------------- 3 tap lowpass filter
-- by Michel Pollet      24 Sep 2002

inSampleRate	:= 44100:
-- 0 to samplerate/2
inCutoff		:= 1000:
-- slider("inCutOff", 100, inSampleRate / 4):
-- from 0.1 to sqrt(2)
inResonance		:= sqrt(2):
--slider("inResonance", 0.1, sqrt(2)):

init =	cc := 1.0 / tan(pi * inCutoff / inSampleRate),
		a1 := 1.0 / ( 1.0 + inResonance * cc + cc * cc ),
		a2 := 2.0 * a1,
		a3 := a1,
		b1 := 2.0 * ( 1.0 - cc * cc ) * a1,
		b2 := ( 1.0 - inResonance * cc + cc * cc ) * a1,
		b3 := 0.0

iir(j) = 	a1 * in[j] + a2 * in[j - 1] + a3 * in[j - 2] -
		b1 * out[j - 1] - b2 * out[j - 2] - b3 * out[j - 3]

max		= 100

in	:= 0[1:max]:;in[5] := 1:;
-- in[j]	= sin(j/4) dim[max];

out	:= 0[1:max]:
init:
j:= 4:
( out[j] := iir(j), j := j + 1 ) while j <= max:

Title := "LowPass 3 tap IIR graph":
Xmin := 0:; Xmax := max:

Xlabel := sprintf("cutoff = %.1fHz", inCutoff) | sprintf(
  " resonance = %.4f", inResonance)| sprintf(
  " at %.1fHz", inSampleRate)| sprintf(
  " - for %.4fms", max * 1000 / inSampleRate):;
plotline out[4:]

newaxis 
inCutoff		:= 5000:
-- slider("inCutOff", 100, inSampleRate / 4):
-- from 0.1 to sqrt(2)
inResonance		:= sqrt(2) /2:

out	:= 0[1:max]:
init:
j:= 4:
( out[j] := iir(j), j := j + 1 ) while j <= max:

Xlabel := sprintf("cutoff = %.1fHz", inCutoff) | sprintf(
   " resonance = %.4f", inResonance)|sprintf(
   " at %.1fHz", inSampleRate)|sprintf(
   " - for %.4fms", max * 1000 / inSampleRate):;
plotline out[4:]