The MPX-1 can use its own internal clock or it can reference an external clock. Depending on your rear panel connections, select ANALOG/INT, ANALOG/EXT or DIG/EXT. When ANALOG/EXT or DIG/EXT is first selected, the MPX-1 will repeatedly try to establish lock with an external digital source. If it is unable to establish lock, an error message will be displayed. Pressing any button will allow you to clear the error message and will return you to the audio screen with ANALOG/INT selected. To select ANALOG/EXT you must connect a source of digital clock at the digital input. Note: the MPX-1 will accept only signals at 44.1 kHz. See page 5-2 of the User Guide.