To get sound into and out of your computer, you must have an audio interface. An audio interface is a device that converts digital audio from your computer to analog waves that speakers can broadcast. In the other direction, an audio interface converts analog waves into digital audio you can save and manipulate on your computer. Either way, an audio interface has only one purpose: It converts sound between the digital and analog domains.
The audio interface that comes with your computer is sufficient for using Logic, but it offers only a stereo eighth-inch input and a stereo eighth-inch output. These connections are usually reserved for attaching headphones to your computer or for basic recording purposes. They do not offer professional sound quality.
To communicate with its audio interface, a computer needs a small software application called a driver. All audio interfaces have their own special drivers, and in most cases you'll have to install that driver before Logic will recognize it. But once that driver's installed, there's very little else you need to do. Logic will automatically recognize your audio interface along with all available inputs and outputs (most audio interface manufacturers have a support section on their Web sites where you can download drivers).
However, situations might arise where Logic unexpectedly uses the wrong audio interface. For example, if Logic starts playing from the speaker on your computer, it's using your computer's built-in audio controller instead of your audio interface. Fortunately, this situation is easy to fixjust select your audio interface's driver from Logic's Audio Drivers Preferences pane.
Logic's Core Audio represents a massive step forward for audio on the Macintosh. In OS X, several applications can stably share a single audio interface at the same time. For example, you can now play a Logic arrangement, preview a video in QuickTime, and audition Apple Loops in iTunesall at the same time!
Selecting an Audio Driver
Increasing Available Audio Tracks
By default, Logic gives you 24 audio track channels. If you need more, you can increase this number using the Max. Number of Audio Tracks setting in the Audio Drivers Preferences pane.