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Topic: I don't understand bitrates in AAC (Read 10255 times) previous topic - next topic
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I don't understand bitrates in AAC

Hello !

I am french and a big noob in using AAC encoding, so my apologies for what will come next ! 

I heard from AAC a long time ago, but have always used MP3 LAME either for audio CD encoding than for video DVD encoding. And I start to learn what it is and how it works a few days ago. I understand the differents versions of AAC (AAC, AAC v2 = HE AAC and AAC v3 = HE v2 AAC ; LC or HE AAC...) and how works the quality setting in encoding, but not the bitrate.

I want to use the Nero AAC encoder to encode AC3 files from DVD in stereo AAC and if, on the first hand, find on Internet many settings for the quality (around 0.5), I can't find which bitrate use ? Some guides talks about 48 or 64 kb/s some others 128 kb/s or more. I read differents wiki and understand a bigger bitrate than 128 kb/s will not be smart in HE AAC, but why use a bitrate setting if it already exists the quality one ?

With MP3 it is simple, CBR, ABR, VBR, and a bitrate for CBR, or V0 to V9 settings vor differents kb/s ranges.

So what is the difference between an AC3 audio file encoded in 64 kb/s and 128 kb/s in the same quality, 0.5 for example ?

Sorry to bother you and thanks in advance ! 

Edit : And I don't understand either the difference between ABR and VBR in AAC. ^^

I don't understand bitrates in AAC

Reply #1 –
Hi Ludo13B,

OK, let's see

There is no AAC v3. There is AAC Low Complexity (LC), High Efficiency AAC (for low bitrates), and HE-AAC v2 (with Parametric Stereo, for even lower bitrates). Among other things like the - rarely used - AAC main profile.

The "quality" option in the nero AAC encoder produces variable bit rate (VBR) files. That's why you can't tell exactly what bit rate the resulting MP4 file will have. If you want a certain constant bit rate (CBR), you can use the "-cbr bitrate in bits per second" option. There is also a "-br bitrate in bits per second" option, which I think produces ABR files (which is something between CBR and VBR  ).

Did I miss anything?

Edit: Yes, I did. The higher the quality value (between 0 and 1), the higher the bitrate. A value of 1 produces, depending on the input signal, something between 256 and 320 kbps, I would guess.

Chris
If I don't reply to your reply, it means I agree with you.

I don't understand bitrates in AAC

Reply #2 –
Thank you Chris for your answer, I think I get it, even if some things remains obscur to me. ^^

At first, I thought LC-AAC was exclusively CBR, and HE-AAC VBR (and ABR). Don't ask me why, I don't know ! 
I think it's due to multitasks, I should not do differents things at the same time. ^^

In the Wiki, "-q" is VBR, with a number between 0 and 1, "-br" is ABR with a bitrate, and "-cbr" is CBR with a bitrate (really ? ^^). So it's a number between 0 and 1 OR a bitrate, but not the two of them. But in some audio profiles for MeGUI (a GUI to encode video and audio), and you said the same to me, the two appears for the same encoding...
So is there a priority for the setting before the number or bitrate (like : "Ok it's -br, so I will pass on the quality and work with the bitrate" or "It's -q so I will ignore the bitrate and work with the number"...) or is the bitrate always taked into account to get a file size near what we expect... but at the cost of quality ?


More, I have no references. For example, if I use the Nero AAC encoder version 1.3.3.0 (the last one). And only stereo ABR* (to encode movies or mini-DV) at quality 0.5 (if it's taked into account).
Will I have artefacts if I use HE-AAC at 96 kbps ? What's the AAC equivalent of LAME ABR 128 kbps ? HE-AAC 64 kbps ? LC-AAC 96 kbps ?
I read a lot of blind listening tests, but the more recent was from 2005 and it's hard for me to know what is the AAC equivalent to LAME settings today...

* ABR : it was used to encode audio on movies with LAME, because VBR wasn't enough reliable for this kind of use, but is it the same for AAC ? It's a different codec, so maybe VBR will be reliable ?

Hum, thanks in advance, maybe it's really simple, but the audio profiles for MeGUI disappointed me. ^^ If you can clearly answer to these questions, I will (maybe ^^) not bother you again !

Ludo

I don't understand bitrates in AAC

Reply #3 –
"Will I have artefacts if I use HE-AAC at 96 kbps ?" The answer is yes (and probably worse quality than use the LC profile), read the threads below.

http://www.hydrogenaudio.org/forums/lofive...php/t40022.html

http://www.hydrogenaudio.org/forums/lofive...php/t53340.html

http://wiki.hydrogenaudio.org/index.php?ti...rametric_stereo

I don't know if there is any problem related to encoding tipe with AAC and the movie, but VBR will produces "contant quality" files when ABR will produce files that "averages at especified bitrate"... If you want to have sure encode only a little piece of the movie like 1 minute and test it to see if there is any problem using VBR in the player...
Sorry by my bad english...
Sorry for my bad english.

I don't understand bitrates in AAC

Reply #4 –
Quote from: C.R.Helmrich on
Among other things like the - rarely used - AAC main profile.


At the risk of having Schuyler grumble at me again, has anyone EVER used the high-complexity profile?  Oh, I mean "main".
-----
J. D. (jj) Johnston

I don't understand bitrates in AAC

Reply #5 –
Quote from: Diow on
"Will I have artefacts if I use HE-AAC at 96 kbps ?" The answer is yes (and probably worse quality than use the LC profile), read the threads below.

http://www.hydrogenaudio.org/forums/lofive...php/t40022.html

http://www.hydrogenaudio.org/forums/lofive...php/t53340.html

http://wiki.hydrogenaudio.org/index.php?ti...rametric_stereo

I don't know if there is any problem related to encoding tipe with AAC and the movie, but VBR will produces "contant quality" files when ABR will produce files that "averages at especified bitrate"... If you want to have sure encode only a little piece of the movie like 1 minute and test it to see if there is any problem using VBR in the player...
Sorry by my bad english...

Thank you for your answer, I made the conclusion to test differents profiles from MeGUI and to compare them with their size and my listening impressions. Unfortunately, I don't know any tools to help me in audio files comparison, because I doubt my ears will hear the difference... But let's try anyway.
Here are the settings of the three profiles I will create and compare :
  • MP3-128ABR - MP3 Lame v3.98.2 - ABR 128 Kbps stereo
  • NDAAC-HE-64Kbps - Nero AAC v1.3.3.0 - ABR HE 64 Kbps stereo (q 0.5)
  • NDAAC-LC-96Kbps - Nero AAC v1.3.3.0 - ABR LC 96 Kbps stereo (q 0.5)

Concerning VBR, I read it can works for several files but have problems with some others, so I prefer to use the most reliable method I can.


Quote from: Woodinville on
Quote from: C.R.Helmrich on
Among other things like the - rarely used - AAC main profile.


At the risk of having Schuyler grumble at me again, has anyone EVER used the high-complexity profile?  Oh, I mean "main".

I am too green in AAC to try some settings not much used. 

And the other questions on my previous post still are on ! 

Thanks !

P.S. : The test between AAC HE 64 Kbps and LC 96 Kbps is it necessary ? Or the advantage will obviously come for one of them ?

I don't understand bitrates in AAC

Reply #6 –
Maybe this won't really be useful to you in regard to your question, but I think you are wasting your time trying to convert AC3 to AAC to save space.

I did some test of this previously, and I could not get a significant reduction in size without nasty artefacts. I think it all boils down to multichannel inefficiency of the AAC codec, but I don't know it's technical details, so cannot be sure of that claim

You may take a try on ogg (maybe aoTuV) instead. I didn't do much testing on it, but afair, it were better than AAC.
Can't wait for a HD-AAC encoder :P

I don't understand bitrates in AAC

Reply #7 –
No, it's interesting, but do you think you will conclude the same way for 2.0 output ? I am not interested in keeping 5.1.

I don't understand bitrates in AAC

Reply #8 –
No for 2.0 you can simply follow the results of the listening tests to see optimal bitrates on different encoders - I just assumed you would keep multichannel...
Can't wait for a HD-AAC encoder :P

I don't understand bitrates in AAC

Reply #9 –
Oh, a lot to comment on!

Quote
I think it all boils down to multichannel inefficiency of the AAC codec

What inefficiency? I don't know any codec which is more efficient than (HE-)AAC for multichannel audio (or (HE-)AAC + MPEG Surround for very low bitrates). The problem might be the tandem coding as you describe it, i.e. coding with AC3 first, then re-encoding with AAC. That should be avoided anyway. So Ludo, in your case of 2.0 audio, AAC should work fine.

Quote
In the Wiki, "-q" is VBR, with a number between 0 and 1, "-br" is ABR with a bitrate, and "-cbr" is CBR with a bitrate (really ? ^^). So it's a number between 0 and 1 OR a bitrate, but not the two of them. But in some audio profiles for MeGUI (a GUI to encode video and audio), and you said the same to me, the two appears for the same encoding...

Did I?  I thought I said the same as the Wiki you quoted. "-q" stands for quality. The nero encoder maps the quality value to a certain target bitrate range internally, depending on the profile (HE-AAC or AAC LC).

Quote
Will I have artefacts if I use HE-AAC at 96 kbps

Yes, but you will also have artifacts with AAC LC at that bitrate, for example, a reduced bandwidth (cutoff around 14 kHz), which will make the audio sound slightly duller than it originally was. Question is, which sound worse, the artifacts from HE-AAC or the ones from AAC LC? Just try both and see what you prefer.

Quote
What's the AAC equivalent of LAME ABR 128 kbps ? HE-AAC 64 kbps ? LC-AAC 96 kbps ?

Don't know for ABR, but for CBR, according to some old tests I cited here, it's LC-AAC around 100 kbps CBR. HE-AAC at 64 kbps in my opinion sounds worse than MP3 at 128 kbps.

Quote
At the risk of having Schuyler grumble at me again, has anyone EVER used the high-complexity profile?  Oh, I mean "main".

Haven't heard of anyone. Is Schuyler reading this?

Chris
If I don't reply to your reply, it means I agree with you.

I don't understand bitrates in AAC

Reply #10 –
Quote from: Ludo13B on
  • MP3-128ABR - MP3 Lame v3.98.2 - ABR 128 Kbps stereo
  • NDAAC-HE-64Kbps - Nero AAC v1.3.3.0 - ABR HE 64 Kbps stereo (q 0.5)
  • NDAAC-LC-96Kbps - Nero AAC v1.3.3.0 - ABR LC 96 Kbps stereo (q 0.5)


Those settings don't appear to be -q0.5 even when one considers transcoding from AC3 audio.  -q0.5 normally results in bitrates around 190kbps VBR and does not use HE instructions, only LC-AAC.  I am not sure what settings your software is using but it doesn't like it is using the standard Nero -q values.

I don't understand bitrates in AAC

Reply #11 –
Ok, my mistake. 
The MeGUI profiles contains information about quality AND bitrate, but when I configure them via MeGUI, it restrict me to chose one OR the other, so lack of understanding filled ! 

I also will see aoTuV who seems to be really efficient for my purpose, stereo and good quality.

Thanks to all for your explanations and advises !

Ludo
 

I don't understand bitrates in AAC

Reply #12 –
Quote from: odyssey on
Maybe this won't really be useful to you in regard to your question, but I think you are wasting your time trying to convert AC3 to AAC to save space.
I did some test of this previously, and I could not get a significant reduction in size without nasty artefacts. I think it all boils down to multichannel inefficiency of the AAC codec, but I don't know it's technical details, so cannot be sure of that claim

Did you test with the latest Nero AAC 1.3.3? There were improvements in last version regarding multichannel encoding.