Using Scream!
tap. Scream! 4.5 also allows you to configure each digitiser to receive triggers from
other digitisers.
To create a new stream with a trigger, open Scream!'s
Digitiser configuration
window
for the relevant digitiser, and click on the
Output control
tab. In the
Output control
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
Triggering
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. However, not
all models can use both methods. Scream! will interrogate the digitiser to discover
which methods are supported by its on-board software.
In addition, some digitisers support external and software triggering.
5.1.4 STA/LTA – Triggering by average values
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 U - December, 2021
