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Questions Answered 4: Optical Output on the Mac

Does it work with pro audio apps? By Dave Nagel
A reader has several questions about the optical output on recent Macs, specifically about how the Toslink port works with pro audio applications and surround/multi-channel mixing.

Question 1. The first question is a simple one: Do all Intel-based Macs have optical audio outputs? Yes they do. Workstations (the quad-processing Xeon machines known as Mac Pros) come with standard Toslink ports, from which you can connect your computer to a home theater receiver or other device with a Toslink port. They also have Toslink in ports, so you can capture audio from various sources just like you would with a standard line-in jack. There aren't all that many pro/semi-pro audio devices that send out an optical signal, but they do exist, like some types of DJ gear.

Apple's non-workstation-class machines also have optical out ports. These include the MacBook, MacBook Pro, iMac and Mac Mini. The optical ports on these machines are actually combo ports. That is, they can be used with standard 1/8-inch miniplugs, or they can be used with mini-Toslink plugs--both in the same port. The miniplug ports require, of course, a mini-Toslink adapter, like the one shown below.

The adapter plugs into the port, and then a standard Toslink cable plugs into that adapter. These adapters are available widely at computer stores and on the Internet. Personally, I use a Belkin adapter. (You can read my review of that by clicking here.)

Question 2. The second question relates to the use of the optical ports in pro audio applications. Specifically, does the Mac recognize the optical output as a multi-channel device? In other words, would it be possible to do surround mixing without separate hardware?

Sadly, not all answers can be positive ones. The optical ports send out one of two types of signals: encoded digital audio or two-channel linear PCM. Encoded digital audio includes things like Dolby Digital and DTS, so this is basically a consumer application of the optical port, as it's used mostly for hooking up your Mac to a home theater receiver and listening to DVDs in true surround sound. (I have written an article on this previously, which you can read by clicking here.)

The linear PCM stream is just a stereo signal carried digitally over optical fiber. This signal can then be read by any device capable of receiving an optical signal, including home theater receivers. (For example, I use my optical port to listen to iTunes music on my receiver. It sounds a lot better than a signal coming from the analog line out.

There are PCM streams that can carry six channels of audio, but devices that can read six-channel PCM streams are rare.

If you open up Audio MIDI Setup (located in /Applications/Utilities), you can see that there are only two basic options for digital output: 2-Channel (at 16-, 20- and 24-bit at 44.1 kHz to 96 kHz) and Encoded Digital Audio. And you can see that the system recognizes only those two channels.

So no, at least at present, multi-channel mixing in applications like Apple Logic Pro still require an external device that has six discreet channels. I, for example, use the PreSonus Firebox, which is a FireWire-based audio interface. You can see in Audio MIDI Setup that the system does recognize that as having six discreet channels in and eight discreet channels out.

And that's the sort of thing you need for multi-channel mixing. Theoretically someone could come out with a virtual output device that would at least allow you to mix surround sound without a six-channel output device, but that apparently hasn't occurred to anyone but me. (Maybe, for example, you'd want to try out some surround mix alternatives on the road, then listen to them when you arrive at your destination. A virtual multi-channel output would be great for that. But perhaps I'm just dreaming.)

Question 3. The final question relates to sending audio over optical and converting it to coaxial using a third-party converters. What makes this option attractive is that converters are available cheap (in the $20 to $30 range), and there are far more pro audio devices that use coaxial than those that use optical.

The answer is that the Mac sends out its optical signal and doesn't care what type of device the cable connects to, as long as you have an optical cable in the Mac's port. There is no difference between the digital signal sent out by the Mac and the signal sent out by, say, a CD or DVD player. If the converter works successfully with your hardware using other types of output devices, there's no reason it shouldn't work with your Mac.

Need a question answered about creative software? Need to know something about your computer? Drop Dave Nagel a line, and you might have your question answered on the pages of DMN with illustrations and a step by step guide to solving your problem. Or maybe you'll just get an answer that will help you with a purchasing decision. And if Dave doesn't know the answer (not blooming likely), he might just hook you up with someone who does. You can reach Dave at dnagel@digitalmedianet.com.

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Related Keywords:mac, audio, toslink, optical, s/pdif, spdif, macbook, mac mini, imac, macbook pro, pro audio, surround sound, surround mixing

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