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I have several albums on DVD with both Dolby Digital 5.1 and DTS 5.1 versions. I'd like to be able to listen to them without dealing with physical disks all the time.

I have no problem using ffmpeg to pull the audio data out and stick it in a file, but I haven't been able to find the magic format that will let it play via Apple TV. I've read that the Apple TV doesn't need to understand the format, it just needs to send the data to a receiver that does, which makes perfect sense, but how do I get it to do that? I can't get a WAV containing DTS to work.

If the solution ends up being to stick the AC3 data in a video container, that's fine, but ideally, I just want to store/play the audio (and ideally, it would be the DTS version).

I have both a 1st and a 3rd generation Apple TV (a.k.a. the Apple TV 2 with 1080p support), so any solution, old or new, will do.

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Yes. The Apple TV can pass through 5.1 AC-3 to a receiver if you set up the file correctly. Apple has the full details in a knowledge base article, but basically you need to set up your mp4/m4v file with a stereo AAC as the default audio track, then put your AC-3 track after it, disabled. On the Apple TV, you need to enable it by turning on Dolby Digital Out in the Audio Settings.

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That worked. Thanks. I’ll keep looking for an audio-only solution, though. :-) FYI, the Dolby Digital setting can be left on “Auto”. –  Rob McBroom Jul 14 '12 at 3:52
    
Actually what may work better for you is encoding your albums to Apple Lossless (ALAC). It can technically handle 5.1 sound, and the Apple TV supports it (although the specs don't explicitly mention multichannel, just support for Apple Lossless). XLD should be able to encode to 5.1 ALAC, but you may need to use something else to convert from AC3 to a format XLD can read (probably wav). –  robmathers Jul 14 '12 at 15:21
    
Yes, I'm aware that once you have a suitable WAV file containing the DTS data, you can convert it to ALAC (though DTS is already compressed and there's not much of a space savings). The trick is creating the WAV file. I've been working on a Python script to do it, but no luck so far. –  Rob McBroom Jul 26 '12 at 2:42
    
One further update to the answer… If you create a video container, it no longer seems to be necessary to include stereo AAC at all. You can have just one AC3 audio track. –  Rob McBroom Jul 26 '12 at 2:44
    
ffmpeg should be able to do multichannel AC3 to wav without issues, have you tried that? –  robmathers Jul 26 '12 at 20:12
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Actually, Apple TV cannot support AC3 5.1 audio track. While, we usually have the demand to get the full AC3 5.1 surround sound to let Apple TV pass-through Dolby Digital to A/V receiver to take advantage of our surround sound system. I searched widely via Google, and found the easiest solution: http://tablet-video-tips.com/convert-videos-to-apple-tv-with-ac3-51-surround-sound-mac/

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ATV version 2 & 3 do support AC-3 5.1 pass-thru. –  jimtut Dec 18 '13 at 4:51
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AppleTV will pass through an AC3 5.1 audio track if it's a secondary audio track (with AAC the primary). You need to embed the audio into an m4v file and make sure all tracks have the same language setting (including "Unknown"), but you can use empty video and AAC tracks and just add the AC3 Passthru track as the only actual data. Then if you mark it as "Music Video" in iTunes, iTunes will treat it as if it's a music file (and it'll show up with other music files). Things get more complex with music that segues between tracks, since iTunes cannot stream "videos" seamlessly one into another like it can with music.

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