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Before recording software instruments in GarageBand on Mac
You can connect a USB or MIDI music keyboard to your computer to play and record software instruments. You can play and record the software instruments built into GarageBand or software instruments you add as add as Audio Units plug-ins. Before you record, you also need to add a software instrument track and choose a sound.
GarageBand’s built-in sounds are sorted into categories, such as Bass, Guitar, Orchestral, Piano, Synthesizer, and so on. When you choose a sound patch from any of these categories, the Smart Controls area updates to show you controls for the patch. You can use these controls to adjust the sound. See Intro to Smart Controls in GarageBand on Mac.