/*
* Copyright (c) 2007 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 };
}
}