Create mixed audio file from two or more mp3 files

Question

I've trying to create a single synchronous audio output file from two or more than two audio files of same length. I have done some research and came to know that this can be achieved through below steps:

1. Only uncompressed audio files can be used for this purpose. i.e. Convert mp3 audio files into WAV format.
2. Mix 2 new wav files; i have followed this approach: Mix audio in android

But the same problem arises, the mixed audio file doesn't play.

I've been trying to add a valid header to the output file as per this answer Writing PCM recorded data into a .wav file (java android).

Here is my Mixer class:

public class SongMixer2
{

private Context mContext;

public SongMixer2(Context context)
{
    mContext = context;
}

public void onCreate()
{
    try
    {
        mixSound();
    } catch (IOException e)
    {
        // TODO Auto-generated catch block
        e.printStackTrace();
    }
}

private void mixSound() throws IOException
{
    AudioTrack audioTrack = new AudioTrack(AudioManager.STREAM_MUSIC,
            44100, AudioFormat.CHANNEL_OUT_STEREO,
            AudioFormat.ENCODING_PCM_16BIT, 44100, AudioTrack.MODE_STREAM);

    InputStream in1 = mContext.getResources().openRawResource(R.raw.brokaw);
    InputStream in2 = mContext.getResources().openRawResource(R.raw.brokaw);

    byte[] arrayMusic1 = null;
    arrayMusic1 = new byte[in1.available()];
    arrayMusic1 = createMusicArray(in1);
    in1.close();

    byte[] arrayMusic2 = null;
    arrayMusic2 = new byte[in2.available()];
    arrayMusic2 = createMusicArray(in2);
    in2.close();

    byte[] output = new byte[arrayMusic1.length];

    audioTrack.play();

    for (int i = 0; i < output.length - 1; i++)
    {
        float samplef1 = arrayMusic1[i] / 128.0f;
        float samplef2 = arrayMusic2[i] / 128.0f;
        float mixed = samplef1 + samplef2;

        // reduce the volume a bit:
        mixed *= 0.8;
        // hard clipping
        if (mixed > 1.0f)
            mixed = 1.0f;
        if (mixed < -1.0f)
            mixed = -1.0f;

        byte outputSample = (byte) (mixed * 128.0f);
        output[i] = outputSample;
    }

    audioTrack.write(output, 0, output.length);
    convertByteToFile(output);
}

public static byte[] createMusicArray(InputStream is) throws IOException
{

    ByteArrayOutputStream baos = new ByteArrayOutputStream();
    byte[] buff = new byte[10240];
    int i = Integer.MAX_VALUE;
    while ((i = is.read(buff, 0, buff.length)) > 0)
    {
        baos.write(buff, 0, i);
    }

    return baos.toByteArray(); // be sure to close InputStream in calling
                                // function

}

public static void convertByteToFile(byte[] fileBytes)
        throws FileNotFoundException
{

    String path = Environment.getExternalStorageDirectory().getPath() + "/mixed.wav";
    File f=new File(path);
    BufferedOutputStream bos = new BufferedOutputStream(
            new FileOutputStream(f));
    try
    {
        bos.write(writeHeader(fileBytes.length), 0, 44);
        bos.write(fileBytes);
        bos.flush();
        bos.close();
    } catch (IOException e)
    {
        // TODO Auto-generated catch block
        e.printStackTrace();
    }
}

private static byte[] writeHeader(int totalDataLen)
{
    byte[] header = new byte[44];

    header[0] = 'R';  // RIFF/WAVE header
    header[1] = 'I';
    header[2] = 'F';
    header[3] = 'F';
    header[4] = (byte) (totalDataLen & 0xff);
    header[5] = (byte) ((totalDataLen >> 8) & 0xff);
    header[6] = (byte) ((totalDataLen >> 16) & 0xff);
    header[7] = (byte) ((totalDataLen >> 24) & 0xff);
    header[8] = 'W';
    header[9] = 'A';
    header[10] = 'V';
    header[11] = 'E';
    header[12] = 'f';  // 'fmt ' chunk
    header[13] = 'm';
    header[14] = 't';
    header[15] = ' ';
    header[16] = 16;  // 4 bytes: size of 'fmt ' chunk
    header[17] = 0;
   /* header[18] = 0;
    header[19] = 0;
    header[20] = 1;  // format = 1
    header[21] = 0;
    header[22] = (byte) channels;
    header[23] = 0;
    header[24] = (byte) (longSampleRate & 0xff);
    header[25] = (byte) ((longSampleRate >> 8) & 0xff);
    header[26] = (byte) ((longSampleRate >> 16) & 0xff);
    header[27] = (byte) ((longSampleRate >> 24) & 0xff);
    header[28] = (byte) (byteRate & 0xff);
    header[29] = (byte) ((byteRate >> 8) & 0xff);
    header[30] = (byte) ((byteRate >> 16) & 0xff);
    header[31] = (byte) ((byteRate >> 24) & 0xff);
    header[32] = (byte) (2 * 16 / 8);  // block align
    header[33] = 0;
    header[34] = RECORDER_BPP;  // bits per sample
    header[35] = 0;*/
    header[36] = 'd';
    header[37] = 'a';
    header[38] = 't';
    header[39] = 'a';
   /* header[40] = (byte) (totalAudioLen & 0xff);
    header[41] = (byte) ((totalAudioLen >> 8) & 0xff);
    header[42] = (byte) ((totalAudioLen >> 16) & 0xff);
    header[43] = (byte) ((totalAudioLen >> 24) & 0xff);
*/

 return header;
}

}

And the Activity:

public class SoundMixerActivity extends Activity 
{
protected void onCreate(Bundle savedInstanceState) 
    {
    super.onCreate(savedInstanceState);
    new SongMixer2(this).onCreate();
}
}

A File 'mixed.wav' is created there in SD card, but it won't play...anyone please help!!

Answer 1

In your header you say the file has an audio alignment of 4 bytes. This would be for 16-bit stereo audio.

In your createMix, however, you assume that the audio is 8bit (a byte) rather than 16-bit. You need to perform your mixing with shorts.

Its also usual to take the average of the samples from each file. ie (sample1 + sample2) / 2.