Biquad filter

The biquad~ object is a biquadratic filter. In technical terms, that’s a second-order IIR filter with two poles and two zeros; in practical terms, it’s a versatile filter that can have a wide variety of characteristics—lowpass, hipass, bandpass, notch, shelf, etc.—depending on the values of the coefficients in the filtering equation. But unless you’re a trained electrical engineer, you probably don’t know exactly how the coefficient values correspond to particular filtering characteristics.

Smooth filter changes

If you want to change the coefficients of biquad~ in real time while a sound is playing, it's usually better to use MSP signals rather than individual Max messages, to avoid causing clicks. In that case, you should replace filtergraph~ with filtercoeff~ and send the frequency, gain, and Q parameters into filtercoeff~ as smooth signals (as shown in the left portion of the example).

Variable-mode filter demo

This patch allows you to try out various filter settings of the biquad~ object, via the filtergraph~ object. For adjusting the parameters you can drag on filtergraph~ with the mouse, or you can send values in its three rightmost inlets for frequency, gain, and Q. The spectroscope~ object tries to draw the spectrum of the signal.

Bandpass filter swept with a LFO

This example demonstrates one of MSP's many filter objects—the resonant bandpass filter reson~—and demonstrates how a parameter of that filter—in this case its center frequency—can be easily modulated by a low-frequency control oscillator (cycle~). The range of cycle~ is between 1 and -1, but that range can be amplified by multiplication, and can be offset by addition.