Subtracting one audio file from another in android

Question

I recorded an audio file A and saved it as testaudio0.gp Then I recorded an audio file B while audio file A was playing at the same time and saved it as testaudio.gp

Of course, in audio file B I hear audio file A as well.

I use the normal MediaPlayer and MediaRecorder classes in Android. The audio files are of the same length. The file size is 6,81Kb for both.

Here is my Code:

final MediaRecorder recorder = new MediaRecorder();
recorder.setAudioSource(MediaRecorder.AudioSource.VOICE_COMMUNICATION);
recorder.setOutputFormat(MediaRecorder.OutputFormat.THREE_GPP);
recorder.setAudioEncoder(MediaRecorder.AudioEncoder.AMR_NB);
recorder.setOutputFile("/sdcard/testaudio.3gp");
try {
    recorder.prepare();
} catch (IOException e) {
    e.printStackTrace();
    Toast.makeText(this, "Error", Toast.LENGTH_SHORT).show();
}

Uri myUri = Uri.parse("/sdcard/testaudio0.3gp"); // initialize Uri here
final MediaPlayer mediaPlayer = new MediaPlayer();
mediaPlayer.setAudioStreamType(AudioManager.STREAM_MUSIC);
try {
    mediaPlayer.setDataSource(getApplicationContext(), myUri);
} catch (IOException e) {
    e.printStackTrace();
}
try {
    mediaPlayer.prepare();
} catch (IOException e) {
    e.printStackTrace();
}
mediaPlayer.start();




recorder.start();   // Recording is now started
new Handler().postDelayed(new Runnable() {
    @Override
    public void run() {
        recorder.stop();
        recorder.reset();   // You can reuse the object by going back to setAudioSource() step
        recorder.release(); // Now the object cannot be reused
        mediaPlayer.stop();
        mediaPlayer.reset();
        mediaPlayer.release();
        runOnUiThread(new Runnable() {
            @Override
            public void run() {
                Toast.makeText(MainActivity.this, "Recording Stopped!", Toast.LENGTH_SHORT).show();
            }
        });

    }
}, 4000);

As you can see I changed recorder.setAudioSource to VOICE_COMMUNICATION since after doing some research on echo cancellation I read that this should help. It seems like it helps a bit, but there is still a very good hearable background audio of file A existing.

I would like to subtract as much as possible of audio file A . I did not find a stack exchange question answering how to do this. Please be patient that I do not understand those complicated mathematical operations to do. A code example would help a lot.

Any help is appreciated. Thank you.

Answer 1

To subtract one audio file A from another audio file B where B contains new signal C together with a playback of file A is conceptually simple ... ignoring all the artifacts of signal A bouncing off walls etc as found in file B ... simply take the negative of audio curve A and overlay it atop audio curve B ... signal A will get erased from audio B leaving you with just the additional signal C

I believe you can do this using the audio workstation Audacity ... yet its not too hard to write custom code to perform the same transformation ... in pseudo code we do this below

For simplicity lets consider signal A to be a sin curve starting at zero radians ... we have all seen this sin curve ... if we synthesize this signal with radians varying from 0 to 2*pi it will generate one cycle of the sin wave ... enough for below demonstration ... now if we simultaneously synthesize the negative of signal A which will happen if we use a phase shift of pi radians this negative curve will be the exact opposite shape

for x = 0; x < 2*pi; x += 2*pi/100 {

     A = sin(x)       // your original data of file A
  negA = sin(x + pi)  //  if you plot this negA will == A * (-1)

  new_signal = cos(5*x) // your new signal which go into file B

     B = A + new_signal // your audio data of file B

     C = B + negA  # this is same as following C = B - A == new_signal
     C = B - A     #  desired output : (signal A + new signal) minus A
                   #  this will result in simply new signal which is your goal
                   #   do you now see that C = new_signal
}