Question

I would like to know if somebody knows a library to changing the playback rate of a track in real time. My idea is to load a track and change its playback rate to half or double. Firstly, I tried with MusicPlayer but is was not possible at all and then I tried with SoundPool. The problem is that with SoundPool I can´t change the rate once the track is loaded. Here is the code I am using (proof of concept):

float j = 1;

@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.main);

    Button b = (Button)findViewById(R.id.Button01);
    b.setOnClickListener(new OnClickListener() {

        @Override
        public void onClick(View v) {
            j = (float) (j +.5);

        }
    });

    AssetFileDescriptor afd;
    try {

        SoundPool sp = new SoundPool(1, AudioManager.STREAM_MUSIC, 0);

        afd = getAssets().openFd("wav/sample.wav");
        int id = sp.load(afd, 1);
        sp.play(id, 1, 1, 1, 0, j);

    } catch (IOException e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
    }
}

When I press the button, the playback rate should increase but it does not happen. Any idea of how change the rate in real time?

Thanks in advance.

Answer 1

you call sp.play once, at the time that function is called the value of j is copied into that function.

Changing j afterward doesn’t change the value of j in the function (this is called pass by value)

To change the rate of play afterward you need to call sp.setRate(StreamID,j) within your button click listener.

Note that the StreamID is returned by the sp.play function, so store this as a variable in the class.

This code should work ( i didn’t test it):

float j = 1.0f;
SoundPool sp = null;
int sID = 0;

@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.main);

    Button b = (Button)findViewById(R.id.Button01);
    b.setOnClickListener(new OnClickListener() {

        @Override
        public void onClick(View v) {
            j = (float) (j +.5f);
            sp.setRate(sID,j);

        }
    });

    AssetFileDescriptor afd;
    try {

        sp = new SoundPool(1, AudioManager.STREAM_MUSIC, 0);

        afd = getAssets().openFd("wav/sample.wav");
        int id = sp.load(afd, 1);
        sID = sp.play(id, 1, 1, 1, 0, j);

    } catch (IOException e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
    }
}

although it will probably crash after a couple button presses, SoundPool only supports rates from 0.5 to 2.0. You should add checking to make sure j stays within those bounds.