Reference > resonator

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resonator(freq, bandwidth) creates a resonator filter centered at frequency freq (in Hz) and with a frequency width bandwidth (in Hz). The resonator filter implements the resonz filter, which is a type of constant gain band pass filter.

• .apply returns a function that applies the filter equation and updates the filter state.
• .set(freq, bandwidth) changes the filter parameters.

Filter equation

output[i] = gain * (input[i] - input[i - 2]) + 2 * q * cosPoleAngle * output[i - 1] - q * q * output[i - 2];
q = Math.exp(- Math.PI * bandwidth / sampleRate);
gain = Math.sqrt((1 - q * q) / 2);
cosPoleAngle = ((1 + q * q) / 2 * q) * Math.cos(2 * Math.PI * freq / sampleRate);

In synthesine, filters are functions with methods for applying and setting the filter, and an internal memory that keeps track of the state of the filter. The technical term for the kind of object is a closure. Every time you create a filter, the state is erased, so it’s a good idea to define your filters only once, in the setup function.

You can find the code for resonator in the synthesine library.