iMac Digital Audio Output and 7.1 Speakers

Discussion in 'Mac Accessories' started by aleksgeo, Jun 6, 2011.

  1. macrumors newbie

    aleksgeo

    Joined:
    Jun 6, 2011
    Location:
    Singapore
    #1
    Hi everyone,

    After searching around, I couldn't find anything that answered my questions in a way that I understood. Apologies if this is a repeat of a previous thread.

    The gist of it is as follows: I'll probably be buying a 27" iMac in the near future (perhaps after the next refresh), which I plan to use, among other things, for watching movies. Some of my movies are in MKVs, with the audio encoded as AC3, DTS, TrueHD or DTS-HD, mostly 5.1 but going up to 7.1 in some cases. I'm aware that iMacs, like most (all?) modern Macs, have optical audio output, by way of the combined analogue/optical 3.5mm jack.

    That in mind, my first question is this: Can I connect any 7.1 sound system that takes optical input to the iMac? I read somewhere (I think another thread on MacRumors) that iMacs only support 5.1 output, but I hope that's not correct.

    Secondly, and more importantly, assuming I've got a 7.1 (or failing that, 5.1) system hooked up to the iMac via an optical cable, the audio going from the iMac to the sound system will be digital. As I mentioned above, I've got movies that have audio streams encoded in various formats. What I don't understand is which device handles the decoding of the audio streams. Is it done on the software side? Say I'm using VLC, does the program decode the audio, which then simply gets pushed to the sound system? Or does the audio get sent to the sound system in whatever format it's encoded, in which case the system would have to be able to decode the given format?

    I hope my questions aren't too confusing to understand, it's rather late right now (4am) where I'm at so I might not be as coherent as I'd like :p
     
  2. macrumors 603

    Joined:
    Aug 15, 2001
    Location:
    The Cool Part of CA, USA
    #2
    First, yes, all modern Macs have optical audio output, and it is compatible with the optical input of any 7.1 system so long as you get an appropriate fiber optic cable. I'm doing exactly that with a Mini and a 7.1 Onkyo system. You can also use a Thunderbolt to HDMI converter to send both video and digital audio through an HDMI cable, but it sounds like you're using the iMac's built-in screen.

    Second, the sound you actually get out of the sound system will depend on what goes into it. The Mac, by default, outputs stereo audio, which can then be post-processed by the receiver (through one of the Dolby modes) to extract surround channels. It might also be able to output 5.1 sound directly (from a game, for example), but I've never tried this. That would be the "5.1 limit" you'd heard about.

    Alternately, some video players can be set to output the raw bitstream audio via the digital output; at that point, if your home theater deck knows how to decode whatever format the audio is in (AC3, TDS, etc), it will play properly with however many channels is in the container. Apple's DVD player will do this with the Dolby or DTS tracks on DVDs, for example, and VLC also has a checkbox in its preferences. Other video players probably have similar settings as well. There is a maximum bitrate that can be sent through a toslink fiber optic cable, but so far as I know nothing other than uncompressed 7-channel hi-def audio runs into that limit.

    I'm not aware of any apps that will on-the-fly decode from, say, TrueHD to PCM (uncompressed), but if you get a 7.1 sound system it's likely it'll support pretty much every format available, so it should just work so long as you set your player(s) to output optical digital audio.
     
  3. thread starter macrumors newbie

    aleksgeo

    Joined:
    Jun 6, 2011
    Location:
    Singapore
    #3
    Thanks for the quick reply Makosuke. Just to make sure I get what you're saying correctly, by default if I watch a movie, the computer will downmix the sound to stereo, pass that to the sound system, and the sound system will then upmix it to 7.1? Unless I enable the appropriate option in the media player program (I think in VLC it's "Use S/PDIF when available"), in which case it'll just pass the raw audio stream through the optical cable and the sound system will decode it itself?
     
  4. macrumors 603

    Joined:
    Aug 15, 2001
    Location:
    The Cool Part of CA, USA
    #4
    More or less; the sound system will upmix it to whatever you have it set to upmix that stereo input to. This could be straight stereo, all-speaker stereo (I like that mode for music), or, most likely, Dolby Pro Logic or one of the later variants of it, which have between 4 and 7 channels, depending on the type.

    This will also be true for "pure" stereo sources (for example, an audio CD) that have some surround information encoded into the stereo channels (try it--many modern audio CDs have echos or other background effects show up on the surround speakers, not the front channels). Of course, even with less-than-7-channel sound sources, you can set your speaker system to output on all eight speakers (for example, sharing the surrounds to the rear surrounds and redirecting low-frequency sounds to the subwoofer).

    Correct (S/PDIF is the bitstream format carried by the toslink optical cable; it can also go through coax cable, but the Mac doesn't have such an output). And this is what you will want to do.
     
  5. thread starter macrumors newbie

    aleksgeo

    Joined:
    Jun 6, 2011
    Location:
    Singapore
    #5
    Great, thanks a lot for the explanation. Cleared up my doubts on that topic very nicely.

    After reading around a bit more on the topic of Mac audio I've got a couple extra questions, though :p

    First, a fairly simple one: Is it possible to output only audio through a HDMI cable using one of those adapters you mentioned, while using the Mac's screen? You mentioned Onkyo so I looked at their website and found a nice 7.1 system that I might consider getting and it has both optical and HDMI inputs. Can I hook that up using a Thunderbolt to HDMI dongle instead of a toslink cable?

    Second, it's my understanding that Macs can't output sound to more than one device at a time. I've got a Macbook Pro and can't really test that since the internal speakers disappear from the audio output device list when I plug in a pair of headphones. But since Macs generally only have one audio output jack, I've been considering getting a USB headset, like the Razer Megadolon. If I've got that plugged in along with external speakers (e.g. that 7.1 Onkyo system) will sound be coming through both, or will I need to toggle between the two? I read a bit about aggregate devices but that doesn't seem like the ideal solution, so I'm hoping there's a way to have sound through both devices at the same time without having to resort to that.
     
  6. macrumors 603

    Joined:
    Aug 15, 2001
    Location:
    The Cool Part of CA, USA
    #6
    Probably, but I haven't read anything specific about someone trying it. My Onkyo shows up to the Mac as a video source even if there's no TV attached to its HDMI out, so my guess would be that you'd more likely end up with a "phantom" second screen on the receiver. In which case you'd have two options if you only wanted to use the Mac's internal display: Set it to a low resolution and just avoid whatever part of the display's edge it was attached to (virtually), or set it to mirror the Mac's internal display.

    Maybe there's a way to entirely disable its video out, though, in which case you could use it for audio only.

    All that said, there's little if any real-world advantage of this versus an optical cable, at least that I can think of. The cables aren't any cheaper, and you don't need the dongle with the optical out.

    The analog headphone jack has always disabled the internal speakers, but I don't actually know what happens when you have an additional USB audio interface attached. My guess is all of the available devices would show up in the pref pane, and you could select which to send sound to. You probably can't do both at once, but you wouldn't need to physically disconnect anything to switch from optical out to, say, USB headphones.
     

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