AAC Surround (from AC3) Issues

[Dayvon]

\\I posted this in the A/V Containers section, because I'm not sure which forum heading I should put this under. Mods - please delete the one with no responses/or wrong spot.//

So I've been in the process of upgrading my compression formats (Xvid -> x264) and am considering switching to the MP4 container. I like the idea of using this more advanced container to in a way more future proof my archives by conforming to AAC & AVC standards.

However, like everyone this presents an audio dilema because AC3 is not a MP4 compliant format. So I've been exprimenting with convert AC3 to AAC surround.

I've run into a problem.

The AAC surround file that I make using BeLight sounds great. No artifacts, accurate surround panning. However, when I mux the MPEG-4 video file and the AAC/MP4 audio file using YAMB, things seem to get weird. First of all, the AAC file is no longer decoded correctly in terms of the center channel being gone. Now, this might be a problem that is inherent to my setup (I have a 4.1 system) and the failure of my sound card to assign the center channel properly. But what really irks me is the quality of the AAC file in the muxed MP4 sounds VERY degraded as opposed to the AAC unmuxed. It is VERY apparent in vocal passages, a kinda of bit-ing that makes voices grainy.

Does anyone have a clue what is going on? Is there a really good AAC decoder (right now using FFDShow realaac decoder) that would fix the channel assignment issues? Why would a sound file sound degraded in quality after muxing?

Here's a quick list of my process and programs used.

DGIndex -> AC3 file -> BeLight -> AAC Surround -> YAMB -> MP4 Video-Audio Muxed File -> Media Player Classic -> Haali Splitter -> FFDShow -> Creative Sound Card -> Logitech z-560's

Thanks for any help you can give. I'd really like to jump to the new improved container, but if I can't get AAC surround at a comparable quality level to the original AC3, I'm sticking to x264-AC3-avi's.

[[)370|\|470!2]

Media Player Classic -> Haali Splitter -> FFDShow -> Creative Sound Card -> Logitech z-560's

...And when you're listening to aac w/o container, what filters being used?

...There could be only two causes:

1. Something wrong with FFDShow aac filtering;
2. MP4BOX has some bugs(which is normal:D)

You may want to try CoreAAC DS filter, to make sure.

[SeeMoreDigital]

It's physically impossible for YAMB to alter the channel mapping of 6 (5.1) channel AAC streams or degrade the quality of the AAC stream (unless it's borked to begin with).

I use Foobar2000 with Nero's AAC filters to encode 6Ch AC3 to AAC. YAMB to generate my muxes to MP4 and FFdshow's filters to decode the AAC and MPEG-4 streams.

I'm also able to play such encodes/MP4 muxes in hardware and decode the 6Ch AAC streams discretely using an external DSS amp.... So it is possible ;)


Cheers

[Dayvon]

Tried using the CoreAAC DS filter. No luck. I'm thinking that either the splitting, the muxing or my sound card are the problem. I'm gonna do somemore messin around. I might setup a link to download a quick example.

[[)370|\|470!2]

If, as you've mentioned before, unmuxed aac sounds fine, then it has nothing to do with
your sound card.

[SeeMoreDigital]

If it helps, you can use my 12 second MPEG-4+6Ch AAC-LC in MP4 (http://81.98.148.105/Uploaded_Files/Doom9_Forum_files/12sec_MPEG-4+6ChAAC-LC_in_MP4.7z) sample as your source.

I use such samples to map my speakers.... so I know they work 100%


Cheers

[Dayvon]

If it helps, you can use my 12 second MPEG-4+6Ch AAC-LC in MP4 (http://81.98.148.105/Uploaded_Files/Doom9_Forum_files/12sec_MPEG-4+6ChAAC-LC_in_MP4.7z) sample as your source.

I use such samples to map my speakers.... so I know they work 100%


Cheers

Hey thanks alot. That clip does help. There is a problem with the assignments. I have to use FFdShow's post-mixer to get the audio to map properly. Using your clip without the post-mixer, the center channel seems to be present, just 20 times quieter than the other channels. This might be bleed over (4.1 system) or would this have to do with the center channel needing to be normalized or compressed? I don't like to normalize or compress so I turn those options off in BeLight. Is that causing my problem?

For some reason, now everything (video-audio muxed .mp4 & audio aac-only) seems to sound low-quality when it didn't before. I'm gonna have to setup another set of tests, and figure out whats going on.

BTW, if anybody knows what LC-AAC surround setting (in BeLight) I should use to get top quality -not larger than the AC3 file- That would be awesome. Maybe just changing that would help.

[SeeMoreDigital]

I have quite a few test files here (http://www.seemoredigital.net/51_Test_Encodes/51_AV_Setup_Test_Files.html) on my web site ;)


Cheers

[Dayvon]

BTW, I think I figured out what I did wrong with the AAC files... I think that they were originally mp4 files that I renamed to aac... yikes. Well anyway, it seems that I found some aac settings that I like the quality of and are a bit smaller than the AC3 files so I'm now good to go.

I really seem to prefer LC AAC as opposed to HE AAC. Call me picky, but i couldnt seem to find a HE AAC setting that has enough quality to it. If anyone has a HE AAC setting that you can't tell much of a difference between that and the original AC3, let me know. I stuck with LC AAC streaming 5.1 settings.