MeGUI: Nero AAC Audio Profiles - No VBR used?

[JakFrost]

I noticed that in the MeGUI STAudio v08 profiles for Nero AAC I do not see any VBR audio profiles and all of them are ABR and also force the encoder to a certain profile feature of AAC, LC, HE, or HEv2+PS.

In the threads that I've been reading regarding AAC quality and the general concensus is that VBR mode should be used for best quality with a q=0.? (0.35-0.45 or 0.5) settings like in this Hydrogenaudio: Nero AAC Recommended Settings (http://www.hydrogenaudio.org/forums/index.php?showtopic=44310).

Now I'm wondering why the audio profiles provided do not have VBR (q=0.?) profiles?

Is there a reason why ABR is preferred over VBR for audio encoding for videos? Is it due to problems with audio & video sync and VBR audio stream?

The reason why I ask is since I'm currently starting to encode AC-3 5.1ch to AAC 5.1ch and I'm trying to determine the best settings for Nero AAC multichannel audio quality.

So far I learned that 24, 32 per channel is recommended for multichannel HE-AAC yielding total bitrates of 128Kbps or 160Kbps for realistic audio encoding sizes and bitrates.

(I understand that there is an overlap between the stereo channel data so the actual bitrate per channel is higher than the 24 or 32 rates in the end due to stereo channel decoupling or bit sharing in AAC codec.)


MeGUI: Nero AAC Audio Profiles - STaudio v8

Nero AAC: NDAAC-HE-64Kbps.xml
Nero AAC: NDAAC-HE-MultiChannel-128kbps.xml
Nero AAC: NDAAC-HE-MultiChannel-160Kbps.xml
Nero AAC: NDAAC-HEPS-32Kbos.xml
Nero AAC: NDAAC-HEPS-48Kbps.xml
Nero AAC: NDAAC-LC-96Kbps.xml
Nero AAC: NDAAC-LC-MultiChannel-192Kbps.xml
Nero AAC: NDAAC-LC-MultiChannel-HQ-256Kbps.xml



*************************************************************
* *
* Nero Digital Audio Reference MPEG-4 & 3GPP Audio Encoder *
* Copyright 2007 Nero AG *
* All Rights Reserved Worldwide *
* *
* Package build date: Aug 6 2007 *
* Package version: 1.1.34.2 *
* *
* See -help for a complete list of available parameters. *
* *
*************************************************************

Quality/bitrate control:
-q <number> : Enables "target quality" mode.
<number> is a floating-point number in 0...1 range.
-br <number> : Specifies "target bitrate" mode.
<number> is target bitrate in bits per second.
-cbr <number> : Specifies "target bitrate (streaming)" mode.
<number> is target bitrate in bits per second.
When neither of above quality/bitrate options is used,
the encoder defaults to equivalent of -q 0.5

[poisondeathray]

This doesn't specifically answer your question(s), but you can create and save custom profiles very easily

[Avenger007]

:readrule: You can't ask what's best (there are always trade-offs).
Try q=0.35 for 5.1 and q=0.45 for 2.0. Go higher if you can hear the difference with higher q values and go lower if it's too much bitrate but don't go lower than q<=0.15 because PS sucks.

[Seraphic-]

:readrule: You can't ask what's best (there are always trade-offs).
Try q=0.35 for 5.1 and q=0.45 for 2.0. Go higher if you can hear the difference with higher q values and go lower if it's too much bitrate but don't go lower than q<=0.15 because PS sucks.

Why would you use a higher Q setting for 2.0 then for 5.1?

[Avenger007]

Well it's more like using lower Q for 5.1 than for 2.0 to keep the bitrate within a sane range. If the bitrate ends up being close to the original then I would use the original instead.

[Sharktooth]

coz the more the channels the better is the encoder efficiency.
abr is preferred to vbr since you can predict the final audio bitrate and that will help calculating the final filesize.

[JakFrost]

coz the more the channels the better is the encoder efficiency.
abr is preferred to vbr since you cand predict the final audio bitrate and that will help calculating the final filesize.

What you wrote is what I understood when I was doing research on the ABR vs VBR usage for audio encoding with video.

I also learned that variable bitrate (VBR) is always used for AAC no matter if it is CBR or ABR, just that the latter encoding types restrict the variance in the bitrate so it doesn't swing so far as normal VBR.

So now the question is, if there is some leeway in the size that the audio stream can be, is it worth going with VBR or is ABR good enough?

I'm still curious to find out if there are audio sync issues with VBR and why MeGUI does not include any VBR audio profiles.

(I'm just starting to use MeGUI for encoding some DVDs and I want to retain good quality 5.1-channel audio, so I'm trying to determine what my standards should be for encoding audio and video going forward. Since I'm trying to do 1-DVD to 1-CD size encodings I do not want to keep the bloated 150 MB AC-3 5.1 448Kbps tracks when I can go to 64 MB HE-AAC 5.1 160Kbps and maintain very good quality audio.)

[fibbingbear]

I just made my own VBR profile, have not noticed any audio sync issues (for the audio sync issues I do have, I I think they're DVDDecryptor issues as opposed to MeGUI or MKV screwing up).

You could ask the same reason why there are so many profiles for fixed bitrate in MeGUI, and far less for CRF. It doesn't really matter that much, just use what works for you. For starting templates, they're great.

If you have leeway, use VBR.

[JakFrost]

I played around with various VBR templates used Q=0.1 all the way through 0.25 and decided against VBR and just stick with the ABR profiles. The average bit rates and sizes of the audio files would vary so drastically between the different audio tracks in the chapters it was impossible to predict the final sizes and bit rates.

I'm sticking with the ABR mode and AAC-HE 64Kbps for 2-channel audio 160Kbps for 5.1-channel.

[tebasuna51]

I played around with various VBR templates used Q=0.1 all the way through 0.25 and decided against VBR and just stick with the ABR profiles. The average bit rates and sizes of the audio files would vary so drastically between the different audio tracks in the chapters it was impossible to predict the final sizes and bit rates.

And, what is the problem?

I'm sticking with the ABR mode and AAC-HE 64Kbps for 2-channel audio 160Kbps for 5.1-channel.

But, to use the true ABR mode you need 2 pass encode.

[Buggle]

I do not really see the problem, just make several encodes and decide on file size which you are going to use. I for instance use the AAC-HE 192kbit profile to encode a file, then use Q3 to encode one and if it differs too much I try to use another Q to get closer.

If you want maximum quality given a certain file size (bitrate), you should use the 2-pass of Nero AAC. Since MeGUI does not give you that option (and if you are not willing to play around with CLI and stuff like that), the only available option to maximize quality within a certain range for size is just trying different Q values. It's only audio, so it won't take that long.