lpf in freq

Second order Butterworth lowpass filter.

> let f = xLine KR 0.7 300 20 RemoveSynth
> in audition (out 0 (lpf (saw AR 200 * 0.1) (fSinOsc KR f 0 * 3600 + 4000)))

Control rate filtering.

> let ctl = lpf (lfPulse KR 8 0 0.5) (mouseX KR 2 50 Exponential 0.1)
> in audition (out 0 (sinOsc AR (ctl * 200 + 400) 0 * 0.1))