BSL PRO 4 Manual - Biopac

Part D — Appendices 337Appendix C - Filter characteristicsFilter typesBSL PRO employs two types of digital filters:(a) Finite Impulse Response (FIR) perform post-acquisition filtering(b) Infinite Impulse Response (IIR) perform filtering calculations online (during an acquisition) or postprocessing(after an acquisition)Although the similarities between the two types of filters outweigh the differences, some importantdistinctions remain.1. First, IIR filters are typically more efficient than FIR filters, which means that IIR filters can filterdata faster than FIR filters, which is why IIR filters are used for online Calculations.2. Second, IIR filters tend to be less accurate than FIR filters. Specifically, IIR filters tend to causephase distortion or “ringing.” When the phase of a waveform is distorted, some data points on awaveform are shifted (either forward or backward in time) more than others. This can result in theintervals between events (such as the Q-R interval or the inter-beat interval in an ECG waveform)being slightly lengthened or shortened compared to the original signal. In practice, however, theeffect of this distortion is usually minimal since the frequencies which are most distorted are alsoattenuated the most. By contrast, FIR filters are phase linear, which means that the interval betweenany two sample points in the filtered waveform will be exactly equal to the distance between thecorresponding sample points in the original waveform.3. Third, IIR filters have a variable Q setting that defines the filter response pattern, but FIR filters donot have a Q component. The optimal Q of an IIR filter is 0.707, with lower values resulting in aflatter response and higher values resulting in a more peaked response. The default Q for all IIRfilters is 0.707 (except for Band pass filters where Q defaults to 5), which is appropriate for nearly allfilter applications.In the examples on the following page, the filter responses of several different types of filters are compared.All of the filters are 50 HZ low pass filters operating on the same data.The first graph shows how the number of filter coefficients in FIR filters (Q) affects the filter’s frequencyresponse. Note that as the number of coefficients (Q) increases, the filter becomes more accurate. A good ruleof thumb is to set Q ≥ 2(f s / f c ), where f s = sampling rate and f c = cutoff frequency.FIR filter performance as a function of number of coefficients (Q)BSL PRO Manual