Guralp Systems CMG-DCM build <10,000 - Manual (Page 80)

CMG-EAM (Platinum Firmware)
The next section of the screen controls the latency mode and
compression mode of the digitiser. Low latency mode is intended for
use with strong motion calculations. The digital filtering is changed
from finite impulse response (FIR) to infinite impulse response (IIR)
and packets are output each second at twenty samples per second in
order to achieve very near real time data.
Low latency mode cannot be selected unless certain preconditions are
met: for example, the first decimator output must be set to two
hundred samples per second. If all preconditions are satisfied, the
Enable low latency operation button is enabled. As an alternative, the
Force low latency operation button can be used to make all necessary
changes before enabling low latency mode.
The Compression mode drop-down menu controls how samples are
packed, affecting both data latency and line utilisation. GCF packets
contain a 32-bit starting value and then a series of differences between
consecutive samples. When the input signal is relatively quiet, these
differences can often be expressed as 8-bit quantities. When the input
signal includes large transients, the differences are transmitted as 32-
bit quantities. For intermediate level signals, 16-bit values can be
used. This is known as compression as it can “compress” four samples
into the space otherwise occupied by a single value. The digitiser can
be configured to limit the amount of compression used.
The difference values are stored in records, which are four bytes long
and, so, may contain four 8-bit differences, two 16-bit differences or
one 32-bit difference. A GCF packet can contain up to 250 records so
the maximum number of samples in a packet is between 250 (when
32-bit differences are used) and 1000 (for eight-bit differences).
Packets must start on whole-second boundaries, so they are not always
filled. In addition, it is possible to configure the digitiser to further
restrict the number of records in a packet in order to decrease latency.
The drop-down menu controls both of these settings and usually offers
the following choices:
80 Issue C