How to Connect Real Instruments to GarageBand on Your Mac
You can connect real instruments, such as electric keyboards and guitars to your Mac for use in GarageBand. Using real instruments with GarageBand produces not only better sound, but realistic practice, as you finger the actual instrument and not an on-screen keyboard/string set.
You can connect a real MIDI keyboard through a USB cable (on most newer gear) or a MIDI adapter (on older equipment). You can connect other MIDI instruments, including guitars, woodwinds, and drums, and record onto a real instrument track in GarageBand. Click the red Record button when you're ready to rock. Move the playhead to just before where you want to start jamming.
MIDI is geek shorthand for Musical Instrument Digital Interface, a standard that has been around for years.
If the high-quality instrument you have in mind is your own singing voice, connect a microphone (in lieu of the Mac’s built-in microphone) to an audio input port on the computer.
Open System Preferences, click Sound, click Input, and then select Line In. Drag the Input volume slider to an appropriate level. Choose Vocals and the instrument that most closely matches your singing style, such as Epic Diva, Helium Breath, or Megaphone. Garage Band will tailor the effects to your voice.
Don’t worry if you don’t know how to characterize your singing voice; one of your other options is No Effects.
Good microphones are also useful when you're recording podcasts. If you choose to use a microphone not for singing but for recording your speaking voice, set the option to No Effects (unless you’re going for a comedic effect).
To add the instrument to a recording, click the Create a New Track (+) button, choose Track→New Track, then select Real Instrument and click Create.
Choose an input source (stereo or mono), depending on how the instrument is connected to the Mac. Select Monitor from the pop-up menu to be able to hear the instrument as you play it, with or without feedback.