BSL PRO Software Guide - Biopac

Appendices 295Appendix G — Filter CharacteristicsFilter typesThe Biopac Student Lab PRO software employs two types of digital filters:1) Finite Impulse Response (FIR) perform all post-acquisition filtering2) Infinite Impulse Response (IIR) perform online calculations (filtering performed during an acquisition)Although the similarities between the two types of filters outweigh the differences, some important distinctionsremain.IIR filters are typically more efficient (faster) than FIR filters.This means that IIR filters can filter data faster than FIR filters, which is why IIR filters are used for onlinecalculations.IIR fillers tend to be less accurate than FIR filters.Specifically, IIR filters tend to cause phase distortion or “ringing.” When the phase of a waveform is distorted,some data points on a waveform are shifted (either forward or backward in time) more than others. This can result inthe intervals between events (such as the Q-R interval or the inter-beat interval in an ECG waveform) being slightlylengthened or shortened compared to the original signal. In practice, however, the effect of this distortion is usuallyminimal since the frequencies which are most distorted are also attenuated the most.By contrast, FIR filters are phase linear, which means that the interval between any two sample points in the filteredwaveform will be exactly equal to the distance between the corresponding sample points in the original waveform.IIR filters have a variable Q settingThe Q setting defines the filter response pattern, but FIR filters do not have a Q component. The optimal Q of an IIRfilter is 0.707, with lower values resulting in a flatter response and higher values resulting in a more peakedresponse. The default Q for all IIR filters is 0.707 (except for Band pass filters where Q defaults to 1), which isappropriate for nearly all filter applications.In the examples on the following page, the filter responses of several different types of filters are compared. All ofthe filters are 50 HZ low pass filters operating on the same data.IIR filters are also used for real-time filtering in the MP UNIT hardware. The following processing rates correspondto the sampling rate:Sample Rates(samples/Sec)Possiblewith MP36/35Possible withMP30MP36/35DSP I.I.R.Filter Proc. Rate(Sample/Sec)MP30DSP I.I.R.Filter Proc. Rate(Sample/Sec)1 YES YES 20,000 2,0002 YES YES 20,000 2,0002.5 NO YES N/A 2,0005 YES YES 20,000 2,00010 YES YES 20,000 2,00020 YES YES 20,000 2,00025 NO YES N/A 2,00050 YES YES 20,000 2,000100 YES YES 20,000 2,000200 YES YES 20,000 2,000250 NO YES N/A 2,000500 YES YES 20,000 2,0001,000 YES YES 20,000 2,0002,000 YES YES 20,000 2,0005,000 YES YES 20,000 N/A10,000 YES YES 20,000 N/A