Description

Zero-delay feedback implementation of a 4 pole (24 dB/oct) low-pass filter based on the Moog ladder filter.

Syntax

asig zdf_ladder ain, xcf, xQ [, 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

asig -- output signal.

asig -- input signal.

xcf -- filter cutoff frequency (i-, k-, or a-rate).

xQ -- filter Q value (i-, k-, or a-rate). Range 0.5-25.0. Self-oscillation occurs at 25.0.

Examples

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

<CsoundSynthesizer>
<CsOptions>
-o dac
</CsOptions>
<CsInstruments>

sr	=	48000
ksmps	=	1
nchnls	=	2
0dbfs	=	1

instr 1	
  asig = random:a(-1.0, 1.0) 
  asig = zdf_ladder(asig, expon(220, p3, 5000), p4)
  outc(asig, asig)
endin

instr 2	
  asig = vco2(0.5, 220)
  asig = zdf_ladder(asig, expon(220, p3, 5000), p4)
  outc(asig, asig)
endin

instr 3	
  asig = vco2(0.5, p4)
  asig = zdf_ladder(asig, expon(5000, p3, 200), 0.5 + p5* 24)
  asig *= madsr(0.05, 0, 1, 0.25)
  outc(asig, asig)
endin


instr play_instr3
  schedule(3, 0, 0.25, mtof(48 + (p4 % 2) * 12), p4 / 16)

  if(p4 < 16) then 
    schedule("play_instr3", 0.25, 0.25, p4 + 1)
  else
    event_i("e", 0.5,0)
  endif 
  turnoff
endin

</CsInstruments>
<CsScore>
i1 0 2 0.5
i1 + . 1 
i1 + . 4 
i1 + . 10 
i1 + . 18 
i1 + . 24.5 
i2 12 2 0.5
i2 + . 1 
i2 + . 4 
i2 + . 10 
i2 + . 18 
i2 + . 24.5 

s
i "play_instr3" 0 0.25 0 
f0 60
</CsScore>
</CsoundSynthesizer>

References

This filter is based on the work of Will Pirkle that employs Vadim Zavalishin's work with bilinear tranforms to create topology-preserving transform (TPT) implementations of analog filters.

Credits

New in Csound 6.09.0