uk.me.berndporr.iirj.LowPassTransform.java Source code

Introduction

Here is the source code for uk.me.berndporr.iirj.LowPassTransform.java

Source

/*
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to You under the Apache License, Version 2.0
 * (the "License"); you may not use this file except in compliance with
 * the License.  You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 * 
 *  Copyright (c) 2009 by Vinnie Falco
 *  Copyright (c) 2016 by Bernd Porr
 */

package uk.me.berndporr.iirj;

import org.apache.commons.math3.complex.Complex;

/**
 * Transforms from an analogue lowpass filter to a digital lowpass filter
 */
public class LowPassTransform {

    private double f;

    private Complex transform(Complex c) {
        if (c.isInfinite())
            return new Complex(-1, 0);

        // frequency transform
        c = c.multiply(f);

        Complex one = new Complex(1, 0);

        // bilinear low pass transform
        return (one.add(c)).divide(one.subtract(c));
    }

    public LowPassTransform(double fc, LayoutBase digital, LayoutBase analog) {
        digital.reset();

        // prewarp
        f = Math.tan(Math.PI * fc);

        int numPoles = analog.getNumPoles();
        int pairs = numPoles / 2;
        for (int i = 0; i < pairs; ++i) {
            PoleZeroPair pair = analog.getPair(i);
            digital.addPoleZeroConjugatePairs(transform(pair.poles.first), transform(pair.zeros.first));
        }

        if ((numPoles & 1) == 1) {
            PoleZeroPair pair = analog.getPair(pairs);
            digital.add(transform(pair.poles.first), transform(pair.zeros.first));
        }

        digital.setNormal(analog.getNormalW(), analog.getNormalGain());
    }

}