LowPassFS.java

/*
 *  Copyright (c) 2007 - 2008 by Damien Di Fede <ddf@compartmental.net>
 *
 *   This program is free software; you can redistribute it and/or modify
 *   it under the terms of the GNU Library General Public License as published
 *   by the Free Software Foundation; either version 2 of the License, or
 *   (at your option) any later version.
 *
 *   This program is distributed in the hope that it will be useful,
 *   but WITHOUT ANY WARRANTY; without even the implied warranty of
 *   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 *   GNU Library General Public License for more details.
 *
 *   You should have received a copy of the GNU Library General Public
 *   License along with this program; if not, write to the Free Software
 *   Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
 */

package ddf.minim.effects;

import ddf.minim.Minim;

/**
 * LowPassFS is a four stage low pass filter. It becomes unstable if the cutoff
 * frequency is set below 60 Hz, so it will report and error and set the cutoff
 * to 60 Hz if you try to set it lower.
 * 
 * @author Damien Di Fede
 * 
 */
public class LowPassFS extends IIRFilter
{
  /**
   * Constructs a low pass filter with a cutoff frequency of <code>freq</code>
   * that will be used to filter audio recorded at <code>sampleRate</code>.
   * 
   * @param freq
   *          the cutoff frequency
   * @param sampleRate
   *          the sample rate of the audio that will be filtered
   */
  public LowPassFS(float freq, float sampleRate)
  {
    super(freq, sampleRate);
  }

  public boolean validFreq(float f)
  {
    if (f < 60)
    {
      Minim.error("This filter quickly becomes unstable below 60 Hz, setting frequency to 60 Hz.");
      return false;
    }
    return true;
  }

  protected void calcCoeff()
  {
    float freqFrac = frequency()/sampleRate();
    float x = (float) Math.exp(-14.445 * freqFrac);
    a = new float[] { (float) Math.pow(1 - x, 4) };
    b = new float[] { 4 * x, -6 * x * x, 4 * x * x * x, -x * x * x * x };
  }
}