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Signal Modifiers: Standard Filters


  ar      lowpass2   asig, kcf, kq[, iskip]


Implementation of resonant second-order lowpass filter.


iskip – initial disposition of internal data space. A zero value will clear the space; a non-zero value will allow previous information to remain. The default value is 0.


asig – input signal to be filtered

kcf – cutoff or resonant frequency of the filter, measured in Hz

kq – Q of the filter, defined, for bandpass filters, as bandwidth/cutoff. kq should be between 1 and 500

lowpass2 is a second order IIR lowpass filter, with k-rate controls for cutoff frequency (kcf) and Q (kq). As kq is increased, a resonant peak forms around the cutoff frequency, transforming the lowpass filter response into a response that is similar to a bandpass filter, but with more low frequency energy. This corresponds to an increase in the magnitude and "sharpness" of the resonant peak. For high values of kq, a scaling function such as balance may be required. In practice, this allows for the simulation of the voltage-controlled filters of analog synthesizers, or for the creation of a pitch of constant amplitude while filtering white noise.


; Orchestra file for resonant filter sweep of a sawtooth-like waveform.
  sr      = 44100
  kr      = 2205
  ksmps   = 20
  nchnls  = 1

          instr 1

  idur    =          p3
  ifreq   =          p4
  iamp    =          p5 * .5
  iharms  =          (sr*.4) / ifreq

; Sawtooth-like waveform
  asig    gbuzz      1, ifreq, iharms, 1, .9, 1

; Envelope to control filter cutoff 
  kfreq   linseg     1, idur * 0.5, 5000, idur * 0.5, 1

  afilt   lowpass2   asig, kfreq, 30

; Simple amplitude envelope
  kenv    linseg     0, .1, iamp, idur -.2, iamp, .1, 0 
          out        asig * kenv


; Score file for above
  f1 0 8192 9 1 1 .25

  i1 0 5 100 1000
  i1 5 5 200 1000


Sean Costello
Seattle, Washington August, 1999
New in Csound version 4.0

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Signal Modifiers: Standard Filters