INSTRUMENT design – simple one-pole filter object
The Oonepole object instantiates a simple one-pole IIR filter, based on designs found in Computer Music: Synthesis, Composition, and Performance by Charles Dodge and Thomas Jerse and the Synthesis ToolKit (STK) authored by Perry Cook and Gary Scavone. The general recursive filter equation used for this object is:
y[n] = a*x[n] + b*y[n-1]
where y[n] and y[n-1] are the current and previous outputs of the equation, respectively, and x[n] is the current input. a and b are the coefficients to the equation that determine what kind of filter is constructed. Simple low-pass and high-pass filters are possible with characteristics depending upon the values of the a and b coefficients.
The OonepoleTrack object does the same thing as Oonepole except that it ‘tracks’ changes to the state of the filter and only performs computations if the cutoff frequency or lag time of the filter has changed. More generally, the older functions reson and resonz do similar kinds of filtering operations. See also the Oequalizer object for filtering capabilities.