The biquad~ Pd object implements a general second-order IIR digital filter. It takes a list of coefficients of the form [-a1 -a2 b0 b1 b2] according to the second-order difference equation previously given in Eq. 1.
The spectrograph~ object provides a frequency-domain spectrogram display.
The biplot~ object provides a plot of the biquad filter response, taking the same list of coefficients as the biquad~ object.
A variety of other filter objects exist in Pd, a few being:
lop~ / onepole~: one-pole lowpass filter with adjustable cutoff frequency
hip~: one-pole highpass filter with adjustable cutoff frequency
lowpass~: lowpass resonant filter with controls for center frequency and Q
lowshelf~: lowshelf filter with controls for shelving frequency, slope and gain
lowres~: an inexpensive resonant lowpass filter
bp~: two-pole bandpass filter
rzero~ / rzero_rev~ / rpole~: raw one-zero and one-pole filters taking filter coefficients
