The Decimate module implements multi-stage filter and decimation routines to process wave data (TYPE_TRACEBUF2) coming from the InRing defined in decimate.d. Decimated trace messages are written to the OutRing with their SCNs changed as specified in decimate.d. The decimated trace message retains its TYPE_TRACEBUF2 type, but it (normally) has a new SCNL name.
Decimate uses FIR filters to reduce the incoming wave frequency content down to the Nyquist frequency of the output wave. FIR filters have the advantage of being strictly causal and stable. However, there is considerable delay introduced by a FIR filter. Fortunately FIR filters have linear phase so the delay does not change the shape of the wave. The delay will be removed from the data by adjusting their timestamps. This adjustment makes the filter acausal: small precursory artifacts will be seen in front of impulsive wave arrivals. This is an inherent characteristic of FIR filters. See "Of Poles and Zeros" by Frank Scherbaum for a method of removing these artifacts.
The decimation is performed as part of the filter routine for efficiency. The FIR filter coefficients are determined from the specified decimation rate, using the Remez Exchange algorithm. This produces an equi-ripple filter. Currently the ripple limits are hard coded to 0.005 in the pass band and 0.0031 in the stop band (see decimate.h in the source directory.) The foefficients zeroes of each filter, as well as the effective delay, are logged on startup. FIR filters have no poles.