Using Scream!
Güralp digitisers initially sample incoming data at a high rate (
e.g.
2000 Hz for the
DM24), which are then filtered and reduced to a lower rate (
decimated
) using an on-
board digital signal processing unit, or DSP. The DSP has several
filtering/decimation stages, which run one after the other. Stages which can produce
output are called
taps
. The Güralp DM24 can output 4 taps simultaneously.
Each configurable tap can be set to a different decimation factor by choosing values
from the drop-down menus on the left. Decimation factors of 2, 4, 5, 8, and 10 are
available. The numbers visible in the drop-down menu of each tap are the data rates
that each of the possible decimation factors will provide, given the settings of the
taps above it. Only integer (Hz) data rates are allowed: thus, for example, if one tap
emits data at 25 Hz, the only possible further decimation factor is 5.
To the right of each decimation factor menu is a grid of check-boxes. These boxes
mark which streams of data to generate at each sample rate. The screen-shot above
shows a possible configuration for a triaxial instrument. Every channel of the
digitiser may be output at any tap; In the example above, all three axes are being
output at Tap 2 (20Hz). The DM24 MkIII has a fourth channel for external input and
calibration, which can also be output at any tap.
If you want to change the names used for the channels, click in the white box
containing a Z in the above picture, and type a letter or number. It will name the
channels with a sequence of letters or numbers beginning with the one you choose
(e.g.
A, B, C; 2, 3, 4; 9, A, B)
,
unless you type
Z
in which case they will revert to Z, N, and
E.
Each combination of channel and tap has two check-boxes:
•
Checking the upper check-box of each pair makes that tap output data
continuously for that component;
•
Checking the lower check-box makes that tap output data only when a trigger
is active.
In the example above, the digitiser will output data continuously for all three
channels at Tap 2, but never for any other taps. If you do not need all the streams to
output at all rates, you should leave boxes unchecked to save communications
capacity.
You cannot check both continuous and triggered output for the same channel and
tap.
When you enable a triggered stream, the digitiser will output data in that stream only
when a particular set of trigger criteria are met. To remind you that the streams are
not continuous, the window shows the data streams (lines) passing through a
“switch”.
38
Issue U - December, 2021
