Question regarding 6 channel AAC tracks

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sasha
Posts: 38
Joined: Mon Jan 29, 2007 3:16 pm

Question regarding 6 channel AAC tracks

Post by sasha »

Let me start by saying that I think that all the developers involved with the development of Handbrake are doing an excellent job and I’m amazed about all the effort that is being put into it.

I’m not sure if this is the correct to place this remark but perhaps somebody can explain/help me with understanding some things regarding surround sound.

Most DVD’s have an AC3 or DTS soundtrack for creating surround sound. Except for the DVD player app all applications that require QuickTime are not able to output these soundtracks because Apple cripples the optical output.

Apple posts some trailers with a 6-channel AAC surround soundtrack which you can only experience in full surround if you soundcard that outputs to a receiver that has 6 discrete inputs.

I have done following:
- purchased a Griffing Firewave and connected it to my surround receiver with 6 discrete inputs
- configured the Firewave via the Audio Midi setup and the included software
- downloaded a 6 channel AAC trailer from the Apple site
- created a 6 channel AAC MP4 file with a SVN version of Handbrake

The good thing is that all channels work and full surround sound is present on both files. The sound quality of both AAC files is kind “empty” especially when compared with the orginal surround track (in the case of the DVD).

My guess is that:
- either my setup is not correct
- A receiver manipulates a original AC3/DTS output to make it sound “full”

My SACD player/cd’s that you also need to hook up with 6 discrete channels sound fantastic.

Am I doing something wrong or are receivers not able to handle 6 channel AAC tracks correctly?

Sorry for the lengthy post.

Mucx
Regular User
Posts: 94
Joined: Mon Feb 12, 2007 12:00 am

Post by Mucx »

You have twigged upon something I have well, grumbled about queitly in the past but wasn't sure enough in my knowledge of things to ask the question which was:

"Work to get 6 channel AAC endcoding is great as it will mean QT compatible files without fancy installed codecs and the file size would be smaller BUT why is there so much excitement when, to me, in the unsure knowledge I have is that there is no hardware available that can take the 6 channel AAC and reproduce it properly like it can for AC3"

I was too scared to ask that one before because it required a bit of research (still not sure after looking) so it is my initial quickfire 'oh hang on a sec' response. To me it seems Amps only or can only properly convert AC3 to full surround...maybe newer ones can do AAC? Is that why Apple dont supply movies with 'proper surround' as it would be admitting that the AAC codec isnt widely cared about by other manufacturers when it comes to Moves et all and their chosen sound format?

I dont know the answers to any of this but I always wondered as great as it is getting AAC 6 channel surround...is it any use to those of us like the poster above, if it cannot be properly fed through an amplifier? Maybe that is why Sashas sound is 'empty'?

Please don't kill me lol...

Mucx

sasha
Posts: 38
Joined: Mon Jan 29, 2007 3:16 pm

Post by sasha »

Perhaps it is not an equal comparison but I’m comparing a 6-channel AAC track in combination with the Griffin Firewave with my SACD player.

When I play a multi-channel SACD the sound is absolutely fantastic. When I use the 6-channel AAC track/Firewave combination the sound quality is “empty/flat”. My receiver treats both set-ups equally, meaning that there are no “surround” options. (6-channels in combination with 6-speakers).

I thought that perhaps it might have to do with the speaker set-up (full range speaker instead a satellite system). Perhaps SACD’s are already ‘enhanced’.

maurj
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Post by maurj »

The audio codec shouldn't make a difference here - AC3 is just another way to encode 6 discrete channels of audio, so is AAC. Your amp may not be able to play AAC itself, but since it's just receiving 6-channels of audio from a FireWave, then the fact that that audio came from a Mac playing an AAC shouldn't make a difference.

What might be happening is that your receiver might be post-processing the six channels in an AC3 source audio to make them sound different. There should be no reason that it couldn't do this for 6-channel AAC too, if it's playing through the same amp. What I mean is: both sources are essentially just 6-channel audio containers.

Are you comparing like for like? By this I mean is your 6-channel AAC ripped from the same SACD via a recent svn version of Handbrake? I don't think that our AC3 -> AAC conversion process should lose anything from the source audio - so something else must be at play here.

Would it be possible for you to provide sample files - a .ac3, and a 6-ch AAC - for me to try on a different amp?

- maurj.

sasha
Posts: 38
Joined: Mon Jan 29, 2007 3:16 pm

Post by sasha »

Hi Maurj,

I was already hoping to hear from the AAC guru himself :-)

I agree with you that it should not make a difference if the receiver is 6-channels from a FireWave or from SACD. I’m still trying to find out if I am doing something wrong with the FireWave or that perhaps the receiver manipulates the sound.

The AC3 track is the original DVD played via the DVD player app (also played via the FireWave). The 6-channel AAC track is created with a relative recent (30-03) SVN version of Handbrake (Casino Royale). I will see if I can upload a clip somewhere that you can download.

I will PM the URL to you.

deckeda
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Posts: 138
Joined: Thu Feb 22, 2007 8:38 am

Re: Question regarding 6 channel AAC tracks

Post by deckeda »

sasha there are some things I want to highlight in your post:
sasha wrote: ... I have done following:
- purchased a Griffing Firewave and connected it to my surround receiver with 6 discrete inputs

... The sound quality of both AAC files is kind “empty” especially when compared with the orginal surround track (in the case of the DVD).

- A receiver manipulates a original AC3/DTS output to make it sound “full”
It seems to me that if you're feeding the amp discrete audio channels, no post processing or decoding is necessary or desired. The FireWave has already done that for you, and all the amp needs to do is send the 6 signals out to the proper speakers.

Another, probably even larger variable is that in the case of AC3, you're normally sending that into the amp for decoding whereas with AAC you're doing the decoding in something else (the FireWave box.) What I'm alluding to is that different hardware imposes it's own sonic signature, or at least has the potential to.

maurj , I'm comforted by your response that they should sound the same but I don't see that as a guarantee, let alone a given. AC3 and AAC, being different codecs, use different algorithms, and therefore their designers made different decisions when deciding what parts of the sound to throw away. Am I right? I wouldn't be surprised they might sound at least a little different, even at the same bitrate.

But sasha, it may be something as simple you comparing an AC-3 that was done at a relatively high bitrate against a low bitrate AAC, yes?

Kurtz
Posts: 37
Joined: Wed Feb 21, 2007 6:52 pm

Bitrate

Post by Kurtz »

Hi
From the sound of it, as already mentioned, it might be something as simple as a low bitrate AAC and a high bitrate AC3. Just for reference what Bitrate are you encoding audio at? Furthermore what is the total bitrate of your movie?

thanks

sasha
Posts: 38
Joined: Mon Jan 29, 2007 3:16 pm

Post by sasha »

Hi guys!
Thanks for your reaction. I have ripped the 6-channel AAC soundtrack with the default Handbrake settings (128 kbps). My guess is that each channel is 128kbps, like the default settings in iTunes.

The total bitrate of the movie according to Quicktime is 1700 kbits/sec. I will do a test at a higher bitrate.

I will use the the following:
complie the latest SVN
Image Qualiy: MP4, x264 (h.264 Main) @2000kbps 2-pass, Anamorphic ON
Audio Quality: 6-channel AAC, samplerate 48Hz, 256kbps

I will let you know!

sasha
Posts: 38
Joined: Mon Jan 29, 2007 3:16 pm

Post by sasha »

Although I have not yet been able to test on my surround system. I think Kurtz and Deckeda are on to something.

I have done a small test with the settings mentioned in my previous post and already on my Macbook Pro I could hear the difference. The sound already was much more "lively". As soon as get home I will test it up via the FireWave/Receiver.

Perhaps jumping to conclusion but the 128kbps setting is shared accross all 6 channels, resulting in 21.3kbps per channel? Regular stereo (like fi the default iTunes settings) would get 64kbps per channel.

Based on the test I'm now creating a full feature with a Audio track @320kbps. I think that will result in 53.3kbps, per channel.

I will let you know asap

hawkman
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Post by hawkman »

sasha wrote:...the 128kbps setting is shared accross all 6 channels...
Yes, this is the case. (Well spotted guys.)

A value of 64kbps per channel is usually a good working estimate for great music quality, and voice/sound effects usually compress much better. HandBrake's current maximum of 320kbps should therefore sound pretty damn good.

sasha
Posts: 38
Joined: Mon Jan 29, 2007 3:16 pm

Post by sasha »

Okay, I have been able to test the new encoded file.

I have used the following settings:
Compiled the latest SVN
Image Quality: MP4, x264 (h.264 Main) @2000kbps 2-pass, Anamorphic ON
Audio Quality: 6-channel discrete AAC @320kbps, @48Hz

Sound has improved dramatically and does not sound as ‘empty’ as with my original encoding. So it was definitely an encoding mistake from my side. I was under the impression that every channel would be encoded at 128kbps instead of 21kbps.

Having said this, and perhaps I am being too critical, but I still think the sound quality is not as “rich” as the original AC3 track from the DVD. As Hawkman mentioned the dialog sound good but in my opinion music and more specifically the LFE channel during certain action sequences lack in “oomph”. Again compared to the original AC3/DVD track.

As a test I encoded the same sequence in stereo. Although now not being surround anymore the bass-sections are quite strong a comparable with the AC3 track.

Could this be related to the Bitrate of the soundtrack? Since I upgraded to Handbrake’s maximum of 320kbps the sound improved so much that perhaps when it is upgraded to 384kbps the overall sound would become “full” again.

Perhaps the developers are already working on this and I can’t even begin to imagine how complex and time-consuming this all is for everybody. In the meantime I will go through my receivers settings as well to see if I can tweak it some more.

Respect to all developers involved!

loyalty_anchored
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Post by loyalty_anchored »

sasha wrote:Could this be related to the Bitrate of the soundtrack? Since I upgraded to Handbrake’s maximum of 320kbps the sound improved so much that perhaps when it is upgraded to 384kbps the overall sound would become “full” again.
I would highly recommend you encode your 5.1 channel AAC at 384 to allocate 64 to each channel. Typically AC3 on commercial DVDs are encoded at 448.

sasha
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Joined: Mon Jan 29, 2007 3:16 pm

Post by sasha »

I would love to encode @ 384kbps but I don't think it is available yet. The SVN build that I compiled yesterday has a maximum of 320kbps.

Having read the Handbrake Surround Sound Guide (http://handbrake.m0k.org/trac/wiki/SurroundSoundGuide) 384kbps is/will be supported at some time.

deckeda
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Joined: Thu Feb 22, 2007 8:38 am

Post by deckeda »

Ouch, I didn't realize it was the total bitrate, not per channel being used. Hell yeah, crank that sucker up.

Having said this, and perhaps I am being too critical, but I still think the sound quality is not as “rich” as the original AC3 track from the DVD

Probably just means you have a good ear. Consider that AC-3 is a lossy codec, and not the best at that .

Then, consider you're re-encoding it with another lossy codec and so throwing away even more.

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