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spf

Steiner-Parker filter.

Spf is a digital implementation of a linear Steiner-Parker analogue filter, which is a second-order multimode filter with three separate inputs for lowpass, highpass, and bandpass and one output.

Syntax

asig spf alp,ahp,abp, xcf, xR[, istor]

Initialization

istor --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

alp, ahp, abp -- lowpass, highpass and bandpass input signals. Different signals can be used as inputs; if the same signal is placed at the lowpass and highpass inputs, the result is a band-reject output for that signal. If the same signal is used for all inputs, an allpass filter results.

xcf -- filter cutoff frequency (a- or k-rate)

xR -- filter damping factor, which controls peaking (for bandpass, R = 1/Q, where Q is the ratio of centre frequency and bandwidth). A value of sqrt(2) (approx 1.414) gives no peaking (Butterworth response), and lower values will make the filter peak and ring. A value of 0 turns the filter into a sinusoidal oscillator. Valid values in the range of 0 - 2. At 2, the filter has real poles and so it is equivalent to two first-order filters in series.

Examples

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

Example of the spf opcode.
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<CsoundSynthesizer>
<CsOptions>
-odac -d
</CsOptions>
<CsInstruments>

0dbfs = 1

// three different inputs
instr 1
 alp = diskin:a("fox.wav",1,0,1)/3
 ahp rand p4/3
 abp vco2 p4,100 
 af expon 100,p3,10000
 a3 spf alp,ahp,abp,af,0.707
 out a3
 a1 = 0
endin

// band-reject
instr 2
 a0 init 0
 anoi rand p4
 af expon 100,p3,10000
 as spf anoi,anoi,a0,af,0.3
 out as
endin

// self-oscillation
instr 3
 a0 init 0
 anoi rand p4
 af expon 100,p3,10000
 as spf anoi,a0,a0,af,0
 out as
endin

</CsInstruments>
<CsScore>
i1 0 10 0.2
i2 10 10 0.5
i3 20 10 0.004
</CsScore>
</CsoundSynthesizer>

See also

Standard Filters: Resonant Low-pass filters

Credits

Author: Victor Lazzarini
Feb 2021