Your Mac's audio devices should all be listed in the left-most column in Audio MIDI Setup, and clicking each will give you a number of configuration options for the input and output channels it hosts. In the configuration, you should have options for selecting the audio format (sampling rate, channel number, and bit-depth of the signal), as well as the options for specifying relative channel output levels. In some devices these options will be configurable, and in others they will not.
Audio MIDI Setup is also helpful in creating aggregate and multi-output devices from your available audio interfaces. Let's say you have an Airplay device plus a USB, ethernet, or Firewire audio interface attached to your Mac, and you'd like to play the same audio on more than one of them. You can do so by creating an aggregate or multi-output device that will treat two or more of these separate physical interfaces as a single virtual one.
To do this, click the plus button under the device list in Audio MIDI Setup, and select the option to create either an Aggregate or Multi-Output device. (Aggregate devices support input and output, while Multi-Output devices support output only.) You can double-click the resulting device in the app's left pane to give it a name. You can then select it to see a list of the available interfaces that can be included in the aggregate device.
With your Aggregate or Multi-Output device configured, you can select it for use either as the main system audio device, or for use in specific programs such as GarageBand.
Audio MIDI Setup also lets you configure audio channels for your devices. Whether you are using those devices separately or in aggregate configurations, your Mac will likely have more than one channel available for input and output. Using the Audio MIDI Setup app, you can configure your devices to handle these different audio channels.
When you select an audio device, or expand an aggregate device to select a sub-device included in it, you should see a list of its supported channels in the right pane. You can use the sliders to change the relative volume of each of these channels and balance them as you wish.
You can also change channel assignments for the speakers you have configured. When you select a device that supports audio output, you can click the Configure Speakers button at the bottom-right of the Audio MIDI Setup window, and then choose from either Stereo or several types of Multichannel speaker setups, some of which may be available as configuration options for your system.
While surround sound systems (such as 5.1, 6.1, and 7.1 configurations) will require hardware that can manage them, with aggregate and multi-channel devices you should be able to set up Quadraphonic, Hexagonal, or Octagonal speaker setups, which will split your standard stereo evenly between four and eight configured channels. Even though OS X will assign a default channel to each speaker, you can use the visual interface to change those assignments. Simply click a speaker to hear the system play a test sound through it, and then choose the desired channel for it from the popup menu under the speaker.
As I say, many of you may never to take advantage of OS X's more advanced audio controls. But, as always, it's nice to know they're there when you need them.
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