How to get the volume of a byte array from a SourceDataLine in Java

Question

I have written a Java programm for peer to peer voice chat, but in order to keep the traffic as low as possible i would like to analyze the captured data and make parts with low volume completely silent. The problem is, that I have no idea how to get the volume from the byte array and how to make parts silent.

Answer 1

You have to slide a time window over your data (say .25 seconds worth of data) and compute the root mean square to see if that period of time is silent or not. Exactly how many bytes constitues .25 seconds depends on the audio rate that your sample is.

So assuming you have you data in byte[] audioData, and that audio data is signed 8 bit PCM data, you'd compute the RMS like below... and then use a value like 1000 as your silence threshold.

/** Computes the RMS volume of a group of signal sizes */
public double volumeRMS(int start, int length) {
    long sum = 0;
    int end = start + length;
    int len = length;
    if (end > audioData.length) {
        end = audioData.length;
        len = end - start;
    }
    if (len == 0) {
        return 0;
    }
    for (int i=start; i<end; i++) {
        sum += audioData[i];
    }
    double average = (double)sum/len;

    double sumMeanSquare = 0;;
    for (int i=start; i<end; i++) {
        double f = audioData[i] - average;
        sumMeanSquare += f * f;
    }
    double averageMeanSquare = sumMeanSquare/len;
    double rootMeanSquare = Math.sqrt(averageMeanSquare);

    return rootMeanSquare;
}