filter algorithm in mxj~

Morgan Sutherland

I'm currently trying to implement the following filter algorithm from
music-dsp (for school ;):

---------------------------------------------------------
r = rez amount, from sqrt(2) to ~ 0.1
f = cutoff frequency
(from ~0 Hz to SampleRate/2 - though many
synths seem to filter only up to SampleRate/4)

The filter algo:
out(n) = a1 * in + a2 * in(n-1) + a3 * in(n-2) - b1*out(n-1) - b2*out(n-2)

Lowpass:
c = 1.0 / tan(pi * f / sample_rate);

a1 = 1.0 / ( 1.0 + r * c + c * c);
a2 = 2* a1;
a3 = a1;
b1 = 2.0 * ( 1.0 - c*c) * a1;
b2 = ( 1.0 - r * c + c * c) * a1;

Hipass:
c = tan(pi * f / sample_rate);

a1 = 1.0 / ( 1.0 + r * c + c * c);
a2 = -2*a1;
a3 = a1;
b1 = 2.0 * ( c*c - 1.0) * a1;
b2 = ( 1.0 - r * c + c * c) * a1;
---------------------------------------------------------

My code compiles, but I'm not getting any sound from the outlet. I
suspect this is because of the algorithm, which uses input and output
values from 'back in time':

out[i] = ( a1 * in[i] + a2 * in[i-1] + a3 * in[i-2] - b1*out[i-1] -
b2*out[i-2] );

Do I need to rewrite this with temp variables?

My code looks like this:

---------------------------------------------------------
import com.cycling74.max.*;
import com.cycling74.msp.*;
import java.lang.Math.*;

public class filt extends MSPPerformer
{
    private float freq = 500.0f;
    private float c = 0.0f;
    private float r = 0.1f;

    private float a1 = 0.0f;
    private float a2 = 0.0f;
    private float a3 = 0.0f;
    private float b1 = 0.0f;
    private float b2 = 0.0f;

private float tin1 = 0.0f, tin2 = 0.0f, to3 = 0.0f, to4 = 0.0f;

private float pi = 3.1415926f;

private double sr;

    private static final String[] INLET_ASSIST = new String[]{
        "input (sig)"
    };
    private static final String[] OUTLET_ASSIST = new String[]{
        "output (sig)"
    };

    public filt(float gain)
    {
        declareInlets(new int[]{SIGNAL, DataTypes.FLOAT});
        declareOutlets(new int[]{SIGNAL});

        setInletAssist(0, "signal input");
        setInletAssist(1, "freq");
        setOutletAssist(OUTLET_ASSIST);
    }

public void inlet(float f)
{

freq = f;

}

    public void dspsetup(MSPSignal[] ins, MSPSignal[] outs)
    {
        //Max gives us the sample-rate 'sr' on load

sr = ins[0].sr;

post("dspsetup was called.'");

}

public void perform(MSPSignal[] ins, MSPSignal[] outs)
{

c = 1.0f / (float)Math.tan((double)(pi * freq / (float)sr));

        a1 = 1.0f / ( 1.0f + r * c + c * c);
         a2 = 2f* a1;
         a3 = a1;
        b1 = 2.0f * ( 1.0f - c*c) * a1;
        b2 = ( 1.0f - r * c + c * c) * a1;

        int i;
        float[] in = ins[0].vec;
        float[] out = outs[0].vec;
        int vec_size = ins[0].n;

for(i = 0; i < vec_size; i++)
{

out[i] = ( a1 * in[i] + a2 * in[i-1] + a3 * in[i-2] - b1*out[i-1] -
b2*out[i-2] );

}
}
}
---------------------------------------------------------

--
Morgan Sutherland