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svfilter

A resonant second order filter, with simultaneous lowpass, highpass and bandpass outputs.

Syntax

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

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.

iskip (optional, default=0) -- initial disposition of internal data space. Since filtering incorporates a feedback loop of previous output, the initial status of the storage space used is significant. A zero value will clear the space; a non-zero value will allow previous information to remain. 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.

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 Microprocessor_s (Hayden Books, 1985).

Examples

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

Example of the svfilter opcode.
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<CsoundSynthesizer>
<CsOptions>
; Select audio/midi flags here according to platform
; Audio out   Audio in
-odac   --limiter=.95 ;;;realtime audio out
; For Non-realtime ouput leave only the line below:
; -o svfilter.wav -W ;;; for file output any platform
</CsOptions>
<CsInstruments>

sr = 44100
ksmps = 32
nchnls = 2
0dbfs  = 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 .4, 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
outs    asum * kenv, asum * kenv

endin

</CsInstruments>
<CsScore>
f1 0 8192 9 1 1 .25

i1  0 5 100 .1 1 0 0  5  ; lowpass sweep
i1  5 5 200 .1 1 0 0 30  ; lowpass sweep, octave higher, higher q
i1 10 5 100 .1 0 1 0  5  ; highpass sweep
i1 15 5 200 .1 0 1 0 30  ; highpass sweep, octave higher, higher q
i1 20 5 100 .1 0 0 1  5  ; bandpass sweep
i1 25 5 200 .1 0 0 1 30  ; bandpass sweep, octave higher, higher q
i1 30 5 200 .2 .4 .6  0  ; notch sweep - notch formed by combining highpass and lowpass outputs
e
</CsScore>
</CsoundSynthesizer>

See also

Standard Filters: Resonant Low-pass filters

Credits

Author: Sean Costello
Seattle, Washington
1999

New in Csound version 3.55

iskip new in version 6.06