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SASLab Manual


Main window : Edit > Filter > Time Domain IIR Filter


The IIR filter tool allows filtering the entire waveform using an Infinite Impulse Response (IIR) filter.



In contrast to FIR filters, IIR filters have (due to their feedback structure) an infinite impulse response and a non-linear phase response. The advantage of IIR filters is that they perform faster than FIR filters. The following IIR filter types are available:

High-Pass Removes signals below cut-off frequency (low-cut).

Low-Pass Removes signals above cut-off frequency (high-cut).

Notch Filter Removes signals at the specified center frequencies. Up to 10 discrete frequencies can be removed at once. The individual center frequencies must then be separated by space characters. The button + harmonics! calculates up to 10 harmonic frequencies based on the entered fundamental frequency.

The cut-off frequencies are expected in kHz and can alternatively be entered by using the measuring cursors of the spectrogram or the curve window. To do this the filter dialog box must be opened and the measuring cursors in the spectrogram or curve window must be moved to the desired location. The numeric value of the cursor is then entered automatically into the selected fields of the filter dialog box.

Filter Type The following filter types (characteristics) can be selected: Chebyshef, Butterworth, Bessel and Gauss.

Order Defines the order (4 or 8) of the filter. 8-th order filters exhibit a narrower transition between pass and stop band.

User-defined The user-defined IIR filter option supports custom filter responses. The IIR filter consists of a freely selectable number of cascaded biquads (up to 14). The coefficients must be defined in a .flt INI text file. This file is divided into different sections. The [main] section contains the property order, which defines the filter order (the maximum order is 28 - note that due to the biquad structure only even order numbers are valid). The sampling_frequency property defines the sampling frequency that applies to the coefficients provided in this file.

The following [BIQUADxx] sections contain the actual filter coefficients. All coefficients (properties a1, a2, a3, b1, b2) for each biquad are listed within that section.

This is a sample of a .flt file (see also the file demo.flt in the SASLab Pro program folder):

[main]
order=6
sampling_frequency=20.0
description=

[BIQUAD1]
a1=1.0
a2=-1.514
a3=0.5141
b1=-0.4303
b2=0.3788

[BIQUAD2]
a1=1.0
a2=1.1206
a3=0.2770
b1=-1.1663
b2=0.4186

[BIQUAD3]
a1=1.0
a2=0.0804
a3=0.4838
b2=0.0876
b1=0.5802


Response Preview

Impulse Shows the impulse response of the current filter settings in a separate curve window.

Step Shows the step response of the current filter settings in a separate curve window.

Frequency Shows the frequency response of the current filter settings in a separate curve window.

Avisoft Bioacoustics last modified on 13 February 2021