.m4v AC3/Dolby Digital

Discussion in 'Apple TV and Home Theater' started by akm3, Mar 6, 2009.

  1. akm3 macrumors 68020

    Nov 15, 2007
    I've been (slowly) encoding with Handbrake my DVD collection, and have carefully chosen audio track 1 to be an AAC 2 channel mixdown of the AC3 5.1, and have a second audio track to be the raw AC3 5.1 stream.

    Now that I've done that, how can I access that 'true dolby digital' stream? Can AppleTV do it? Can my Mac Mini do it? How about an xBox I'm streaming via Connect 360? Other options?

    Obviously whatever is doing the decoding needs to have a digital output to a receiver or preamp that can handle the AC3/Dolby Digital signal; but how do I get my device to recognize and select and pass through that signal to my receiver?
  2. GreatDrok macrumors 6502a


    May 1, 2006
    New Zealand
    I've been doing the same. You need to make sure the option for Dolby Digital output is enabled, and then you simply connect your ATV to a DD amp, either via the HDMI connection if your amp has the input or good old optical. The ATV will pass the DD encoded track to your amp and all 5.1 channels will light up. I have an HDMI upscaling Yamaha amp (RX-V861) and it decodes the 5.1 input from my ATV just fine over the HDMI connection.

    Connect 360 to Xbox 360 doesn't support the DD tracks unfortunately and that was just one of the reasons (noise and a sucky interface being the others) for me switching from the Xbox to the ATV.

    The ATV will always play the AAC stereo track since it doesn't do DD decoding itself which is why you need both.
  3. evildarklordude macrumors newbie

    Feb 20, 2008
    New York, NY
    Hi akm3,

    If you have an Apple TV, it will automatically pass through the ac3 track to the receiver through its HDMI and/or optical connection. The Handbrake determined their preset by studying how 5.1 AC3 passthrough was implemented in the intro movie that ships with the Apple TV. As I understand it, Quicktime (what a mac mini would use) is currently incapable of passing through the ac3, regardless of whether the ac3 track is the primary or secondary track in the file. Quicktime will play the AAC track for your sound.

    Hope this helps,


    P.S. I'm not an expert at these things, just an avid HTPC guy. I would appreciate it if the community corrects any mistakes I made. :)
  4. akm3 thread starter macrumors 68020

    Nov 15, 2007

    Thank you both for your answers. I might have been hosing myself, I'm not using 'The AppleTV' preset, but really manually hacking together my own settings. I don't know what my 'Base' preset is.

    I choose Constant Quality 63%, audio 1 is AAC audio 2 is AC-3, and I choose 'loose' anamorphic.

    Since I don't have an AppleTV yet (waiting for the upgrade) I wonder how I can test if my encoding strategy works or not...
  5. evildarklordude macrumors newbie

    Feb 20, 2008
    New York, NY

    If you have QuickTime (I don't know whether you need QuickTime pro) you should be able to open the file you created in it. Once you've opened it, press apple+j and it will pop up details of the embedded tracks. Your video should be h.264 encoded with bitrate less than 5000 b/s. The aac track should be enabled and the ac3 track should be disabled. As long this is true, you should have no problem running it on the apple tv. As before, my disclaimer of not being an expert holds.

    Good luck,
  6. Cave Man macrumors 604

    Cave Man

    Feb 12, 2007
    Neander Valley, Germany; just outside Duesseldorf
    If there is an AC3 track your Apple TV will automatically pass it through the HDMI and optical output to your receiving device (usually a receiver or TV). Of course, that device must be able to decode AC3. If by "true dolby digital" you mean True-HD, then you can forget about that. Optical does not have the bandwidth for it. It can only do 7.1 compressed (i.e., DD, DTS).

    Yes, with Plex it's easy. It will even do DTS for you. But with QuitTime apps (including Front Row), it's a pain. I wouldn't even try it.

    The digital audio is passed through - the ATV nor Mac (nor any other device) does any decoding. That is the responsibility of the the receiver or TV. If it is connected by HDMI, then it should automatically detect the digital audio. If it is by optical cable, you usually have to tell the receiver to expect an optical signal for that input. For example, my Onkyo receiver has two optical inputs. Each of those also has RCA inputs. I have to set the receiver to use the optical input, otherwise it defaults to the RCA analog inputs. Once set, it is permanent (until changed by me).

    QuitTime can pass through AC3, it's just a pain in the @$$ to get it done. This is one (of many) reasons why Plex is far superior to QT/FR - it does it as the default setting (and DTS).

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