Example on how to filter a live signal

[kevinmamaqi]

Please, could you provide an example in how to filter a live signal using a butterworth iirFilter?

[axpersich]

I did it this way:

let Fili = require('fili')
let data = [
    1,1,1,1,1,1,1,1,1,1,
    2,2,2,2,2,2,2,2,2,2,
    3,3,3,3,3,3,3,3,3,3,
    4,4,4,4,4,4,4,4,4,4,
    3,3,3,3,3,3,3,3,3,3,
    2,2,2,2,2,2,2,2,2,2,
    1,1,1,1,1,1,1,1,1,1,
] // data.length = 70

// returns data elements one by one (fifo)
function getData(){
    let res = null;
    if (data && data[0])
        res = data[0];
    data = data.slice(1);
    return res;
}

function createFilter(){
    const calc = new Fili.CalcCascades();
    const coeffs = calc.bandstop({
        order: 2,
        characteristic: 'butterworth',
        Fs: 250,
        Fc: 50,
        BW: 0.5
    });
    return new Fili.IirFilter(coeffs);
}

const WINDOW_LEN = 5;
const WINDOW_STEP = 2;

const filter = createFilter();
let window = []; // store incoming data here

while ( true ){ // your live data source
    let data = getData();
    if(!data) break;
    
    window.push(data);

    // sufficient amount of data gathered -> filter it
    if (window.length >= WINDOW_LEN){
        const filtered = filter.multiStep(window);
        console.log(filtered); // filtered data
        window = window.slice(WINDOW_STEP) // shift window
    }
}

Edit: just noticed - it is probably not a good idea to name window as "window" if working in a browser!

[hansmaad]

@axpersich How should we use BW for bandpass/bandstop? What does BW: 0.5 mean. If Fc: 50 (= center at 50 Hz?)

[axpersich]

Hey @hansmaad, it has been a while since I've worked with filters, so take the following with a grain of salt.

I've used the above code to filter out the 50Hz eletricity hum (that's why Fc=50). The hum is not steady at 50Hz (see attached img), thus, it is a good idea to filter out frequencies which are a little bit above and a little bit below the 50Hz as well - this is what the BW=0.5 does. So the final filtering is done in 49.5-50.5Hz range (= Fc +- BW).

[hansmaad]

@axpersich Thank you very much for your answer. From my testing I got the impression that [BW] is not Hz. It's more like a scale. Its influence is bigger on higher frequencies.
This is original FFT and filtered with a bandpass at 110 Hz and BW=0.05

This one is 910 Hz with same BW

The band seems to be bigger at higher frequencies. I'd like to understand better how to use this BW.