AC3 5.1 to AAC 5.1

[MntTopLivin]

This pertains to using both versions 2.5.6 and 2.6, when converting a MKV with AC3 5.1 to a MP4 with AAC 5.1. The settings are a COPY of the video and, for audio, bitrate 384 and adding DRC, but for 2.5.6 using FAAC and 2.6 using AACLAV. For 2.6 the rear sound is set to the left speaker and front is right, but it does not do this on 2.5.6 (FAAC). However on 2.5.6 all the speakers are playing on the proper channels, except for rear right, which is playing on the lfe channel. I notice this by using a channel spectrum, since I currently do not have a home surround system.  My plan is to have all my files as MP4, since they play nice with WMC extender (XBox) and other players, while keeping AAC 5.1. Yes they do play on the Xbox, but whether the audio is 5.1 or a downmix I can not confirm at this time. Are these bugs with the encoders and if not are these problems fixable?

[Jan Gruuthuse]

My best guess you keep the original AC3 and move it in position to track 1 as 0 would be the default and have a 5.1 AAC stereo downmix in track 0.
Rumor has it that MP4 5.1 AAC is not supported on XBox, can't confirm: no xbox.

MPEG-4 Part 2 support

The Xbox 360 console supports the following for MPEG-4:
File extensions: .mp4, .m4v, .mp4v, .mov, .avi
Containers: MPEG-4, QuickTime
Video profiles: MPEG-4 Part 2 (Simple Profile and Advanced Simple Profile)
Video bit rate: 5 Mbps with resolutions of 1280 Ãâ€" 720 at 30 fps
Audio profiles: AAC, 2-channel, Low Complexity
Audio max bit rate: No restrictions

source: The Xbox 360 console supports most movie formats
make a small sample video and test with this if this works for all your players.

[MntTopLivin]

Thanks for the reply, but I think you missed my problem. When converting from AC3 5.1 to AAC 5.1, using either 2.5.6 or 2.6, there are problems. For 2.6 the rear sound is set to the left speaker and front is right, but it does not do this on 2.5.6 (FAAC). However on 2.5.6 all the speakers are playing on the proper channels, except for rear right, which is playing on the lfe channel. Is this a problem with the encoder or is it fixable?

[Jan Gruuthuse]

It all depends on the used settings? Download this 64 MB test file with ac3 5.1 track. https://rapidshare.com/files/95641368/AC3-51.zip
Check out the attached screenshot.
In avidemux 2.6 when this video is loaded, select: in
Main Menu -> Audio -> Audio Track -> Audio filters
several settings possible

[MntTopLivin]

Thanks for trying, but you are still misunderstanding the problem. I know how to use the settings. After much research, I believe the problem is with the encoder. It does not properly map the channels when going from AC3 to AAC.

[Jan Gruuthuse]

Sorry for trying to understand your problem? Perhaps you did overlook this posting: No downmixing for 6ch AC3 streams? in your research? The stereo download is no longer available at rapdishare. But you now have the original BBC 5.1 AC3 test TV transmission to test with.

[MntTopLivin]

I am not downmixing; am not going from 5.1 to 2.0. I am going from AC3 5.1 to AAC 5.1. When using Avidemux 2.5.6, which uses FAAC, the Lfe and R channel are swapped.

[Jan Gruuthuse]

Developers may have to revisit the 2.5.6 code? All I could find regarding this would be this:

Channel Arrangement for Multi Channel 5.1 Audio Formats

               Channel NÃ,°:
Format :   0    1    2    3    4    5
5.1 WAV    FL   FR   FC   LFE  SL   SR
5.1 AC3    FL   FC   FR   SL   SR   LFE
5.1 DTS    FC   FL   FR   SL   SR   LFE
5.1 AAC    FC   FL   FR   SL   SR   LFE
5.1 AIFF   FL   SL   FC   FR   SR   LFE

source: Understanding Channel Arrangements for 5.1 Audio Formats

[MntTopLivin]

It seems to your finding, the only channels changing positions are the FL, FC and FR. Not quite sure why the conversion is shifting the others. I have a test file and it plays Lfe as SL, SL as SR and SR as Lfe. The AC3 is the original and AAC has the wrong mapping. http://www.immdee.com/surroundtestfile.zip

[Jan Gruuthuse]

Matrix may be not 100% correct? It would explain what is going on. Hope developers have a look at this. And find a fix?