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.

<CsoundSynthesizer>
<CsOptions>
; Select audio/midi flags here according to platform
; Audio out   Audio in    No messages
-odac           -iadc     -d     ;;;RT audio I/O
; For Non-realtime ouput leave only the line below:
; -o lpf18.wav -W ;;; for file output any platform
</CsOptions>
<CsInstruments>

; 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 = p4
  ivol = p5
  aout lpf18 asine, kfco, kres, kdist

  out aout * ivol
endin


</CsInstruments>
<CsScore>

; Table #1, a sine wave.
f 1 0 16384 10 1

; different distortion and volumes to compensate
i 1 0 4 0.2 30000
i 1 4.5 4 0.9 27000
e


</CsScore>
</CsoundSynthesizer>

Credits

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

Audio rate parameters introduced in version 6.02

October 2013.