BSL PRO Software Guide - Biopac

202 Part DFIR Digital Filters DialogBand-pass low: 0.125 times the sampling rate Any values, but low and highhigh: 0.25 times the sampling rate cannot be set to the same valueNumber ofCoefficients TIPShow Filter ResponseThe Number of Coefficients determines how well the filter will match the desired CutoffFrequency (or range). By default, this is set to 39. Filters that use a small number ofcoefficients tend to be less accurate than filters that use a large number of coefficients.Entering a larger value will result in a more accurate filter; however, as the number ofcoefficients increases, so does the processing time required to filter the data. To see howchanging this value affects the way data is filtered, it can be useful to examine the filterresponse patterns.A good rule of thumb is to use a number of coefficients greater than or equal totwo times the sampling rate divided by the lowest cutoff frequency specified. Forexample, if running a low pass filter at 1Hz on data sampled at 100 Hz, chooseat least (2 x 100/1) or 200 coefficients in the filter. Additional coefficients willimprove the response.The Show Filter Response option instructs the software to produce a plot of the filtered dataand generates a new graph window for the response plot. You must save the response plotseparately.Don’t modifywaveformFilter entire waveThe Don’t modify waveform option is useful when used with the Show filter responseoption. When both boxes are checked, the software will produce a plot showing the filterresponse, but will not modify the waveform. This allows you to repeatedly specify differentfilter options (without modifying the waveform) until the desired frequency response isachieved.The Filter entire wave option will filter the entire wave and replace the original.‣ If you want to keep the original, you need to duplicate it prior to filtering.Visit the online support center at www.biopac.com