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.
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Note |
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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 215. 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 */