Affects: Filter operation
Use the Zero-lag selection to select whether or not you want "zero-lag" (two pass) filtering to be performed. The filtering algorithm used is a version of the "Second-order, zero-lag Butterworth filter", but you can select whether or not the reverse pass is to be performed. If only the forward pass is made, phase-shift distortion will be introduced. If you select "zero-lag" filtering, it will take twice as long, but the reverse pass will eliminate the phase-shift. In either case, end-point extrapolation is used to start off the filter; points equal to the average of the first few points (and of the last few points) of the waveform are temporarily prepended (and appended) to the waveform.
By disabling this option, you suppress the reverse pass of the filtering algorithm, which cancels out the phase-shift distortion of the filter's forward pass. If you don't care about this phase-shift, suppressing the reverse pass will make the filter run twice as fast.
Note that when used with a low cutoff frequency, the reverse pass of zero-lag filtering can actually cause some smearing of the signal such that the onset of activity appears to occur before it actually does in the original signal. Because of the potential for erroneous results in your analysis that can be introduced by this, the Zero-lag option is now turned off by default. If you turn it on, use care to watch for and avoid such distortion of your signal, particularly when filtering aggressively.
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