To mix two byte arrays with 16 bit sound samples, you should first convert those arrays to int arrays, i.e., sample-based arrays, then add them (to mix) and then convert back to byte arrays. When converting from byte array to int array, you need to make sure you use the correct endianness (byte order).
Here's some code that lets you mix two arrays. At the end there is some sample code (using sine waves) that demonstrates the approach. Note that this is may not be the ideal way of coding this, but a working example to demonstrate the concept. Using streams or lines, as Phil recommends is probably the smarter overall approach.
Good luck!
import javax.sound.sampled.AudioFileFormat;
import javax.sound.sampled.AudioFormat;
import javax.sound.sampled.AudioInputStream;
import javax.sound.sampled.AudioSystem;
import java.io.ByteArrayInputStream;
import java.io.File;
import java.io.IOException;
public class MixDemo {
public static byte[] mix(final byte[] a, final byte[] b, final boolean bigEndian) {
final byte[] aa;
final byte[] bb;
final int length = Math.max(a.length, b.length);
// ensure same lengths
if (a.length != b.length) {
aa = new byte[length];
bb = new byte[length];
System.arraycopy(a, 0, aa, 0, a.length);
System.arraycopy(b, 0, bb, 0, b.length);
} else {
aa = a;
bb = b;
}
// convert to samples
final int[] aSamples = toSamples(aa, bigEndian);
final int[] bSamples = toSamples(bb, bigEndian);
// mix by adding
final int[] mix = new int[aSamples.length];
for (int i=0; i<mix.length; i++) {
mix[i] = aSamples[i] + bSamples[i];
// enforce min and max (may introduce clipping)
mix[i] = Math.min(Short.MAX_VALUE, mix[i]);
mix[i] = Math.max(Short.MIN_VALUE, mix[i]);
}
// convert back to bytes
return toBytes(mix, bigEndian);
}
private static int[] toSamples(final byte[] byteSamples, final boolean bigEndian) {
final int bytesPerChannel = 2;
final int length = byteSamples.length / bytesPerChannel;
if ((length % 2) != 0) throw new IllegalArgumentException("For 16 bit audio, length must be even: " + length);
final int[] samples = new int[length];
for (int sampleNumber = 0; sampleNumber < length; sampleNumber++) {
final int sampleOffset = sampleNumber * bytesPerChannel;
final int sample = bigEndian
? byteToIntBigEndian(byteSamples, sampleOffset, bytesPerChannel)
: byteToIntLittleEndian(byteSamples, sampleOffset, bytesPerChannel);
samples[sampleNumber] = sample;
}
return samples;
}
private static byte[] toBytes(final int[] intSamples, final boolean bigEndian) {
final int bytesPerChannel = 2;
final int length = intSamples.length * bytesPerChannel;
final byte[] bytes = new byte[length];
for (int sampleNumber = 0; sampleNumber < intSamples.length; sampleNumber++) {
final byte[] b = bigEndian
? intToByteBigEndian(intSamples[sampleNumber], bytesPerChannel)
: intToByteLittleEndian(intSamples[sampleNumber], bytesPerChannel);
System.arraycopy(b, 0, bytes, sampleNumber * bytesPerChannel, bytesPerChannel);
}
return bytes;
}
// from https://github.com/hendriks73/jipes/blob/master/src/main/java/com/tagtraum/jipes/audio/AudioSignalSource.java#L238
private static int byteToIntLittleEndian(final byte[] buf, final int offset, final int bytesPerSample) {
int sample = 0;
for (int byteIndex = 0; byteIndex < bytesPerSample; byteIndex++) {
final int aByte = buf[offset + byteIndex] & 0xff;
sample += aByte << 8 * (byteIndex);
}
return (short)sample;
}
// from https://github.com/hendriks73/jipes/blob/master/src/main/java/com/tagtraum/jipes/audio/AudioSignalSource.java#L247
private static int byteToIntBigEndian(final byte[] buf, final int offset, final int bytesPerSample) {
int sample = 0;
for (int byteIndex = 0; byteIndex < bytesPerSample; byteIndex++) {
final int aByte = buf[offset + byteIndex] & 0xff;
sample += aByte << (8 * (bytesPerSample - byteIndex - 1));
}
return (short)sample;
}
private static byte[] intToByteLittleEndian(final int sample, final int bytesPerSample) {
byte[] buf = new byte[bytesPerSample];
for (int byteIndex = 0; byteIndex < bytesPerSample; byteIndex++) {
buf[byteIndex] = (byte)((sample >>> (8 * byteIndex)) & 0xFF);
}
return buf;
}
private static byte[] intToByteBigEndian(final int sample, final int bytesPerSample) {
byte[] buf = new byte[bytesPerSample];
for (int byteIndex = 0; byteIndex < bytesPerSample; byteIndex++) {
buf[byteIndex] = (byte)((sample >>> (8 * (bytesPerSample - byteIndex - 1))) & 0xFF);
}
return buf;
}
public static void main(final String[] args) throws IOException {
final int sampleRate = 44100;
final boolean bigEndian = true;
final int sampleSizeInBits = 16;
final int channels = 1;
final boolean signed = true;
final AudioFormat targetAudioFormat = new AudioFormat(sampleRate, sampleSizeInBits, channels, signed, bigEndian);
final byte[] a = new byte[sampleRate * 10];
final byte[] b = new byte[sampleRate * 5];
// create sine waves
for (int i=0; i<a.length/2; i++) {
System.arraycopy(intToByteBigEndian((int)(30000*Math.sin(i*0.5)),2), 0, a, i*2, 2);
}
for (int i=0; i<b.length/2; i++) {
System.arraycopy(intToByteBigEndian((int)(30000*Math.sin(i*0.1)),2), 0, b, i*2, 2);
}
final File aFile = new File("a.wav");
AudioSystem.write(new AudioInputStream(new ByteArrayInputStream(a), targetAudioFormat, a.length),
AudioFileFormat.Type.WAVE, aFile);
final File bFile = new File("b.wav");
AudioSystem.write(new AudioInputStream(new ByteArrayInputStream(b), targetAudioFormat, b.length),
AudioFileFormat.Type.WAVE, bFile);
// mix a and b
final byte[] mixed = mix(a, b, bigEndian);
final File outFile = new File("out.wav");
AudioSystem.write(new AudioInputStream(new ByteArrayInputStream(mixed), targetAudioFormat, mixed.length),
AudioFileFormat.Type.WAVE, outFile);
}
}
