Audio file track ordering inside an MP4 file

[buzzby]

Hopefully this has a simple answer.

I am doing a lot of remuxing from MKV to mp4 for my new AppleTV (related thread (http://forum.doom9.org/showthread.php?p=1459788) ) and just wondering if the order of the audio files is important. For a TV episode I am keeping the AC3 audio and adding a AAC track. The video stream is track 1 but does it matter what audio track is next? When i say does it matter this is purely in the context of having the file run within iTunes and on AppleTV.

Thanks

[CarlEdman]

Generally, no. But note that many players will default to playing the first audio track or the first audio track of the preferred language (e.g., English), so it is generally a good idea to put the main track in the main language/track first.

iTunes (and given the common heritage, I would not be surprised if AppleTV too) tends to play *all* non-disabled audio tracks. If you want to keep multiple tracks, be sure to mark any tracks but the one you want played as disabled. Other players do not have this problem.

Also, why are you keeping both the AC3 and AAC track? Generally, AAC tracks are preferable because they are smaller at the same quality. If your source is AC3 and you want to avoid the quality degradation of any conversion, have you checked whether you can just keep the AC3 track in the mp4? It is not quite standard, but widely supported.

[Ritsuka]

I guess the AppleTV wants first the AAC track and then the AC3 track. Make sure to add them to the same alternate group. And of course to enable only the first audio track.

[buzzby]

Thanks for the responses. I am wanting to keep the AC3 files just as a temporary measure until I am confident my conversion is the best it can be. Once I am happy with by conversion process I will drop the AC3 files to save some space.

I am building the MP4 file with MP4Box how can i enable or disable audio tracks? As iTunes and AppleTV cannot play AC3 is this something to worry about?
Thanks again.

[Ritsuka]

AppleTV can pass through the AC3 track to a receiver, so if you want 5.1 you should keep the AC3 track too.

[CarlEdman]

Current versions of mp4box have command line switches to enable and disable individual tracks. Try "mp4box -h general" for details.