Guralp Systems CMG-CD24 User Manual

Page 30

CMG-CD24

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 find out

from the digitiser whether its on-board software supports each
method.

3.1.3.1 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
checking the boxes in the Channel column of the table. If any of the

checked channels passes the trigger condition, the trigger will activate,
and will not detrigger until all of the checked 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 trigger.

Issue C