Filters

An Arduino finite impulse response and infinite impulse response filter library.

New, Improved Arduino-Filters library

I created a new repository with an updated and improved version of this library.
You can find it here: Arduino-Filters.

Digital filters

This library implements digital finite impulse response filters (FIR) and infinite impulse response filters (IIR). Double precision floating point arithmetic is used.

FIR

The difference equation for FIR filters is given by

To initialize it in code, you can use:

const double b_coefficients[] = { b_0, b_1, b_2, ... , b_N };
FIRFilter fir(b_coefficients);

IIR

The difference equation for IIR filters is given by

const double b_coefficients[] = { b_0, b_1, b_2, ... , b_P };
const double a_coefficients[] = { a_0, a_1, a_2, ... , a_Q };
IIRFilter iir(b_coefficients, a_coefficients);

BiQuad Filters

When dealing with high-order IIR filters, they can get unstable.
To prevent this, BiQuadratic filters (second order) are used.

Both Direct Form 1 and 2 are implemented.

const double b_coefficients[] = { b_0, b_1, b_2 };
const double a_coefficients[] = { a_0, a_1, a_2 };
BiQuadFilterDF1 biquad1(b_coefficients, a_coefficients);
BiQuadFilterDF2 biquad2(b_coefficients, a_coefficients);

Optionally, you can specify a gain:

const double b_coefficients[] = { b_0, b_1, b_2 };
const double a_coefficients[] = { a_0, a_1, a_2 };
const double gain = 1;
BiQuadFilterDF1 biquad1(b_coefficients, a_coefficients, gain);
BiQuadFilterDF2 biquad2(b_coefficients, a_coefficients, gain);

Second Order Sections

Instead of manually cascading BiQuad filters, you can use a Second Order Sections filter (SOS).

const double sosmatrix[][6] = {
    {b1_0, b1_1, b1_2,  a1_0, a1_1, a1_2 },
    {b2_0, b2_1, b2_2,  a2_0, a2_1, a2_2 }
};
const double gainarray[] = {1, 1};
SOSFilter<2> filter(sosmatrix, gainarray);

Here, <2> is the number of BiQuad filters in the SOS filter.

Cascades of filters

You can apply multiple filters together in a cascade:

Cascade<N> cascade({&filter1, &filter2, ..., &filterN});

Usage

The library provides a Filter interface with a filter method that takes the new (raw) value as an intput, and outputs the new (filtered) value.

double raw_value = getRawValue();
double filtered_value = fir.filter(raw_value);

You can use multiple filters on the input.

double filtered_value = fir.filter(iir.filter(raw_value));