/* * 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 * Copyright (c) 2019 by Ralph J. Steinhagen */ package de.gsi.math.filter.iir; import org.apache.commons.math3.complex.Complex; /** * Transforms from an analogue lowpass filter to a digital lowpass filter */ public final class LowPassTransform { // NOPMD - nomen est omen private LowPassTransform() { // utility class } private static Complex transform(final Complex in, final double f) { if (in.isInfinite()) { return new Complex(-1, 0); } Complex c; // frequency transform c = in.multiply(f); final Complex one = new Complex(1, 0); // bilinear low pass transform return one.add(c).divide(one.subtract(c)); } public static void transform(final double fc, final LayoutBase digital, final LayoutBase analog) { final double f; digital.reset(); // pre-warp f = Math.tan(Math.PI * fc); final int numPoles = analog.getNumPoles(); final int pairs = numPoles / 2; for (int i = 0; i < pairs; ++i) { final PoleZeroPair pair = analog.getPair(i); digital.addPoleZeroConjugatePairs(transform(pair.poles.first, f), transform(pair.zeros.first, f)); } if ((numPoles & 1) == 1) { final PoleZeroPair pair = analog.getPair(pairs); digital.add(transform(pair.poles.first, f), transform(pair.zeros.first, f)); } digital.setNormal(analog.getNormalW(), analog.getNormalGain()); } }