kfco -- the filter cutoff frequency in Hz. Should be in the range 0 to sr/2.
kres -- the amount of resonance. Self-oscillation occurs when kres is approximately 1. Shoujld usually be in the range 0 to 1, however, values slightly greater than 1 are possible for more sustained oscillation and an "overdrive" effect.
kdist -- amount of distortion. kdist = 0 gives a clean output. kdist > 0 adds tanh() distortion controlled by the filter parameters, in such a way that both low cutoff and high resonance increase the distortion amount. Some experimentation is encouraged.
lpf18 is a digital emulation of a 3 pole (18 dB/oct.) lowpass filter capable of self-oscillation with a built-in distortion unit. It is really a 3-pole version of moogvcf, retuned, recalibrated and with some performance improvements. The tuning and feedback tables use no more than 6 adds and 6 multiplies per control rate. The distortion unit, itself, is based on a modified tanh function driven by the filter controls.
Note: This filter requires that the input signal be normalized to one.
Here is an example of the lpf18 opcode. It uses the files lpf18.orc and lpf18.sco.
Example 15-1. Example of the lpf18 opcode.
/* lpf18.orc */
; Initialize the global variables.
sr = 44100
kr = 4410
ksmps = 10
nchnls = 1
; Instrument #1.
instr 1
; Generate a sine waveform.
; Note that its amplitude (kamp) ranges from 0 to 1.
kamp init 1
kcps init 440
knh init 3
ifn = 1
asine buzz kamp, kcps, knh, ifn
; Filter the sine waveform.
; Vary the cutoff frequency (kfco) from 300 to 3,000 Hz.
kfco line 300, p3, 3000
kres init 0.8
kdist init 0.3
aout lpf18 asine, kfco, kres, kdist
out aout * 30000
endin
/* lpf18.orc */
/* lpf18.sco */
; Table #1, a sine wave.
f 1 0 16384 10 1
; Play Instrument #1 for four seconds.
i 1 0 4
e
/* lpf18.sco */