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# lpf18

lpf18 -- A 3-pole sweepable resonant lowpass filter.

## Description

Implementation of a 3 pole sweepable resonant lowpass filter.

## Syntax

ar lpf18 asig, kfco, kres, kdist

## Performance

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.

## Examples

Here is an example of the lpf18 opcode. It uses the files lpf18.orc and lpf18.sco.

Example 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 */

## Credits

Author: Josep M Comajuncosas
Spain
December 2000

Example written by Kevin Conder with help from Iain Duncan. Thanks goes to Iain for helping with the example.

New in Csound version 4.10

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