Generate And Play A Tone In Android hacking

 /***********************************************************************************
  *            Generate And Play A Tone In Android hacking
  *   声明:
  *      1. 源代码url: http://stackoverflow.com/questions/2413426/playing-an-arbitrary-tone-with-android
  *      2. 主要功能是能够让Android手机发出一定频率的声音;
  *      3. 当然我们也可以用来做各种波形发生器,这只是一个简单的Demo源码程序;
  *
  *                              2015-4-27 周一 晴 深圳 平山村 曾剑锋
  **********************************************************************************/
 public class PlaySound extends Activity {
     // originally from http://marblemice.blogspot.com/2010/04/generate-and-play-tone-in-android.html
     // and modified by Steve Pomeroy <steve@staticfree.info>
     /* 声音持续3秒 */
     private final int duration = ;       // seconds
     /* 采用率是8000 */
     private final int sampleRate = ;
     /* 总共采样多少个点 */
     private final int numSamples = duration * sampleRate;
     /* 利用总共采样的点数来生成数据数组 */
     private final double sample[] = new double[numSamples];
     /* 声音频率 */
     private final double freqOfTone = ; // hz

     /* 主要是因为采用的16位的数据 */
     private final byte generatedSnd[] = new byte[ * 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
         final Thread thread = new Thread(new Runnable() {
             public void run() {
                 genTone();
                 handler.post(new Runnable() {

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

     /**
      * 生成上面参数对应的声音数据，保存在数组中
      */
     void genTone(){
         // fill out the array
         for (int i = ; i < numSamples; ++i) {
             /**
              * 算法解析：
              *     1. sampleRate/freqOfTone：每个周期才多少个点；
              *     2. i / (sampleRate/freqOfTone)：当前第i个点在整个周期的
              *         采样点的比重（不知怎么表达 :) ）；
              *     3. 2 * Math.PI * i / (sampleRate/freqOfTone)：当前第i个点对应的弧度制；
              *     4. Math.sin(2 * Math.PI * i / (sampleRate/freqOfTone))：-1到1的sin函数值；
              */
             sample[i] = Math.sin( * Math.PI * i / (sampleRate/freqOfTone));
         }

         // convert to 16 bit pcm sound array
         // assumes the sample buffer is normalised.
         /**
          * 将上面-1到1范围的sin函数值,转换成-32767到32767范围的值,
          * 这个值是16位的值,放在对应的数组中
          */
         int idx = ;
         for (final double dVal : sample) {
             // scale to maximum amplitude
             final short val = (short) ((dVal * ));
             // in 16 bit wav PCM, first byte is the low order byte
             generatedSnd[idx++] = (byte) (val & 0x00ff);
             generatedSnd[idx++] = (byte) ((val & 0xff00) >>> );

         }
     }

     void playSound(){
         final AudioTrack audioTrack = new AudioTrack(AudioManager.STREAM_MUSIC,
                 sampleRate, AudioFormat.CHANNEL_OUT_MONO,
                 AudioFormat.ENCODING_PCM_16BIT/*16位数据*/, generatedSnd.length,
                 AudioTrack.MODE_STATIC);
         //将所有数据写出，相当于是发出声音
         audioTrack.write(generatedSnd, , generatedSnd.length);
         audioTrack.play();
     }
 }