[java-dev] filter algorithm in mxj~

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