Guralp Systems CMG-3ESPCD User Manual
Page 45
CMG-3ESPCD Digital Broadband Seismometer
Using Scream!
There are two triggering algorithms which Güralp digitisers can use: STA/LTA
and level-triggering. In addition, software and external triggering options are
available.
5.1.3.1 STA/LTA
The STA/LTA algorithm applies a simple ( short-term average ÷ long-term
average ) calculation to the triggering stream. It works by identifying sections
of an incoming data stream when the signal 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 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 the intervals over which the two
averages are calculated, in seconds. 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
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Issue B - November 2013