Akai MPC X User Manual page 285

Use the Bars field to define the length of your loop. Regardless
of how much or how little audio you record, this is how long
your loop will be.
Use the Sync button to sync or un-sync the looper with
sequence playback. When on, the Looper will stay in step with
your current sequence. When you play or record into the Looper,
it will wait until the sequence starts playing Bar 1 to start.
Use the Record To selector to determine the loop recording
behavior:
Play: Before recording, you must first tap the /Play button
on the screen, which will start playing the Looper.
Overdub: Before recording, you must first tap the Rec/
Record button in the lower-right corner to record-arm the
Looper.
Use the Output Gain slider to set the output signal level of the
Looper.
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