svf.ni

# State Variable Filter (Double Sampled, Stable)
    based on information from Andrew Simper, Laurent de Soras,
    and Steffan Diedrichsen
svf = proc s :
    * = fmul
    - = fsub
    + = fadd
    % = fmod
    / = fdiv
    min = fmin
    pow = fpow

    NOTCH = 0
    LOW = 1
    HIGH = 2
    BAND = 3

    fs = var 44100.0 # sampling frequency
    fc = var 1000.0 # cutoff frequency normally something like: 440.0*pow(2.0, (midi_note - 69.0)/12.0)
    res = var 0.5 # resonance 0 to 1
    drive = var 0.0 # internal distortion 0 to 0.1
    mode = var HIGH # 0=notch, 1=low, 2=high, 3=band
    freq = s.l s.r ? * 2.0 (sin (* pi (min 0.25 (/ fc (* fs 2.0))))) # the fs*2 is because it's double sampled
    damp = s.l s.r ? min (* 2.0 (- 1.0 (pow res 0.25))) (min 2.0 (- (/ 2.0 freq) (* freq 0.5)))

    channel = proc s :
        V1 = var 0.0
        V3 = var 0.0
        tick = proc v_1 v_3 :
            v_0 = - s (* damp v_3)
            v_1 = + v_1 (* freq v_3)
            v_2 = - v_0 v_1
            v_3 = - (+ (* freq v_2) v_3) (* drive (* v_3 (* v_3 v_3)))
            (SELF)
        t1 = tick V1 V3
        t2 = tick t1.v_1 t1.v_3
        next V1 t2.v_1
        next V3 t2.v_3
        out = or
            (case mode NOTCH) ? + t1.v_0 t2.v_0
            (case mode LOW) ? + t1.v_1 t2.v_1
            (case mode HIGH) ? + t1.v_2 t2.v_2
            (case mode BAND) ? + t1.v_3 t2.v_3
        * 0.5 out

    l = channel s.l
    r = channel s.r
    (SELF)