BIOPAC® Systems, Inc. Logo

IIR vs. FIR filters

IIR filters

IIR filters are much more efficient than FIR filters, but can introduce linear phase distortions. The IIR filters in BIOPAC hardware are emulated by the software IIR filters.

  • IIR filters can be set up as online calculation channels for real-time digital filtering.
  • IIR filters have a variable Q setting that defines the filter response pattern (but FIR filters do not have a Q component).
    • The optimal Q of an IIR filter is 0.707, with lower values resulting in a flatter response and higher values resulting in a more peaked response.  By definition, any single order bi-quad filter with a Q of 0.707 is a Butterworth filter.
  • The default Q for low pass and high pass IIR filters in AcqKnowledge is 0.707.  The Q for band pass and band stop filters is 5. These values are appropriate for nearly all filter applications.

FIR filters

FIR filters require more computing power than IIR filters. Their filter response can be adjusted so that it is superior to IIR filters (i.e., they can be tweaked to do a better job at attenuating particular frequencies).

  • FIR filters are available only in the software; in AcqKnowledge versions older than 4.4, they cannot be set up as calculation channels and must be applied as transformations once the acquisition is over.
  • Number of coefficients for FIR filters—The following rule of thumb will help to determine the correct number of coefficients to use when setting up an FIR filter: The number of coefficients should be greater than or equal to four times the sampling rate divided by the lowest cutoff frequency specified.  Additional coefficients will improve the response.
    • For example, if running a low pass filter at 1Hz on data sampled at 100Hz, choose at least (4 x 100/1) or 400 coefficients in the filter. Select the “Show Filter Response” option to see the filter response (a graph of attenuation vs. frequency).

For more information on IIR and FIR filters please refer to the AcqKnowledge and BSL PRO manuals.

Page last modified 29Dec2014

Associated Applications

Associated Disciplines

Spotlight On
free BIOPAC webinar

Multiparticipant or Large Groups | Recording from Multiple Participants Part II

Get targeted solutions specific to the demands of recording data from large groups of people at the same time. Learn about Workflow when recording from multiple participants; Synchronizing all the data; Setting up participants efficiently; Tips and tricks for planning ahead; Using biopotential data, FNIRS, eye tracking, and more; Data monitoring and batch analysis techniques.

Register Now
Latest News

New Citations | BIOPAC in Stimulus Presentation Studies

BIOPAC provides software and hardware that allows research teams to study stimulus presentation data. Here are a few notable studies using BIOPAC equipment for research on stimuli’s effects using ECG and SCL data. Your Face Scares Me This study addressed the impact of individual differences in social anxiety by examining the effects of perceptual load and stimulus valence when […]

Electrocardiography Guide Now Available

BIOPAC’s comprehensive Introductory ECG Guide addresses fundamental to advanced concerns to optimize electrocardiography data recording and analysis. Topics include: ECG Complex; Electrical and Mechanical Sequence of a Heartbeat; Systole and Diastole; Configurations for Lead I, Lead II, Lead III, 6-lead ECG, 12-lead ECG, precordial leads; Ventricular Late Potentials (VLPs); ECG Measurement Tools; Automated Analysis Routines for extracting, […]

Read All
Request a Demonstration
Request a Demonstration