lpf18
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## lpf18

lpf18 — A 3-pole sweepable resonant lowpass filter.

## Description

Implementation of a 3 pole sweepable resonant lowpass filter.

## Syntax

    ares lpf18 asig, xfco, xres, xdist [, iskip]

## Initialization

iskip (optional, default=0) -- Skip initialization if present and non-zero.

## Performance

xfco -- the filter cutoff frequency in Hz. Should be in the range 0 to sr/2.

xres -- the amount of resonance. Self-oscillation occurs when xres is approximately 1. Should usually be in the range 0 to 1, however, values slightly greater than 1 are possible for more sustained oscillation and an “overdrive” effect.

xdist -- amount of distortion. kdist = 0 gives a clean output. xdist > 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
Before version 6.04 this filter requires that the input signal be normalized to one.

## Examples

Here is an example of the lpf18 opcode. It uses the file lpf18.csd.

Example 550. Example of the lpf18 opcode.

See the sections Real-time Audio and Command Line Flags for more information on using command line flags.

    <CsoundSynthesizer>
    <CsOptions>
    ; Select audio/midi flags here according to platform
    ; Audio out   Audio in
    -odac             ;;;RT audio out
    ; For Non-realtime ouput leave only the line below:
    ;-o lpf18.wav -W ;;; for file output any platform
    </CsOptions>
    <CsInstruments>

    ; by Kevin Conder with help from Iain Duncan

    sr = 44100
    ksmps = 32
    nchnls = 2
    0dbfs  = 1

    instr 1

    kamp init 1                         ; Note that its amplitude (kamp) ranges from 0 to 1.
    kcps init 440
    knh init 3
    ifn = 1
    asine buzz kamp, kcps, knh, ifn     ; Generate a sine waveform.
    kfco line 300, p3, 3000             ; Filter the sine waveform.
    kres init 0.8                       ; Vary the cutoff frequency (kfco) from 300 to 3,000 Hz.
    kdist = p4
    ivol = p5
    aout lpf18 asine, kfco, kres, kdist
    outs aout * ivol, aout * ivol

    endin

    </CsInstruments>
    <CsScore>
    f 1 0 16384 10 1    ; sine wave.

    ; different distortion and volumes to compensate
    i 1 0 4     0.2         .8
    i 1 4.5 4   0.9         .7
    e
    </CsScore>
    </CsoundSynthesizer>

## See also

Standard Filters: Resonant Low-pass filters

## Credits

Author: Josep M Comajuncosas
Spain
December 2000

New in Csound version 4.10

Audio rate parameters introduced in version 6.02

October 2013.

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