Digital filters are broadly classified into finite impulse response (FIR) and infinite impulse response (IIR) filters. A linear-phase FIR filter does not introduce phase distortion between frequency components, although it is generally more computationally demanding. It does introduce group delay; however, because the delay is nearly constant across all frequencies, the waveform is delayed as a whole without phase distortion.
IIR filters are computationally more efficient, but they generally introduce frequency-dependent phase distortion and group delay. Our products use forward–backward filtering (filtfilt) to eliminate the phase distortion and group delay introduced by the IIR filter. However, this operation produces an overall magnitude response equal to the square of the original filter’s magnitude response. Furthermore, filtfilt does not automatically correct the intrinsic group-delay structure of the input signal or phase and group-delay distortions already introduced by the measurement system.
Homer3 also uses filtfilt to achieve zero-phase filtering, but it does not compensate the cutoff frequencies or attenuation specifications for the squared magnitude response resulting from forward–backward filtering.
Therefore, when preservation of the signal’s phase relationships and accurate control of the magnitude response are particularly important in fNIRS signal analysis, a properly designed linear-phase FIR filter is recommended, despite its greater computational cost.