Android通过lame进行音频格式转换

之前项目中遇到过手机录音问题，Android端录制的格式为amr，但是却没法在iOS端播放，于是找到了通过lame库进行格式转换的方法，我们通过AudioRecord录制出原生的pcm格式音频，然后通过lame库进行格式转换，这里需要用到NDK，所以我们需要配置NDK开发环境。主要就是以下，具体不细说了。

接下来在原来的项目中build.gradle中添加如下代码

android {    externalNativeBuild {        cmake {            path "src/main/cpp/CMakeLists.txt"        }    }}

然后在main文件夹下创建cpp/CMakeLists.txt文件

# Sets the minimum version of CMake required to build the native library.# 这里的3.4.1是我这里的cmake版本  这个可以直接创建一个ndk项目进行查看cmake_minimum_required(VERSION 3.4.1)add_library( # Sets the name of the library.        lame        # Sets the library as a shared library.        SHARED        # Provides a relative path to your source file(s).        lame.cpp        )find_library( # Sets the name of the path variable.        log-lib        # Specifies the name of the NDK library that        # you want CMake to locate.        log)target_link_libraries( # Specifies the target library.        lame        # Links the target library to the log library        # included in the NDK.        ${log-lib})

然后在cpp文件夹下创建一个lame.cpp ，添加如下代码测试

#include <jni.h>#include <string>extern "C"JNIEXPORT jstring JNICALLJava_com_we_convertdemo_Convert_stringFromJNI(JNIEnv *env, jobject) {    std::string hello = "Hello from C++";    return env->NewStringUTF(hello.c_str());}

接着在我们的Java项目中创建一个Convert类添加如下代码:

public class Convert {    static {        System.loadLibrary("lame");    }    public native String stringFromJNI();}

然后在activity当中调用测试一下通过即可。
lame包下载地址https://sourceforge.net/projects/lame/files/lame/
接下来解压该文件后找到libmp3lame文件夹，将该文件夹下面的.c和.h文件复制到项目的cpp文件夹下（该文件夹里面的文件夹不用管），接着把include目录下的lame.h文件也放到cpp目录当中。然后把所有的.c文件添加到CMakeList.txt文件的add_library中。同时对当中的部分文件做修改
1.在 fft.c 文件中删除 #include “vector/lame_intrin.h”
2.在set_get.h 文件中修改 #include 为 #include“lame.h”
3.在 util.h 文件中将”extern ieee754_float32_t fast_log2(ieee754_float32_t x);”修改为 “extern float fast_log2(float x);”
修改完上诉文件后，我们编译一下会报错，接下来需要添加如下配置

cmake {                    cFlags "-DSTDC_HEADERS"          cppFlags "-fexceptions", "-frtti"     }

接下来我们在Convert类中添加如下三个方法：

 public native int init(String pcmPath,int audioChannels,int bitRate,int sampleRate,String mp3Path);    public native void encode();    public native void destroy();

然后实现创建两个编码类Mp3Encoder.h和Mp3Encoder.cpp
代码如下

#include <cstdio>#include "lame.h"class Mp3Encoder {private:    FILE* pcmFile;    FILE* mp3File;    lame_t lameClient;public:    Mp3Encoder();    ~Mp3Encoder();    int Init(const char* pcmFilePath,int channels,int bitRate,int sampleRate,const char* mp3FilePath);    void Encode();    void Destory();};---------------------------------------------------------------------------------------#include "Mp3Encoder.h"Mp3Encoder::Mp3Encoder() {}Mp3Encoder::~Mp3Encoder() {}int Mp3Encoder::Init(const char *pcmFilePath, int channels, int bitRate, int sampleRate,                     const char *mp3FilePath) {    int ret  = -1;    pcmFile = fopen(pcmFilePath,"rb");    if (pcmFile){        mp3File = fopen(mp3FilePath,"wb");        if (mp3File){            lameClient = lame_init();            lame_set_in_samplerate(lameClient,sampleRate);            lame_set_out_samplerate(lameClient,sampleRate);            lame_set_num_channels(lameClient,channels);            lame_set_brate(lameClient,bitRate/1000);            lame_init_params(lameClient);            ret = 0;        }    }    return ret;}void Mp3Encoder::Encode() {    int buffersize = 1024*256;    short* buffer = new short[buffersize/2];    short* leftBuffer = new short[buffersize/4];    short* rightBuffer = new short[buffersize/4];    unsigned char* mp3_buffer = new unsigned char[buffersize];    size_t readBufferSize = 0;    while ((readBufferSize=fread(buffer,2,buffersize/2,pcmFile))>0){        for (int i = 0; i < readBufferSize; ++i) {            if (i%2==0){                leftBuffer[i/2] = buffer[i];            } else{                rightBuffer[i/2] = buffer[i];            }        }        size_t  wroteSize = lame_encode_buffer(lameClient,leftBuffer,rightBuffer                ,readBufferSize/2,mp3_buffer,buffersize);        fwrite(mp3_buffer,1,wroteSize,mp3File);    }    delete[] buffer;    delete[] leftBuffer;    delete[] rightBuffer;    delete[] mp3_buffer;}void Mp3Encoder::Destory() {    if(pcmFile){        fclose(pcmFile);    }    if (mp3File){        fclose(mp3File);        lame_close(lameClient);    }}

然后把我们把所有的.c文件和.cpp文件添加到CMakeLists.txt的add_library中

add_library( # Sets the name of the library.        lame        # Sets the library as a shared library.        SHARED        # Provides a relative path to your source file(s).        lame.cpp        Mp3Encoder.cpp        bitstream.c        encoder.c        fft.c        gain_analysis.c        id3tag.c        lame.c        mpglib_interface.c        newmdct.c        presets.c        psymodel.c        quantize.c        quantize_pvt.c        reservoir.c        set_get.c        tables.c        takehiro.c        util.c        vbrquantize.c        VbrTag.c        version.c        )

具体代码已经上传到github上可以下载查看
https://github.com/honghengqiang/ConvertDemo.git

参考文档：
https://developer.android.com/studio/projects/gradle-external-native-builds
https://developer.android.com/ndk/guides/cpp-support
以及《音视频开发进阶指南》

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