Android Audio上层实现
一. 从AudioTrack类说起
AudioTrack适用于解码后的PCM流数据格式，参考AudioTrack类的最基本构造器的参数列表：
public AudioTrack(int streamType, int sampleRateInHz, int channelConfig, int audioFormat, int bufferSizeInBytes, int mode)
{
... ...
}
streamType参数定义在AudioSystem类中, 由AudioManager类向app层返还调用
frameworks/base/media/java/android/media
... ...
public static final int STREAM_DEFAULT = -1; //默认的audio流以及音量
public static final int STREAM_VOICE_CALL = 0; //通话中
public static final int STREAM_SYSTEM = 1; //系统提示音(低电量，锁屏)
public static final int STREAM_RING = 2; //来电铃声，短信声
public static final int STREAM_MUSIC = 3; //音乐播放声
public static final int STREAM_ALARM = 4; //警报声
public static final int STREAM_NOTIFICATION = 5; //消息提示音
public static final int STREAM_BLUETOOTH_SCO = 6; //通话中，连接上蓝牙
public static final int STREAM_SYSTEM_ENFORCED = 7; //某些国家强制规定的系统提示音
public static final int STREAM_DTMF = 8; //双向多频拨号盘，拨号声
public static final int STREAM_TTS = 9; //麦克风输入声
public static final int STREAM_ACCESSIBILITY = 10; //可扩展的其他声音
... ...
sampleRateInHz，声音采样频率，介于4000-44100Hz即可。channelConfig参数定义在AudioFormat类中，如下
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public static final int CHANNEL_CONFIGURATION_INVALID = 0; //非法无效的channel配置
public static final int CHANNEL_CONFIGURATION_DEFAULT = 1; //默认channel配置
public static final int CHANNEL_CONFIGURATION_MONO = 2; //单声道channel配置
public static final int CHANNEL_CONFIGURATION_STEREO = 3; //双声道channel配置... ...
audioFormat参数也在AudioFormat类中，不仅包含了PCM流格式，还包含了编码格式
... ...
public static final int ENCODING_PCM_16BIT = 2; // 16bit/点进行采样
public static final int ENCODING_PCM_8BIT = 3; // 8bit/点进行采样
public static final int ENCODING_PCM_FLOAT = 4; // float/点进行采样
public static final int ENCODING_AC3 = 5;
public static final int ENCODING_E_AC3 = 6;
public static final int ENCODING_DTS = 7;
public static final int ENCODING_DTS_HD = 8;
public static final int ENCODING_MP3 = 9;
public static final int ENCODING_AAC_LC = 10;
public static final int ENCODING_AAC_HE_V1 = 11;
public static final int ENCODING_AAC_HE_V2 = 12;... ...
bufferSize参数定义在AudioTrack类中，对每次写入缓冲区的数据大小进行限制，并初步判定sampleRateInHz, channelConfig以及audioFormat输入是否合理：
... ...
static public int getMinBufferSize(int sampleRateInHz, int channelConfig, int audioFormat){ ... }
... ...
mode参数定义在AudioSystem类中，设定手机所处状态
... ...
public static final int MODE_INVALID = -2; //非法无效模式配置
public static final int MODE_CURRENT = -1; //当前默认模式
public static final int MODE_NORMAL = 0; //正常模式
public static final int MODE_RINGTONE = 1; //耳机模式
public static final int MODE_IN_CALL = 2; //来电模式public static final int MODE_IN_COMMUNICATION = 3; //通话模式
public static final int NUM_MODES = 4;... ...
二. AudioTrack 样例
我们以双声道，8000Hz采样频率，播放音乐作为预设条件
int mBufferSize = AudioTrack.getMinBufferSize( );
AudioTrack mAudioTrack = new AudioTrack(AudioManager.STREAM_MUSIC,
8000, AudioFormat.CHANNEL_CONFIGURATION_STEREO,
AudioFormat.ENCODING_PCM_16BIT, mBufferSize, AudioSystem.MODE_NORMAL);
mAudioTrack.play();
byte[] audiodata = new byte[100];
... ... //初始化audiodata
mAudioTrack.write(audiodata, 0, audiodata.length) //取决于要写入多少audiodata
mAudioTrack.stop();
mAudioTrack.release(); //释放AudioTrack资源，否则不容易被GC回收