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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