Generate And Play A Tone In Android

Update: Thanks to xxv for spotting a bug in my encoding. He posted the updated code on stackoverflow, here is the link and made a comment here to let me know. Basically I was getting the byte order wrong in the encoding and forgetting to factor in the sampleRate into the wav- oops and sorry! I have update the code here.

Original post.

I threw this together the other night just to figure out how to do some very simple sound synthesis. I am not going to go into details about how the code works as it is pretty obvious for the most part.

It basically generate a sine wave at a given frequency then converts this into a 16bit pcm format and finally plays it. I used a thread to make sure my generation does not block.

Sound synthesis is something I would like to spend some more time messing with. I have written some Haskell code that lets me play with some synthesis ideas, nothing elaborate. One day I will get round to porting it to Java and making some synthesis apps for android.

Here is the code.


public class GenerateSound extends Activity {
private final int duration = 3; // seconds
private final int sampleRate = 8000;
private final int numSamples = duration * sampleRate;
private final double sample[] = new double[numSamples];
private final double freqOfTone = 100; // hz

private final byte generatedSnd[] = new byte[2 * numSamples];

Handler handler = new Handler();

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

@Override
protected void onResume() {
super.onResume();

// Use a new tread as this can take a while
Thread thread = new Thread(new Runnable() {
   public void run() {
     genTone();
     handler.post(new Runnable() {

public void run() {
playSound();
}
});
   }  
 });
 thread.start();
}

void genTone(){
// fill out the array
for (int i = 0; i < numSamples; ++i) {
sample[i] = Math.sin(2 * Math.PI * i / (sampleRate/freqOfTone));
}

// convert to 16 bit pcm sound array
// assumes the sample buffer is normalised.
int idx = 0;
for (double dVal : sample) {
short val = (short) (dVal * 32767);
generatedSnd[idx++] = (byte) (val & 0x00ff);
generatedSnd[idx++] = (byte) ((val & 0xff00) >>> 8);
}
}

void playSound(){
AudioTrack audioTrack = new AudioTrack(AudioManager.STREAM_MUSIC,
8000, AudioFormat.CHANNEL_CONFIGURATION_MONO,
AudioFormat.ENCODING_PCM_16BIT, numSamples,
AudioTrack.MODE_STATIC);
audioTrack.write(generatedSnd, 0, numSamples);
audioTrack.play();
}
}