Shure MX690 - Manual (Page 14)

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The Master List is an index of all frequencies in a band, including
those not assigned to a Group or Channel. The Master List can
only be accessed through the receiver, so the transmitter must
be synchronized via the IR port. When a Master List frequency is
in use, “MASTER LIST” appears on the display. Refer to the SLX
Wireless System User Guide for more information.
Note: Wireless devices such as cellular phones and two-way
radios may interfere with wireless microphones. Keep these and
other potential sources of interference away from the transmitters
and receivers.
Master List Frequencies