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Signal Modifiers: Standard Filters

svfilter

  alow, ahigh, aband  svfilter  asig, kcf, kq[, iscl]

Description

Implementation of a resonant second order filter, with simultaneous lowpass, highpass and bandpass outputs.

Initialization

iscl – coded scaling factor, similar to that in reson. A non-zero value signifies a peak response factor of 1, i.e. all frequencies other than kcf are attenuated in accordance with the (normalized) response curve. A zero value signifies no scaling of the signal, leaving that to some later adjustment (see balance). The default value is 0.

Performance

svfilter is a second order state-variable filter, with k-rate controls for cutoff frequency and Q. As Q is increased, a resonant peak forms around the cutoff frequency. svfilter has simultaneous lowpass, highpass, and bandpass filter outputs; by mixing the outputs together, a variety of frequency responses can be generated. The state-variable filter, or "multimode" filter was a common feature in early analog synthesizers, due to the wide variety of sounds available from the interaction between cutoff, resonance, and output mix ratios. svfilter is well suited to the emulation of "analog" sounds, as well as other applications where resonant filters are called for.

asig – Input signal to be filtered.

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

kq – Q of the filter, which is defined (for bandpass filters) as bandwidth/cutoff. kq should be in a range between 1 and 500. As kq is increased, the resonance of the filter increases, which corresponds to an increase in the magnitude and "sharpness" of the resonant peak. When using svfilter without any scaling of the signal (where iscl is either absent or 0), the volume of the resonant peak increases as Q increases. For high values of Q, it is recommended that iscl be set to a non-zero value, or that an external scaling function such as balance is used.

svfilter is based upon an algorithm in Hal Chamberlin's Musical Applications of Microprocessors (Hayden Books, 1985).

Example

  ; Orchestra file for resonant filter sweep of a sawtooth-like waveform. 
  ; The seperate outputs of the filter are scaled by values from the score,
  ; and are mixed together.
  sr     = 44100
  kr     = 2205
  ksmps  = 20
  nchnls = 1
  
instr 1
  
  idur     = p3
  ifreq    = p4
  iamp     = p5
  ilowamp  = p6              ; determines amount of lowpass output in signal
  ihighamp = p7              ; determines amount of highpass output in signal
  ibandamp = p8              ; determines amount of bandpass output in signal
  iq       = p9              ; value of q
  
  iharms   =        (sr*.4) / ifreq
  
  asig    gbuzz     1, ifreq, iharms, 1, .9, 1             ; Sawtooth-like waveform
  kfreq   linseg    1, idur * 0.5, 4000, idur * 0.5, 1     ; Envelope to control filter cutoff
  
  alow, ahigh, aband   svfilter  asig, kfreq, iq
  
  aout1   =         alow * ilowamp
  aout2   =         ahigh * ihighamp
  aout3   =         aband * ibandamp
  asum    =         aout1 + aout2 + aout3
  kenv    linseg    0, .1, iamp, idur -.2, iamp, .1, 0     ; Simple amplitude envelope
          out       asum * kenv
  
endin
  
  
  ; Score file for above
  f1 0 8192 9 1 1 .25
  
  i1  0 5 100 1000 1 0 0  5  ; lowpass sweep
  i1  5 5 200 1000 1 0 0 30  ; lowpass sweep, octave higher, higher q
  i1 10 5 100 1000 0 1 0  5  ; highpass sweep
  i1 15 5 200 1000 0 1 0 30  ; highpass sweep, octave higher, higher q
  i1 20 5 100 1000 0 0 1  5  ; bandpass sweep
  i1 25 5 200 1000 0 0 1 30  ; bandpass sweep, octave higher, higher q
  i1 30 5 200 2000 .4 .6  0  ; notch sweep - notch formed by combining highpass and lowpass outputs
  e

Author

Sean Costello
Seattle, Washington
1999
New in Csound version 3.55


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Signal Modifiers: Standard Filters