Guralp Systems CMG-DM24 - Manual (Page 28)

Operator’s Guide DM-24 Digitiser
Issue G January 2003 26
The three tick boxes down the left side of the Trigger Setup window (Z, N, E)
allows the user to choose the channels which will be tested for a trigger
condition.
The second column of boxes, the user must set the averaging intervals for the
short and long term averages (STA and LTA). Typically, the time interval for
the short term average should be about as long as the signals you want to
trigger on, while the long term average should be taken over a much longer
interval. Both the STA and LTA values are recalculated continually, even
during a trigger.
The third column of boxes are for defining the STA/LTA ratio which will
constitute a trigger for each of the components selected for triggering. The
system declares a trigger when any one of the triggering components exceeds
this value. The trigger ratio is continuously recalculated for all components and
the system will detrigger when all the components selected for triggering have
fallen below their respective ratio values.
The final column of two boxes on the right-hand side allows the user to select
the pre-trigger and post-trigger data intervals. These values determine the
minimum length of data that will be saved prior to the trigger condition, and
how much data will be saved after the trigger condition has lapsed. Regardless
of the intervals chosen, the data for the triggered streams will begin on an even
second.
If the box Common Values for parameters is ticked, a trigger parameter
entered for one component will be used for all selected components.
(BANDPASS, STA, LTA, RATIO, PRE-TRIG, POST-TRIG)
More details are given later in Chapter 7. DIGITISER STA/LTA TRIGGER
OPTIONS.