Question

I am trying to read data of a .wav file both in java and matlab and save as an array of bytes.

In java the code looks as follows:

 public byte[] readWav2(File file) throws UnsupportedAudioFileException, IOException {

     AudioFormat audioFormat;

     AudioInputStream inputAIS = AudioSystem.getAudioInputStream(file);
     audioFormat = inputAIS.getFormat();
     ByteArrayOutputStream baos = new ByteArrayOutputStream();

     // Read the audio data into a memory buffer.
     int nBufferSize = BUFFER_LENGTH * audioFormat.getFrameSize();


        byte[] abBuffer = new byte[nBufferSize];
        while (true) {

            int nBytesRead = inputAIS.read(abBuffer);

            if (nBytesRead == -1) {
                break;
            }
            baos.write(abBuffer, 0, nBytesRead);
        }

        byte[] abAudioData = baos.toByteArray();


        return abAudioData;
    }

In matlab I am using the wavread function:

[Y, FS] = wavread('sound.wav', 'native');

But the results I am getting are different.

In java the first 20 bytes:

53, 0, 19, 0, -71, -1, -80, -1, -99, -1, 10, 0, 87, 0, -69, -1, 123, -1, -77, -1

In matlab:

53, 19, -71, -80, -99, 10, 87, -69, -133, -77, 38, 143, 13, -100, 39, 45, -52, -83, -82, 56

Why every second byte in java is 0 or -1 where in matlab there isn’t? Even though I skip the 0’s and -1’s where in java there is 123 for matlab there is -133? Why is it different?

Answer 1

Java is returning you 16-bit signed PCM data. Since each sample is 16 bits and a byte holds 8 bits, each sample spans two bytes in Java. What Matlab returns you is an array of the 16-bit samples directly.

Basically, the data is the same. It’s just laid out differently in memory.

To access the samples in an easier way from Java you could do some bitwise arithmetic, like this:

int firstSample = (abAudioData[0]&0xFF) | (abAudioData[1]<<8);

Another way to read the samples is with the java.nio buffers:

ByteBuffer bb = ByteBuffer.wrap(abAudioData);
bb.order(ByteOrder.LITTLE_ENDIAN);
ShortBuffer sb = bb.asShortBuffer();

int firstSample = sb.get();