Guralp Systems Strong Motion - Manual (Page 14)

Strong Motion User Guide
is a set of four cascaded first-order IIR filters. Its roll off is
comparitively more shallow and begins at a lower frequency.
The phase shift observed in illustration 2 is actually a linear
phase shift introduced by a pure time delay. The FIR filter used
has a completely flat phase response, but is acausal. A pure
time delay must be used so that the filter can be realised. This
simply means that the output sample y(t) does not correspond
to the input sample u(t) but instead to the sample u(t-n).
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Illustration 2: Bode plot of normal 200Hz to 20Hz pre-decimation
FIR filter.