使用AudioTrack播放MP3 左右声道控制 之移植Libmad到android平台

前一段时间公司有需求要控制左右喇叭播放音乐测试，所有自己就做了demo 顺便和大家分享一下。
众所周知，Android的audiotrack只能播放原始的音频，也就是PCM数据，若是播放mp3编码格式的音频的话，就是 出现沙沙的噪音。所以，可以使用第三方库Libmad来对mp3文件解码称为PCM数据，再送给audiotrack播放即可。

1、Libmad简介

      Libmad是一个开源的高精度 MPEG 音频解码库，支持 MPEG-1（Layer I, Layer II 和 LayerIII（也就是 MP3）。LIBMAD 提供 24-bit 的 PCM 输出，完全是定点计算，非常适合没有浮点支持的平台上使用。使用 libmad 提供的一系列 API，就可以非常简单地实现 MP3 数据解码工作。在 libmad 的源代码文件目录下的 mad.h 文件中，可以看到绝大部分该库的数据结构和 API 等。

2、实现过程

2.1、下载Android平台下的Libmad工程

作者已经在项目里下载好了，直接引用就ok。
下载的project可以直接用NDK编译通过的，但是要使用还是要写jni层供Java层调用

但是Android.mk要改成如下形式。

#ifeq ($(strip $(BUILD_WITH_GST)),true)  
LOCAL_PATH:= $(call my-dir)  
include $(CLEAR_VARS)  
LOCAL_SRC_FILES:= \version.c \fixed.c \bit.c \timer.c \stream.c \frame.c  \synth.c \decoder.c \layer12.c \layer3.c \huffman.c \FileSystem.c \NativeMP3Decoder.cLOCAL_ARM_MODE := arm
LOCAL_LDLIBS += -L$(SYSROOT)/usr/lib -llog
LOCAL_MODULE:= libmad  
LOCAL_SHARED_LIBRARIES := libutils
LOCAL_C_INCLUDES := \$(LOCAL_PATH)/android  
LOCAL_CFLAGS := -DHAVE_CONFIG_H -DFPM_ARM -ffast-math -O3 
include $(BUILD_SHARED_LIBRARY)   
#endif  

2.2、C代码编写API

需要注意一点的是，得到音频的Samplerate（采样率）要先进行一次readSamples操作才能发采样率读出。

2.2.1 头文件 NativeMP3Decoder.h

/* DO NOT EDIT THIS FILE - it is machine generated */
#include <jni.h>
/* Header for class com_mediatek_factorymode_NativeMP3Decoder */#ifndef _Included_com_mediatek_factorymode_NativeMP3Decoder
#define _Included_com_mediatek_factorymode_NativeMP3Decoder
#ifdef __cplusplus
extern "C" {
#endif
/** Class:     com_mediatek_factorymode_NativeMP3Decoder* Method:    initAudioPlayer* Signature: (Ljava/lang/String;I)I*/
JNIEXPORT jint JNICALL Java_com_mediatek_factorymode_NativeMP3Decoder_initAudioPlayer(JNIEnv *, jobject, jstring, jint);/** Class:     com_mediatek_factorymode_NativeMP3Decoder* Method:    getAudioBuf* Signature: ([SI)I*/
JNIEXPORT jint JNICALL Java_com_mediatek_factorymode_NativeMP3Decoder_getAudioBuf(JNIEnv *, jobject, jshortArray, jint);/** Class:     com_mediatek_factorymode_NativeMP3Decoder* Method:    closeAduioFile* Signature: ()V*/
JNIEXPORT void JNICALL Java_com_mediatek_factorymode_NativeMP3Decoder_closeAduioFile(JNIEnv *, jobject);/** Class:     com_mediatek_factorymode_NativeMP3Decoder* Method:    getAudioSamplerate* Signature: ()I*/
JNIEXPORT jint JNICALL Java_com_mediatek_factorymode_NativeMP3Decoder_getAudioSamplerate(JNIEnv *, jobject);#ifdef __cplusplus
}
#endif
#endif

实现代码比较多 这里就不贴了 可以查看项目源码。

2.3、文件读写操作

2.3.1 头文件 FileSystem.h

####

#include <stdlib.h>typedef    signed long            T_S32;      /* signed 32 bit integer */#define T_pFILE         T_S32
#define     T_hFILE             T_S32#define _CREATE 0//"wb+"//0
#define _RDONLY 1//"rb" // 1
#define _WRONLY 2//"wb"// 2
#define _RDWR   3//"rb+"// 3#define _FMODE_READ     _RDONLY
#define _FMODE_WRITE    _WRONLY
#define _FMODE_CREATE   _CREATE
#define _FMODE_OVERLAY   _RDWR#define _FSEEK_CURRENT  1
#define _FSEEK_END      2
#define _FSEEK_SET      0
#define _FOPEN_FAIL     -1#define SEEK_CURRENT  1
#define SEEK_CUR  1
#define SEEK_END      2
#define SEEK_SET      0#define FS_SEEK_SET   0

2.3.2 实现 FileSystem.c

#include <unistd.h>
#include <sys/stat.h>
#include <sys/time.h>
#include <sys/types.h>
#include <stdlib.h>
#include <fcntl.h>
#include"FileSystem.h"int file_open(const char *filename, int flags)
{
int access;T_pFILE fd = 0;if (flags ==  _CREATE) {access = O_CREAT | O_TRUNC | O_RDWR;} else if (flags == _WRONLY) {access = O_CREAT | O_TRUNC | O_WRONLY;} else if (flags == _RDONLY){access = O_RDONLY;} else if (flags == _RDWR){access = O_RDWR;} else{return -1;}#ifdef O_BINARYaccess |= O_BINARY;
#endiffd = open(filename, access, 0666);if (fd == -1)return -1;return fd;
}
int file_read(T_pFILE fd, unsigned char *buf, int size)
{return read(fd, buf, size);
}
int file_write(T_pFILE fd, unsigned char *buf, int size)
{return write(fd, buf, size);
}int64_t file_seek(T_pFILE fd, int64_t pos, int whence)
{if (whence == 0x10000) {struct stat st;int ret = fstat(fd, &st);return ret < 0 ? -1 : st.st_size;}return lseek(fd, pos, whence);
}
int file_close(T_pFILE fd)
{return close(fd);
}

2.4、通过ndk-build 编译即可。关于jni调用这里不做介绍。

3、Android AudioTrack简介

      在Android中播放声音可以用MediaPlayer和AudioTrack两种方案的，但是两种方案是有很大区别的，MediaPlayer可以播放多种格式的声音文件，例如MP3，AAC，WAV，OGG，MIDI等。而AudioTrack只能播放PCM数据流。事实上，两种本质上是没啥区别的，MediaPlayer在播放音频时，在framework层还是会创建AudioTrack，把解码后的PCM数流传递给AudioTrack，最后由AudioFlinger进行混音，传递音频给硬件播放出来。利用AudioTrack播放只是跳过Mediaplayer的解码部分而已。Mediaplayer的解码核心部分是基于OpenCORE 来实现的，支持通用的音视频和图像格式，codec使用的是OpenMAX接口来进行扩展。因此使用audiotrack播放mp3文件的话，要自己加入一个音频解码器，如libmad。否则只能播放PCM数据，如大多数WAV格式的音频文件。

4、简单Demo程序

下面提供一个Audiotrack播放mp3的demo，mp3没有经过加密的，解码部分是由libmad完成。（若是要播放加密音频文件，可以操作的libmad解码文件流即可。）

4.1 NativeMP3Decoder.java

package com.mediatek.factorymode;public class NativeMP3Decoder {static {  System.loadLibrary("mad");  }  public native int initAudioPlayer(String file,int StartAddr);  public native int getAudioBuf(short[] audioBuffer, int numSamples);  public native void closeAduioFile();  public native int getAudioSamplerate();  }

4.2 TrumpetActivity.java

public class TrumpetActivity extends Activity implements OnClickListener {private String TAG = "TrumpetActivity";private String assertFolderName = "Android";private String assertMusicName = "rich.mp3";@SuppressLint("SdCardPath")private String targetFolderPath = Environment.getExternalStorageDirectory().getPath() + File.separator + assertFolderName;private String decodeFilePath = Environment.getExternalStorageDirectory().getPath()+ File.separator+ assertFolderName+ File.separator+ assertMusicName;private Thread mThread;private boolean mThreadFlag;private short[] audioBuffer;private AudioTrack mAudioTrack;private int samplerate;private int mAudioMinBufSize;private int initRet;private NativeMP3Decoder mMP3Decoder;private Button PlayMusic, PauseMusic;private Button leftSoundChannel, lightSoundChannel;@Overridepublic void onCreate(Bundle savedInstanceState) {super.onCreate(savedInstanceState);setContentView(R.layout.trumpet_layout);FileHelper.copyFolderFromAssets(TrumpetActivity.this, assertFolderName,targetFolderPath);PlayMusic = (Button) findViewById(R.id.playMusic);PauseMusic = (Button) findViewById(R.id.PauseMusic);leftSoundChannel = (Button) findViewById(R.id.leftsoundchannel);lightSoundChannel = (Button) findViewById(R.id.rightsoundchannel);PlayMusic.setOnClickListener(this);PauseMusic.setOnClickListener(this);leftSoundChannel.setOnClickListener(this);lightSoundChannel.setOnClickListener(this);mMP3Decoder = new NativeMP3Decoder();initRet = mMP3Decoder.initAudioPlayer(decodeFilePath, 0);if (initRet == -1) {Log.i(TAG, "Couldn't open file '" + decodeFilePath + "'");} else {Log.i(TAG, "Couldn't open file else'" + decodeFilePath + "'");mThreadFlag = true;initAudioPlayer();audioBuffer = new short[1024 * 20];mThread = new Thread(new Runnable() {@Overridepublic void run() {while (mThreadFlag) {if (mAudioTrack != null) {if (mAudioTrack.getPlayState() != AudioTrack.PLAYSTATE_PAUSED) {if (mAudioTrack != null) {if (mAudioTrack.getPlayState() != AudioTrack.PLAYSTATE_STOPPED) {mMP3Decoder.getAudioBuf(audioBuffer,mAudioMinBufSize);mAudioTrack.write(audioBuffer, 0,mAudioMinBufSize);}}} else {try {Thread.sleep(1000);} catch (InterruptedException e) {e.printStackTrace();}}}}}});mThread.start();try {Thread.sleep(50);} catch (InterruptedException e) {e.printStackTrace();}mAudioTrack.play();}}public void setChannel(boolean left, boolean right) {if (null != mAudioTrack) {mAudioTrack.setStereoVolume(left ? 1 : 0, right ? 1 : 0);mAudioTrack.play();}}@SuppressWarnings("deprecation")private void initAudioPlayer() {samplerate = mMP3Decoder.getAudioSamplerate();System.out.println("samplerate = " + samplerate);Log.d("bfp", "samplerate:" + samplerate);samplerate = samplerate / 2;mAudioMinBufSize = AudioTrack.getMinBufferSize(samplerate,AudioFormat.CHANNEL_CONFIGURATION_STEREO,AudioFormat.ENCODING_PCM_16BIT);mAudioTrack = new AudioTrack(AudioManager.STREAM_MUSIC,samplerate, AudioFormat.CHANNEL_CONFIGURATION_STEREO,AudioFormat.ENCODING_PCM_16BIT, mAudioMinBufSize,AudioTrack.MODE_STREAM);}@Overrideprotected void onDestroy() {mThreadFlag = false;try {Thread.sleep(200);} catch (InterruptedException e) {e.printStackTrace();}if (mAudioTrack != null) {if (mAudioTrack.getPlayState() == AudioTrack.PLAYSTATE_PLAYING) {mAudioTrack.stop();mAudioTrack.release();}}mAudioTrack = null;audioBuffer = null;mMP3Decoder.closeAduioFile();super.onDestroy();}@Overridepublic void onClick(View v) {switch (v.getId()) {case R.id.playMusic:if (initRet == -1) {Toast.makeText(getApplicationContext(),"Couldn't open file '" + decodeFilePath + "'",Toast.LENGTH_SHORT).show();} else {if (mAudioTrack.getPlayState() == AudioTrack.PLAYSTATE_STOPPED) {mAudioTrack.play();} else if (mAudioTrack.getPlayState() == AudioTrack.PLAYSTATE_PAUSED) {mAudioTrack.play();} else {Toast.makeText(getApplicationContext(),"Already in play", Toast.LENGTH_SHORT).show();}}break;case R.id.PauseMusic:if (initRet == -1) {Log.i("conowen", "Couldn't open file '" + decodeFilePath + "'");Toast.makeText(getApplicationContext(),"Couldn't open file '" + decodeFilePath + "'",Toast.LENGTH_SHORT).show();} else {if (mAudioTrack.getPlayState() == AudioTrack.PLAYSTATE_PLAYING) {mAudioTrack.pause();} else {Toast.makeText(getApplicationContext(), "Already stop",Toast.LENGTH_SHORT).show();}}break;case R.id.leftsoundchannel:setChannel(true, false);break;case R.id.rightsoundchannel:setChannel(false, true);break;default:break;}}
}

4.3 main.xml

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"android:layout_width="fill_parent"android:layout_height="fill_parent"android:orientation="vertical" ><LinearLayout
        android:layout_width="match_parent"android:layout_height="wrap_content" ><Button
            android:id="@+id/playMusic"android:layout_width="0dp"android:layout_height="wrap_content"android:layout_weight="1"android:text="@string/PlayMusic" /><Button
            android:id="@+id/PauseMusic"android:layout_width="0dp"android:layout_height="wrap_content"android:layout_weight="1"android:text="@string/PauseMusic" /></LinearLayout><LinearLayout
        android:layout_width="match_parent"android:layout_height="wrap_content" android:orientation="horizontal"><Button
            android:id="@+id/rightsoundchannel"android:layout_width="0dp"android:layout_height="wrap_content"android:layout_weight="1"android:text="@string/lightSoundChannel" /><Button
        android:id="@+id/leftsoundchannel"android:layout_width="0dp"android:layout_height="wrap_content"android:layout_weight="1"android:text="@string/leftSoundChannel" /></LinearLayout></LinearLayout>

源码链接地址：http://download.csdn.net/detail/feipeng_/9724644