BSL PRO Software Guide - Biopac

201Digital FiltersThere is a fair amount of arcane terminology and theory surrounding the use and implementation of digitalfilters. For a general overview, see Appendix D — Filter Characteristics.Two types of post-processing filters are available under the Transform > Digital filters submenu: FIR (finiteimpulse response) filters and IIR (infinite impulse response) filters.FIR filters are linear phase filters, which means that there is no phase distortion between the original signaland filtered waveforms.IIR filters are not phase linear filters, but are much more efficient than FIR filters in processing data. TheIIR filters are useful for approximating the results of standard biquadric filters of the form:2(as bs c)2(xs ys z)IIR filters can be used to mimic filters commonly implemented in electronic analog circuitry and are also usedfor online filtering.FIR FiltersTo access the FIR filter dialog box, click the Transform menu, scroll toselect Digital Filters, drag right to FIR and drag right again for the filteroptions. When you select an FIR filter, the corresponding Digital Filterdialog box will be generated, allowing you to specify a number of differentfiltering options.In most cases, the default settings are optimal, although specific situations may require different settings. Thedialog will reflect data for the selected area, for example “50 samples at 50.000 samples/sec.”FIR Digital Filters DialogWindowAt the top of the dialog box is a Window pop-up menu, which allows you to choose from avariety of filtering algorithms. By default, the filter default is set to a “Blackman” type.These different Windows (described in detail in Appendix D) allow you to “fine tune” thefilter response.A “window” refers to a computation that spans a fixed number of adjacent data points.Window functions are used to eliminate discontinuities that may result at the edges of thefixed span of points for the FIR filter.Cutoff FrequencyThe second option is to provide a Cutoff Frequency, or threshold, for the filter function.Filter Type Default Cutoff Frequency Cutoff rangeLow-pass 0.125 times the sampling frequency Between 0.0000000001Hz and 0.5times the sampling rateHigh pass 0.25 times the sampling rate Between 0.0000000001Hz and 0.5times the sampling rate