AAC is different.  Even iTunes AAC CBR will give bitrate changes.  It just CBRs the music to the standard of AAC.  There is probably something in the wiki about this and also in the AAC Tech forum. 

I can't remember the entire details behind the whole thing but it safe to state that CBR in AAC is a different beast than CBR in other formats.  Where people are expecting one bitrate, in AAC it gets as close to the target bitrate as possible with minor VBR tuning.  Probably allows the CBR to sound better than old method CBR...who knows...I don't. 

Check the wiki, see if it states anything...if not you can wait until I post again probably later with the information from the threads I read it from that state why it is like that and blah blah.
Zune 80, Tak -p4 audio library, Lossless=Choice

Encoding with Nero AAC CBR - Why isn't the bitrate constant?

Reply #5 –
Quote from: Unit1 on
Thanks for the replies

I am testing an AAC decoder and would like to throw at it many different combination of files.  I will also be using FAAC-VBR and FAAC-CBR encoded files.

If Nero is quasi-ABR, why does it have a ABR setting? I guess the question would be: what's the difference between ABR and CBR in Nero AAC encoder?


It really isn't a quasi-ABR.  I guess I meant to say that the bitrate will slightly fluctuate but it will remain very close to your target bitrate.  Hence, it performs like ABR but there is a lot less fluctuation in its CBR setting than its ABR or VBR settings.

Gow pretty much hit the nail on the head.  By definition, the AAC format has different settings than mp3.  With mp3 (and probably WMA and OGG but I am not sure), you can set a CBR bitrate and every frame of that music file will be encoded at that bitrate.  With AAC, you can set a CBR bitrate and most frames of that song will be encoded at the bitrate you choose.  There might be some frames here and there encoded at higher or lower bitrates depending on the complexity of your music file.  Overall, the bitrate of the AAC file will remain very close to the bitrate you pick.

I forget the fine details of it all but this performance was defined when the AAC format was made.  It will yield slightly higher quality files for a given CBR bitrate than previous formats.  I guess that is what they were aiming for.

Encoding with Nero AAC CBR - Why isn't the bitrate constant?

Reply #6 –
With most modern formats, CBR is defined as a constant bitrate over a sliding window (buffer). That means that there can be local fluctuations, but when you consider the given window the bitrate is constant.
Even mp3 does this, but for backward compatibility reasons it features a strange behavior that is hiding it (the bit reservoir) by adding its own bitstream intermediate level.

So when you have a look at an AAC stream that is CBR, if you only consider single frames that is perfectly normal that they have different sizes. The resulting stream is still perfectly CBR (as defined by the standard), and not "quasi-abr".
The last audio format from the MPEG natural audio family to feature a constant bitrate for every frame was Layer II.

Encoding with Nero AAC CBR - Why isn't the bitrate constant?

Reply #7 –
How does Apple's AAC encoder manage to always hit the target bitrate, where Nero's AAC encoder just gets close? Does Apple fudge with the stream somehow to make it appear as a constant bit rate?

I only ask because I use AAC in video files for iPod, which can only handle a max of 160kps and not 1 kps more.

(sorry I know this thread is a few months old)

Encoding with Nero AAC CBR - Why isn't the bitrate constant?

Reply #8 –
Even iTunes' CBR AAC and Winamp's CBR AAC are "quasi-ABR".  It's just that they put the target bitrate value into the AAC bitstream to give the decoder a nice round number to display.  Nero does not, so Winamp guesses the frame's bitrate using bits-in-frame * sample rate / 1024000
 

Encoding with Nero AAC CBR - Why isn't the bitrate constant?

Reply #9 –
The newest version of iTunes will actually display the proper average bitrate of CBR/ABR/VBR AAC files.  So 128kbps "CBR" files can actually come out having average bitrates of 125kbps or even 130kbps.