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 = 3;       // seconds
    /* 采用率是8000 */
    private final int sampleRate = 8000;
    /* 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 = 440; // hz

    /* 主要是因为采用的16位的数据 */
    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
        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 = 0; 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(2 * 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 = 0;
        for (final double dVal : sample) {
            // scale to maximum amplitude
            final short val = (short) ((dVal * 32767));
            // in 16 bit wav PCM, first byte is the low order byte
            generatedSnd[idx++] = (byte) (val & 0x00ff);
            generatedSnd[idx++] = (byte) ((val & 0xff00) >>> 8);

        }
    }

    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, 0, generatedSnd.length); 
        audioTrack.play();
    }
}