我正在尝试在Java中实现一个低通滤波器。我的要求非常简单,我需要消除特定频率(单一维度)之外的信号。似乎Butterworth滤波器很适合我的需求。
现在重要的是CPU时间应该尽可能地少。滤波器将处理接近一百万个样本,我们的用户不喜欢等待太长时间。是否有任何现成的Butterworth滤波器实现具有最优算法进行过滤?
我正在尝试在Java中实现一个低通滤波器。我的要求非常简单,我需要消除特定频率(单一维度)之外的信号。似乎Butterworth滤波器很适合我的需求。
现在重要的是CPU时间应该尽可能地少。滤波器将处理接近一百万个样本,我们的用户不喜欢等待太长时间。是否有任何现成的Butterworth滤波器实现具有最优算法进行过滤?
我有一个页面描述了一种非常简单、非常低CPU的低通滤波器,也能够适应帧速率的变化。我用它来平滑用户输入,并经常用于绘制帧速率图表。
http://phrogz.net/js/framerate-independent-low-pass-filter.html
简而言之,在您的更新循环中:
// If you have a fixed frame rate
smoothedValue += (newValue - smoothedValue) / smoothing
// If you have a varying frame rate
smoothedValue += timeSinceLastUpdate * (newValue - smoothedValue) / smoothing
当smoothing
值为1
时,不会发生平滑处理,而较高的值会越来越平滑结果。
该页面有几个用JavaScript编写的函数,但公式与语言无关。
这里是一个使用Apache Math库中的傅里叶变换的低通滤波器。
public double[] fourierLowPassFilter(double[] data, double lowPass, double frequency){
//data: input data, must be spaced equally in time.
//lowPass: The cutoff frequency at which
//frequency: The frequency of the input data.
//The apache Fft (Fast Fourier Transform) accepts arrays that are powers of 2.
int minPowerOf2 = 1;
while(minPowerOf2 < data.length)
minPowerOf2 = 2 * minPowerOf2;
//pad with zeros
double[] padded = new double[minPowerOf2];
for(int i = 0; i < data.length; i++)
padded[i] = data[i];
FastFourierTransformer transformer = new FastFourierTransformer(DftNormalization.STANDARD);
Complex[] fourierTransform = transformer.transform(padded, TransformType.FORWARD);
//build the frequency domain array
double[] frequencyDomain = new double[fourierTransform.length];
for(int i = 0; i < frequencyDomain.length; i++)
frequencyDomain[i] = frequency * i / (double)fourierTransform.length;
//build the classifier array, 2s are kept and 0s do not pass the filter
double[] keepPoints = new double[frequencyDomain.length];
keepPoints[0] = 1;
for(int i = 1; i < frequencyDomain.length; i++){
if(frequencyDomain[i] < lowPass)
keepPoints[i] = 2;
else
keepPoints[i] = 0;
}
//filter the fft
for(int i = 0; i < fourierTransform.length; i++)
fourierTransform[i] = fourierTransform[i].multiply((double)keepPoints[i]);
//invert back to time domain
Complex[] reverseFourier = transformer.transform(fourierTransform, TransformType.INVERSE);
//get the real part of the reverse
double[] result = new double[data.length];
for(int i = 0; i< result.length; i++){
result[i] = reverseFourier[i].getReal();
}
return result;
}
正如Mark Peters在评论中所说:需要过滤大量数据的过滤器应该用C或C++编写。但是你仍然可以利用Java。只需查看Java Native Interface (JNI)即可。由于C/C++编译为本机机器代码,它将比在Java虚拟机(JVM)中运行字节码要快得多,JVM实际上是一个虚拟处理器,将字节码转换为本地机器的本机代码(取决于CPU指令集,如x86、x64、ARM等)。
我知道这是一个老问题,但我想补充一些之前似乎没有提到的内容。
首先,你应该意识到:
当你需要在应用程序中使用滤波器时,你需要选择某种类型的滤波器,选择特定的滤波器设计方法,应用该方法找到满足你约束条件的滤波器系数,最后将这些系数复制到你的滤波器实现中。
选择滤波器类型和应用设计方法是你一次性完成的,使用适当的软件,如Matlab、Octave或Scilab。这可能涉及一些试验和观察滤波器获得的特性,如频率响应(幅度和相位)和/或冲激响应,以查看它们是否符合你的规格。一旦你确定了解决方案,你将有一组常数系数。这些数字,或这些数字的某些线性组合,是你需要复制到你的程序(Java或其他)中作为常数表的全部内容。
在您的程序中,您只需要一个应用某些滤波器实现的函数,该函数使用这些系数对输入流样本(以及可能的先前输出样本)进行线性组合,以在每个时间点产生新的输出样本。我从http://www.dspguide.com/采用了这个。我对Java还很陌生,所以代码可能不太好看,但它可以工作。
/*
* To change this license header, choose License Headers in Project Properties.
* To change this template file, choose Tools | Templates
* and open the template in the editor.
*/
package SoundCruncher;
import java.util.ArrayList;
/**
*
* @author 2sloth
* filter routine from "The scientist and engineer's guide to DSP" Chapter 20
* filterOrder can be any even number between 2 & 20
* cutoffFreq must be smaller than half the samplerate
* filterType: 0=lowPass 1=highPass
* ripplePercent is amount of ripple in Chebyshev filter (0-29) (0=butterworth)
*/
public class Filtering {
double[] filterSignal(ArrayList<Float> signal, double sampleRate ,double cutoffFreq, double filterOrder, int filterType, double ripplePercent) {
double[][] recursionCoefficients = new double[22][2];
// Generate double array for ease of coding
double[] unfilteredSignal = new double[signal.size()];
for (int i=0; i<signal.size(); i++) {
unfilteredSignal[i] = signal.get(i);
}
double cutoffFraction = cutoffFreq/sampleRate; // convert cut-off frequency to fraction of sample rate
System.out.println("Filtering: cutoffFraction: " + cutoffFraction);
//ButterworthFilter(0.4,6,ButterworthFilter.Type highPass);
double[] coeffA = new double[22]; //a coeffs
double[] coeffB = new double[22]; //b coeffs
double[] tA = new double[22];
double[] tB = new double[22];
coeffA[2] = 1;
coeffB[2] = 1;
// calling subroutine
for (int i=1; i<filterOrder/2; i++) {
double[] filterParameters = MakeFilterParameters(cutoffFraction, filterType, ripplePercent, filterOrder, i);
for (int j=0; j<coeffA.length; j++){
tA[j] = coeffA[j];
tB[j] = coeffB[j];
}
for (int j=2; j<coeffA.length; j++){
coeffA[j] = filterParameters[0]*tA[j]+filterParameters[1]*tA[j-1]+filterParameters[2]*tA[j-2];
coeffB[j] = tB[j]-filterParameters[3]*tB[j-1]-filterParameters[4]*tB[j-2];
}
}
coeffB[2] = 0;
for (int i=0; i<20; i++){
coeffA[i] = coeffA[i+2];
coeffB[i] = -coeffB[i+2];
}
// adjusting coeffA and coeffB for high/low pass filter
double sA = 0;
double sB = 0;
for (int i=0; i<20; i++){
if (filterType==0) sA = sA+coeffA[i];
if (filterType==0) sB = sB+coeffB[i];
if (filterType==1) sA = sA+coeffA[i]*Math.pow(-1,i);
if (filterType==1) sB = sB+coeffA[i]*Math.pow(-1,i);
}
// applying gain
double gain = sA/(1-sB);
for (int i=0; i<20; i++){
coeffA[i] = coeffA[i]/gain;
}
for (int i=0; i<22; i++){
recursionCoefficients[i][0] = coeffA[i];
recursionCoefficients[i][1] = coeffB[i];
}
double[] filteredSignal = new double[signal.size()];
double filterSampleA = 0;
double filterSampleB = 0;
// loop for applying recursive filter
for (int i= (int) Math.round(filterOrder); i<signal.size(); i++){
for(int j=0; j<filterOrder+1; j++) {
filterSampleA = filterSampleA+coeffA[j]*unfilteredSignal[i-j];
}
for(int j=1; j<filterOrder+1; j++) {
filterSampleB = filterSampleB+coeffB[j]*filteredSignal[i-j];
}
filteredSignal[i] = filterSampleA+filterSampleB;
filterSampleA = 0;
filterSampleB = 0;
}
return filteredSignal;
}
/* pi=3.14...
cutoffFreq=fraction of samplerate, default 0.4 FC
filterType: 0=LowPass 1=HighPass LH
rippleP=ripple procent 0-29 PR
iterateOver=1 to poles/2 P%
*/
// subroutine called from "filterSignal" method
double[] MakeFilterParameters(double cutoffFraction, int filterType, double rippleP, double numberOfPoles, int iteration) {
double rp = -Math.cos(Math.PI/(numberOfPoles*2)+(iteration-1)*(Math.PI/numberOfPoles));
double ip = Math.sin(Math.PI/(numberOfPoles*2)+(iteration-1)*Math.PI/numberOfPoles);
System.out.println("MakeFilterParameters: ripplP:");
System.out.println("cutoffFraction filterType rippleP numberOfPoles iteration");
System.out.println(cutoffFraction + " " + filterType + " " + rippleP + " " + numberOfPoles + " " + iteration);
if (rippleP != 0){
double es = Math.sqrt(Math.pow(100/(100-rippleP),2)-1);
// double vx1 = 1/numberOfPoles;
// double vx2 = 1/Math.pow(es,2)+1;
// double vx3 = (1/es)+Math.sqrt(vx2);
// System.out.println("VX's: ");
// System.out.println(vx1 + " " + vx2 + " " + vx3);
// double vx = vx1*Math.log(vx3);
double vx = (1/numberOfPoles)*Math.log((1/es)+Math.sqrt((1/Math.pow(es,2))+1));
double kx = (1/numberOfPoles)*Math.log((1/es)+Math.sqrt((1/Math.pow(es,2))-1));
kx = (Math.exp(kx)+Math.exp(-kx))/2;
rp = rp*((Math.exp(vx)-Math.exp(-vx))/2)/kx;
ip = ip*((Math.exp(vx)+Math.exp(-vx))/2)/kx;
System.out.println("MakeFilterParameters (rippleP!=0):");
System.out.println("es vx kx rp ip");
System.out.println(es + " " + vx*100 + " " + kx + " " + rp + " " + ip);
}
double t = 2*Math.tan(0.5);
double w = 2*Math.PI*cutoffFraction;
double m = Math.pow(rp, 2)+Math.pow(ip,2);
double d = 4-4*rp*t+m*Math.pow(t,2);
double x0 = Math.pow(t,2)/d;
double x1 = 2*Math.pow(t,2)/d;
double x2 = Math.pow(t,2)/d;
double y1 = (8-2*m*Math.pow(t,2))/d;
double y2 = (-4-4*rp*t-m*Math.pow(t,2))/d;
double k = 0;
if (filterType==1) {
k = -Math.cos(w/2+0.5)/Math.cos(w/2-0.5);
}
if (filterType==0) {
k = -Math.sin(0.5-w/2)/Math.sin(w/2+0.5);
}
d = 1+y1*k-y2*Math.pow(k,2);
double[] filterParameters = new double[5];
filterParameters[0] = (x0-x1*k+x2*Math.pow(k,2))/d; //a0
filterParameters[1] = (-2*x0*k+x1+x1*Math.pow(k,2)-2*x2*k)/d; //a1
filterParameters[2] = (x0*Math.pow(k,2)-x1*k+x2)/d; //a2
filterParameters[3] = (2*k+y1+y1*Math.pow(k,2)-2*y2*k)/d; //b1
filterParameters[4] = (-(Math.pow(k,2))-y1*k+y2)/d; //b2
if (filterType==1) {
filterParameters[1] = -filterParameters[1];
filterParameters[3] = -filterParameters[3];
}
// for (double number: filterParameters){
// System.out.println("MakeFilterParameters: " + number);
// }
return filterParameters;
}
}