iMac Digital Audio Output and 7.1 Speakers

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.

aleksgeo said:

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?

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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).

aleksgeo said:

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?

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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.

aleksgeo said:

Is it possible to output only audio through a HDMI cable using one of those adapters you mentioned, while using the Mac's screen?

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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.

aleksgeo said:

It's my understanding that Macs can't output sound to more than one device at a time.

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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.