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FIR filter phase coefficient. This function returns a phase shift coefficient that applies to data that has been decimated with the FIR filter-based decimate function. If a time series has been decimated, and then turned into frequency data (via a fourier transform, eg), then this phase coefficient can be applied to the frequency data to recover the phase of the original data. To get the phase correction in radians for a particular frequency point f, the phase coefficient must be multiplied by the frequency f, and divided by the original sampling frequency of the (pre-decimated) data. The returned phase coefficient is positive, so when it is added to the phase of the frequency data, the phase lag added by the FIR filter is removed.

Returns:

double the phase coefficient, defined as above

Parameters:

flag - FIR filter option
dec_factor - decimation factor; MUST be a power of 2

Author:

PF, May 1999

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