Guralp Systems CMG-DM24S12AMS - Manual (Page 28)

CMG-DM24S12AMS Operator's guide
If a digitiser has both Enable External Trigger Output and Enable
External Trigger Input selected, it will record data when it receives a
trigger from Scream! as if it had triggered itself, but it will not report that
trigger back to Scream!. It will only send a trigger to Scream! if its own
triggering criteria are satisfied.
Note that Scream! must be up and running for external triggering to
work. If you shut down the internal PC, you will need a copy of Scream!
running on a connected network to pass on trigger signals. Because this
introduces a degree of latency, you should allow a reasonable pre-
trigger period for high data-rate capture (see below).
Pre-trigger and post-trigger recording
In order to capture all of a seismic event, it is often useful to be able to
record data immediately preceding the trigger. The DM24 has an
internal buffer of some seconds which allows this data to be added to
the triggered stream. Pre-trigger data is particularly useful for
emergent-type signals, where the system does not trigger until one
phase after the first arrival.
In addition, to ensure that the coda of each event is included, some
seconds of data are recorded after the system detriggers. In the example
diagram above, the system detriggers well before the event has finished,
so a post-trigger value of 20 seconds is used to capture the remainder of
the event. The example also uses a pre-trigger period of 10 seconds.
The two boxes at bottom right of the Triggering pane allow the user to
set the pre-trigger and post-trigger data intervals, in seconds. These
values determine the minimum length of time during which data will be
saved before the trigger condition occurs, and after it has lapsed.
Regardless of the intervals chosen, the data in the triggered streams will
begin on an even second.
28 Issue B