IIRFilter¶
Apply IIR filter to signal.
Together with the FIR filter, this is one of the two default ways to select or remove frequency bands in a signal. The IIR filter delays the signal less than the FIR filter, at the cost of increased signal phase distortion, especially in the region between passband (retained frequencies) and stopband (suppressed frequencies). The IIR filter is also less CPU intensive for the same amount of filtering. This filter maintains internal state to filter seamlessly across chunk boundaries. On the first few samples the filter will exhibit some minor artifacts, but is initialized to have minimal step response. Note that filter instability is not a concern with this implementation, since it is realized as a cascade of second-order sections (or biquads), rather than a simple direct-form I or II filter design. More Info... Version 1.1.0
Ports/Properties¶
data¶
Data to process.
axis¶
Axis along which to filter. Rarely anything other than 'time' is used here.
order¶
Filter order. If unspecified, this will be auto-determined based on the tightness of the transition bands and the desired attenuation strength, except for the 'freeform' filter mode and the 'Bessel' filter design, where it must be given. A larger order (e.g., 8 or higher) will result in larger signal delay, but can handle steeper and more complex frequency responses.
frequencies¶
Transition frequencies. For a lowpass/highpass filter, you can either give the cutoff frequency, or two freqencies to determine the rolloff curve. For a bandpass/bandstop filter, you can give two cutoff frequencies, or 4 frequencies to determine the rolloff curve. The syntax for 'free-form' filters is [(frequency1, gain1), (frequency2, gain2), ...], and you can use the special frequency value -1 as a stand-in for the highest frequency at which a filter can be designed for the given sampling rate (the signal's Nyquist frequency).
mode¶
Filter mode. Low/highpass lets low/high frequencies through, bandpass lets frequencies in a limited band through, and bandstop (a.k.a. notch) removes frequency in a limited band. The freeform mode (rarely used) can design arbitrary filters by specifying frequencies in the spectrum and corresponding desired gain factors.
design¶
Filter design rule to use. A Butterworth filter has a flat passband response, Chebychev Type-I has a steeper rolloff than Butterworth at the expense of larger passband ripples, and Chebychev Type-II also has steeper rolloff but larger stopband ripples. The Elliptic filter has maximally steep rolloff at the cost of larger ripple in both stopband and passband. The Bessel filter has a maximally linear phase response, meaning that the signal in the rolloff region is least distorted. For a freeform filter, the Yule-Walker method is always used and the design parameter is ignored.
stop_atten¶
Minimum attenuation in stopband. This is the minimum acceptable attenuation, in dB, in the stopband, which is ideally infinitely suppressed, but in practice 30-80 dB are enough, depending on the amplitudes of the signals to attenuate.
pass_loss¶
Maximum attenuation in passband. This is the maximum amount of signal attenuation, in dB, that is tolerated in the passband (which is ideally flat, i.e., 0 dB deviation).
ignore_nans¶
If activated the channels containing nan values are ignored.
offline_filtfilt¶
If the signal is not streaming (i.e ., offline), then perform a zero-phase filtfilt operation. As a result, the signal experiences zero time shift, and consequently no time-stamp correction is necessary.
set_breakpoint¶
Set a breakpoint on this node. If this is enabled, your debugger (if one is attached) will trigger a breakpoint.
metadata¶
User-definable meta-data associated with the node. Usually reserved for technical purposes.