Guralp Systems CMG-CD24 - Manual (Page 28)

CMG-CD24
The CD24 initially samples incoming data at 2000 Hz. These data 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 (and the outputs from those stages) are
called taps. The CD24 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; currently, as illustrated, all three axes are being output at Tap
2 (20Hz).
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. The upper
check-box of each pair activates continuous output, whilst the lower
activates triggered output. 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. This
is shown diagrammatically as data passing through a switch. In the
example above, we might want the high-rate data from Tap 0 to be
generated only when an event registers at some other tap. To do this,
tick one or more of the lower set of check-boxes for Tap 0.
With this configuration uploaded, Tap 2 will continue to produce
output at all times, but Tap 0 will also emit data whenever the trigger
28 Issue C