The best ringtones are those that are edited within an inch of their digital lives. And it's not difficult to do, though you have a couple of ways to approach the task.
The first method is to trim the beginning or the end of the track by dragging its bottom edge toward the center of the track (a Trim icon will appear as you hover your cursor over the track's bottom corner). Play the track to the point where you want the ringtone to start — when the vocalist comes in, for instance. Then click the track's bottom-left corner and drag it to the right, to that point. A readout will show you the edge's time position as well as the overall length of the track. Now click somewhere in the middle of the track and drag it to the left so that it starts at 0:00. Then drag the right edge of the track to the left so that it ends somewhere before the 0:40 mark.
Alternatively, you can move the playhead to the point where you want the ringtone to begin, click the track in the workflow pane, and choose Edit > Split Regions at Playhead (or press Command-T). Click the portion that you don't want to keep (the stuff to the left of the split) and press Delete. Move the playhead where you'd like the ringtone to end, click the track, and once again split the track. Delete the material to the right of the split, and drag the remaining track to the 0:00 mark. Complete the job by dragging the right edge of the cycle bar so that it aligns with the end of your track.
The helpful thing about cycling being turned on is that you'll hear how the ringtone will loop on your phone. If you find that the transition between the end of the track and the beginning sounds abrupt or there's too much silence between them, adjust the track's length — adding or subtracting content from the end and getting rid of any silence at the beginning. If you can get a sense of where the track's beats fall, try to end the track so that it plays the last measure's entire complement of beats — most songs have four beats per measure.
Exporting the ringtone
Now that you've created the perfect ringtone, it's time to send it to iTunes. Just choose Share > Ringtone to iTunes. GarageBand will save the ringtone as an M4R file, iTunes will launch, and the ringtone will appear under iTunes' Tones heading in the Library pane.
If your iPhone isn't configured to sync wirelessly, connect it to your Mac using the sync cable. Drag your ringtone to the iPhone's Tones entry and then click the Sync button that appears at the bottom of the iTunes window. The ringtone will be copied to your phone.
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