Guralp Systems Scream - Manual (Page 82)

Scream! 4.5
amplitude increases. The purpose of taking a short term average,
rather than triggering on signal amplitude directly, is to make it less
likely that spurious spikes or generally elevated background noise will
trigger the device. Averaging also introduces an element of frequency
selectivity into the triggering process.
You can select which tap is tested for the trigger from the Data source
drop-down menu. The tap does not have to output data to Scream! for
you to be able to use it here.
Any or all of the channels available at that tap may be used to
determine a trigger. You can select which channels are considered by
ticking the boxes in the Channel column of the table. If any of the
ticked channels passes the trigger condition, the trigger will activate,
and will not de-trigger until all of the ticked channels have fallen
below their respective ratio values.
The STA and LTA columns allow you to set, in seconds, the intervals
over which the two averages are calculated. 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 Ratio column determines by what factor the STA and LTA must
differ for the trigger to be passed. Finding the ratio most suited to your
needs is best done by experiment. Too high a value will result in
events being missed, while too low a value will result in spurious
non-seismic noise triggering the system. Like the averages, their ratio
is continuously recalculated for all components.
Note: None of the boxes are allowed to be empty, and
so you will need to enter the new value before removing
the old one. Alternatively, you can use the
and
cursor keys to change the values.
For example, setting the STA to 1 second, the LTA to 10 seconds and
the Ratio to 4 would give rise to the trigger behaviour depicted in the
following illustration:
82 Issue K