BiquadFilters.js

npm i biquadjs

A simple javascript adaptation of a set of useful Biquad filters from an obscure arachnoid.com python tutorial. See the example html for usage. I combed the internet for this and I use these for EEG signal filtering. These will work in nodejs with imports, or just paste it into your html page and delete the exports.

Features:

• Low pass
• High pass
• Band pass
• Notch
• Peak
• High shelf
• Low shelf

Set Q-factor and other parameters as needed, you will want to experiment a bit for the notch, peak, and bandpass filters, the defaults otherwise are butterworth (1/root(2)) Q factors while I set relatively effective values for the aforementioned special cases. dbGain only applies to the shelf filters, which provide amplification above or below the set frequency.

Main class: Biquad

let lowpass = new Biquad('lowpass',20,512); //20Hz lowpass at 512Hz. 
//let notch = makeNotchFilter(...) //macros
//let bandpass = makeBandpassFilter(...) //macros

Secondary class: BiquadChannelFilterer - bundles biquad presets in a configurable way for easy device stream transforming.

There is a ready-made macro to use a list of filters, preset for getting biosignal data like EEG and ECG by applying notch and bandpass etc.

let filterer = new BiquadChannelFilterer({
    sps:512,
    useBandpass:true,
    bandpassLower:3, //hz
    bandpassUpper:45 //hz
});
let result = filterer.apply(signal_step);

All settings:

export type FilterSettings = {
  sps:number, //required
  useSMA4?:boolean,
  useNotch50?:boolean,
  useNotch60?:boolean,
  useLowpass?: boolean,
  lowpassHz?:number,
  useHighpass?:boolean,
  highpassHz?:number,
  useBandpass?: boolean,
  bandpassLower?:number,
  bandpassUpper?:number,
  useDCBlock?: boolean,
  DCBresonance?: number,
  useScaling?: boolean,
  scalar?:number,
  offset?:number, //+=
  trimOutliers?:boolean,
  outlierTolerance?:number
}

• Scaling factor just applies a scalar e.g. ADC -> Voltage conversion.
• Filtering toggles whether the filter is applied when looping over it in a bigger program
• Samplerate is fixed, calculate correctly or it won't work very well. You can start/stop streams just fine and the filters will correct otherwise.
• Returns the result with filterer.apply(signal_step), where the signal step is the latest amplitude in a time domain sequence (which can be real time, see https://app.brainsatplay.com)

Functions:

let classinstance = new Biquad(type,freq,sps,Q,dbGain) Create a new filter, these keep the latest sample and filter values so make a new one for each additional filter. types: 'lowpass','highpass','bandpass','notch','peak','lowshelf','highshelf'. Set the frequency and sample rate, with default butterworth Q values. dBGain only applies to the shelf filters. See below for notch and bandpass filter macros. I should add a peak filter macro..

let output = classinstance.applyFilter(signal_step) Apply the filter to the next sample in the sequence. Returns the filtered amplitude.

let z = classinstance.zResult(freq) Returns the z-transfer function values (i.e. how much each frequency amplitude gets multiplied/reduced by the filter)

let dcblocker = new class DCBlocker(r) Create a DC blocking filter which better highlights oscillations. Default r = 0.995. Use output = dcblocker.applyFilter(y) for each signal step.

makeNotchFilter(frequency,sps,bandwidth) Macro to generate a notch filter with the correct Q factor for the specified bandwidth. Returns a Biquad class instance.

makeBandpassFilter(freqStart,freqEnd,sps,resonance) Macro to generate a bandpass filter. Returns a Biquad class instance. The resonance value has a default setting that works pretty well, but I recommend experimenting. Having the resonance value being around 10^floor(log10(center frequency)) works well at least at low frequencies, I used 1 and 9.75 for a 3-45Hz bandpass filter in different cases while experimenting, for example. Applying 4 in a row seems to work well to get much sharper cancellation, though for some reason the output gets scaled by 1/n filters so you just need to multiply by how many filters you used to restore the amplitudes to the right values.