Guralp Systems CMG-6TD User Manual
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Configuration with Scream!
system is triggered, are all selectable by the user, permitting maximum
flexibility of operation and the most efficient use of available storage space.
The 6TD can be set up for triggered output, that is, to output certain data
streams only when a particular trigger criterion is met. The trigger criteria
can be tested with data from the same or some other stream. For example,
you could use a later tap (with a lower sample rate) as a trigger for output
from an earlier, more detailed tap. Scream! 4 also allows you to configure
each digitiser to receive triggers from other digitisers.
To create a new stream with a trigger, open Scream!'s Configuration window
for the relevant digitiser, and click on the Output Control tab. In this pane, a
tap which gives rise to a triggered stream has a tick in the lower row of its grid
of check-boxes. You cannot configure the trigger criteria until you have
selected at least one stream to be affected by the trigger.
Once you have decided which streams should be output when the trigger is
activated, you will be able to click on the
button to describe the
trigger condition. Alternatively, click on the Triggering tab at the top of the
window. Either action will open the Triggering pane.
There are two triggering algorithms which Güralp digitisers can use:
STA/LTA, where the ration of the short-term average to the long-term average
is monitored, and level, where the raw signal level is monitored.
5.1.3.1 STA/LTA triggering
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.
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Issue F - February 2014