AppleTV3 & AAC 5.1

Discussion in 'Apple TV and Home Theater' started by shaunreinson, Dec 16, 2013.

  1. shaunreinson macrumors newbie

    Mar 10, 2013
    I'm confused about the way the AppleTV3 handles Surround sound (and I do understand a lot about this stuff) so would appreciate some clarification - if anyone can help....

    AC3 -

    I'm fine with this no problems. Its passed through on the Optical output so my Yamaha sound-bar can decode the discrete channels. I'm also guessing its on the HMDI cable as well (?) Great.

    AAC 5.1 -

    This can be contained in MP4/M4V files and is a discrete format with better compression than AC3.

    How does the AppleTV3 deal with this ?

    I've read in various places that its down mixed to a stereo 2.0 PCM signal and that this 2.0 contains the matrixed DPLII data that my yamaha sound bar can still decode for a surround effect.

    I've also read that it down mixes the LF & RF discrete signals ONLY into a 2.0 PCM signal that would (probably) not reveal any surround info when passed through a DPLII decoder

    I'm guessing this signal is contained on the HMDI cable, so what signal is on the Optical cable ?

    I"ve googled this many times, and its quite conflicting and I'm still none the wise so any help and practical advice would be great.

    The reason I'm asking is I have a lot of AAC5.1 movies that I'd be happy to watch if its down mixed to a DPLII signal - just try to figure out how to set it all up.
  2. priitv8 macrumors 68040

    Jan 13, 2011
    It doesn't.
    Your AppleTV-compatible movies (and this is defined in Apple documents) shall only contain AAC stereo and AC3 5.1 surround sound tracks.
    All others are simply ignored.
  3. Idgit macrumors 6502


    Mar 14, 2004
    If you want surround sound from your movies containing AAC 5.1, use MP4Tools to convert the AAC 5.1 to AC3 5.1.
  4. shaunreinson thread starter macrumors newbie

    Mar 10, 2013
    Well, I answered my own question by creating a video with AAC 5.1 audio and no AC3 or other tracks then ran it through my AppleTV

    The answer is that it down-mixes the audio to a 2.0 DPLII so any surround sound receiver can decode.

    If you want to run the test yourselves on your setup - then see this youtube video with full instructions I created - together with a download link for the video to put on your equipment.
  5. priitv8 macrumors 68040

    Jan 13, 2011
    I just tried it on my AV Receiver and it does not look like DPL II downmix!
    It looks more like a stereo downmix. I can not hear any sound from my surround speakers, nor from LFE Sub.
    DPL II downmix should be a 4-channel matrix: L, R, C and SR (Surround Rear, mono). If I switch on DPL II decoder, then C channel obviously comes from centre speaker, because DPL decoder directs identical signal from L and R (the sum) into C.
  6. shaunreinson thread starter macrumors newbie

    Mar 10, 2013
    You could be right - I've amended the Youtube comments accordingly.

    I'm using a Yamaha sound-bar for my surround, and get no LFE either.

    Could do with finding some Apple specs on this...
  7. priitv8 macrumors 68040

    Jan 13, 2011
    Well, I'd say it looks like
    L = LF + LR + C/2
    R = RF + RR + C/2
    (i.e. front and rear channels are summed on either side.
    I was about to say all other channels are simply dropped except front L and R, but then again, I get the rear sound always from front speakers, thus I assume they are summed. Because Centre is still at centre, so it's component must be in both L and R.

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