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IirFilter

Namespace: RTPROCESSINGLIB  ·  Library: DSP Library

Python equivalent

mne.filter.filter_data (method='iir') in MNE-Python.

#include <dsp/iirfilter.h>

class UTILSLIB::IirFilter

Butterworth IIR filter design and application using second-order sections.

All filter types (LP, HP, BP, BS) are supported via the classical analogue-prototype → bilinear-transform design route with pre-warped cutoff frequencies. The resulting biquad cascade is numerically stable for high filter orders.

Typical usage:

// Design a 4th-order zero-phase Butterworth bandpass 1–40 Hz at 1000 Hz sampling rate
auto sos = IirFilter::designButterworth(4, IirFilter::BandPass, 1.0, 40.0, 1000.0);

// Apply to one channel
Eigen::RowVectorXd filtered = IirFilter::applyZeroPhase(rawChannel, sos);

// Apply to all channels of a matrix
Eigen::MatrixXd matFiltered = IirFilter::applyZeroPhaseMatrix(rawMatrix, sos);

Static Methods​

designButterworth(iOrder, type, dCutoffLow, dCutoffHigh, dSFreq)​

Design a Butterworth filter as a cascade of second-order sections.

Parameters:

  • iOrder : int Filter order (≥ 1). For BP/BS the effective order is 2*iOrder.

  • type : FilterType Filter type: LowPass, HighPass, BandPass, or BandStop.

  • dCutoffLow : double Lower cutoff frequency in Hz. Used for LP, HP, BP, BS.

  • dCutoffHigh : double Upper cutoff frequency in Hz. Used for BP and BS only.

  • dSFreq : double Sampling frequency in Hz.

Returns:

  • QVector< IirBiquad > — QVector of IirBiquad sections (multiply their transfer functions to get H(z)).

applySos(vecData, sos)​

Apply a biquad cascade to one data row (causal, single-pass).

Parameters:

  • vecData : const Eigen::RowVectorXd & Input row vector.

  • sos : const QVector< IirBiquad > & Second-order sections from designButterworth().

Returns:

  • Eigen::RowVectorXd — Filtered row vector (same length as vecData).

applyZeroPhase(vecData, sos)​

Apply a biquad cascade with zero-phase (forward + backward pass) to one row.

The effective order is doubled and there is no phase distortion.

Parameters:

  • vecData : const Eigen::RowVectorXd & Input row vector.

  • sos : const QVector< IirBiquad > & Second-order sections from designButterworth().

Returns:

  • Eigen::RowVectorXd — Zero-phase filtered row vector (same length as vecData).

applyZeroPhaseMatrix(matData, sos)​

Apply zero-phase filtering to every row of a matrix (each row = one channel).

Parameters:

  • matData : const Eigen::MatrixXd & Input matrix (n_channels x n_samples).

  • sos : const QVector< IirBiquad > & Second-order sections from designButterworth().

Returns:

  • Eigen::MatrixXd — Filtered matrix (n_channels x n_samples).

Authors of this file​