Guralp Systems Strong Motion - Manual (Page 13)

Güralp Systems Ltd.
4 Calculation Details
This section describes the concept of the strong motion
calculations. The document SWA-RFC-STMN holds the details of
the algorithms used and the packet format.
4.1 Low Latency Waveform Data
Using the setup given in section 2, the digitiser will transmit
low latency data blocks. These are 20sps blocks that are
emitted every second. They can be turned off with the
command “0 triggers” and on with “$77 triggers” (or simply “7
triggers” for a 2-channel instrument).
Turning off the transmission of the blocks does not affect the
strong motion calculations. The low latency streams are always
computed, even if they are not transmitted or stored in flash,
as they are used by the strong motion calculations to produce a
strong motion data block.
The low latency data uses a different pre-decimation filter to
the normal 20sps data. You will see a difference between the
two if they are turned on simultaneously. There are two
differences between the normal and the low latency filter:
●
The low latency filter has a high pass effect, with a corner
frequency at approximately 0.001Hz (1000s).
●
The low latency filter is implemented as an IIR (Infinite
Impulse Response) rather than an FIR (Finite Impulse
Response) filter. This implementation was chosen
because an FIR filter requires many taps to produce a
good cut-off, but more taps introduce more latency. An
IIR filter can be implemented with far fewer taps, allowing
the data to be retrieved in much shorter order.
The low latency filter has a different phase and freqency
response to the normal filter. Bode plots (filter response
magnitude and phase plotted against frequency) for both types
of filter are displayed below.
The Bode plots (see illustrations 2 and 3) show the magnitude
and phase shift of the response of each set of filters to a range
of frequencies. The normal pre-decimation filter (a cascaded
divide by 2 and divide by 5 FIR filter) has a sharp cut-off point
and rolls off very quickly. The low latency pre-decimation filter
July 2008 13