Android OpenSL ES 开发:Android OpenSL 介绍和开发流程说明

一、Android OpenSL ES 介绍

OpenSL ES (Open Sound Library for Embedded Systems)是无授权费、跨平台、针对嵌入式系统精心优化的硬件音频加速API。它为嵌入式移动多媒体设备上的本地应用程序开发者提供标准化, 高性能,低响应时间的音频功能实现方法,并实现软/硬件音频性能的直接跨平台部署,降低执行难度,促进高级音频市场的发展。简单来说OpenSL ES是一个嵌入式跨平台免费的音频处理库。

 

Android的OpenSL ES库是在NDK的platforms文件夹对应android平台先相应cpu类型里面,如:

Android OpenSL ES 开发:Android OpenSL 介绍和开发流程说明

二、Android OpenSL ES 开发流程

OpenSL ES 的开发流程主要有如下6个步骤:

1、 创建接口对象

2、设置混音器

3、创建播放器(录音器)

4、设置缓冲队列和回调函数

5、设置播放状态

6、启动回调函数

其中4和6是播放PCM等数据格式的音频是需要用到的。

2.1 OpenSL ES 开发最重要的接口类 SLObjectItf

 通过SLObjectItf接口类我们可以创建所需要的各种类型的类接口,比如:

  • 创建引擎接口对象:SLObjectItf engineObject
  • 创建混音器接口对象:SLObjectItf outputMixObject
  • 创建播放器接口对象:SLObjectItf playerObject

以上等等都是通过SLObjectItf来创建的。

2.2 使用SLObjectItf来创建具体的接口对象实例

OpenSL ES中也有具体的接口类,比如(引擎:SLEngineItf,播放器:SLPlayItf,声音控制器:SLVolumeItf等等)。

2.3 创建引擎并实现

OpenSL ES中开始的第一步都是声明SLObjectItf接口类型的引擎接口对象engineObject,然后用方法slCreateEngine创建一个引擎接口对象;创建好引擎接口对象后,需要用SLObjectItf的Realize方法来实现engineObject;最后用SLObjectItf的GetInterface方法来初始化SLEngnineItf对象实例。如:

SLObjectItf engineObject = NULL;//用SLObjectItf声明引擎接口对象
SLEngineItf engineEngine = NULL;//声明具体的引擎对象实例
 
void createEngine()
{
    SLresult result;//返回结果
    result = slCreateEngine(&engineObject, 0, NULL, 0, NULL, NULL);//第一步创建引擎
    result = (*engineObject)->Realize(engineObject, SL_BOOLEAN_FALSE);//实现(Realize)engineObject接口对象
    result = (*engineObject)->GetInterface(engineObject, SL_IID_ENGINE, &engineEngine);//通过engineObject的GetInterface方法初始化engineEngine
}

2.4 利用引擎对象创建其他接口对象

其他接口对象(SLObjectItf outputMixObject,SLObjectItf playerObject)等都是用引擎接口对象创建的(具体的接口对象需要的参数这里就说了,可参照ndk例子里面的),如:

//混音器
SLObjectItf outputMixObject = NULL;//用SLObjectItf创建混音器接口对象
SLEnvironmentalReverbItf outputMixEnvironmentalReverb = NULL;////创建具体的混音器对象实例
 
result = (*engineEngine)->CreateOutputMix(engineEngine, &outputMixObject, 1, mids, mreq);//利用引擎接口对象创建混音器接口对象
result = (*outputMixObject)->Realize(outputMixObject, SL_BOOLEAN_FALSE);//实现(Realize)混音器接口对象
result = (*outputMixObject)->GetInterface(outputMixObject, SL_IID_ENVIRONMENTALREVERB, &outputMixEnvironmentalReverb);//利用混音器接口对象初始化具体混音器实例
 
//播放器
SLObjectItf playerObject = NULL;//用SLObjectItf创建播放器接口对象
SLPlayItf playerPlay = NULL;//创建具体的播放器对象实例
 
result = (*engineEngine)->CreateAudioPlayer(engineEngine, &playerObject, &audioSrc, &audioSnk, 3, ids, req);//利用引擎接口对象创建播放器接口对象
result = (*playerObject)->Realize(playerObject, SL_BOOLEAN_FALSE);//实现(Realize)播放器接口对象
result = (*playerObject)->GetInterface(playerObject, SL_IID_PLAY, &playerPlay);//初始化具体的播放器对象实例

最后就是使用创建好的具体对象实例来实现具体的功能。

三、OpenSL ES 使用示例

首先导入OpenSL ES和其他必须的库:

-lOpenSLES -landroid 

3.1 播放assets文件

创建引擎——>创建混音器——>创建播放器——>设置播放状态

JNIEXPORT void JNICALL
Java_com_renhui_openslaudio_MainActivity_playAudioByOpenSL_1assets(JNIEnv *env, jobject instance, jobject assetManager, jstring filename) {
 
    release();
    const char *utf8 = (*env)->GetStringUTFChars(env, filename, NULL);
 
    // use asset manager to open asset by filename
    AAssetManager* mgr = AAssetManager_fromJava(env, assetManager);
    AAsset* asset = AAssetManager_open(mgr, utf8, AASSET_MODE_UNKNOWN);
    (*env)->ReleaseStringUTFChars(env, filename, utf8);
 
    // open asset as file descriptor
    off_t start, length;
    int fd = AAsset_openFileDescriptor(asset, &start, &length);
    AAsset_close(asset);
 
    SLresult result;
 
 
    //第一步,创建引擎
    createEngine();
 
    //第二步,创建混音器
    const SLInterfaceID mids[1] = {SL_IID_ENVIRONMENTALREVERB};
    const SLboolean mreq[1] = {SL_BOOLEAN_FALSE};
    result = (*engineEngine)->CreateOutputMix(engineEngine, &outputMixObject, 1, mids, mreq);
    (void)result;
    result = (*outputMixObject)->Realize(outputMixObject, SL_BOOLEAN_FALSE);
    (void)result;
    result = (*outputMixObject)->GetInterface(outputMixObject, SL_IID_ENVIRONMENTALREVERB, &outputMixEnvironmentalReverb);
    if (SL_RESULT_SUCCESS == result) {
        result = (*outputMixEnvironmentalReverb)->SetEnvironmentalReverbProperties(outputMixEnvironmentalReverb, &reverbSettings);
        (void)result;
    }
    //第三步,设置播放器参数和创建播放器
    // 1、 配置 audio source
    SLDataLocator_AndroidFD loc_fd = {SL_DATALOCATOR_ANDROIDFD, fd, start, length};
    SLDataFormat_MIME format_mime = {SL_DATAFORMAT_MIME, NULL, SL_CONTAINERTYPE_UNSPECIFIED};
    SLDataSource audioSrc = {&loc_fd, &format_mime};
 
    // 2、 配置 audio sink
    SLDataLocator_OutputMix loc_outmix = {SL_DATALOCATOR_OUTPUTMIX, outputMixObject};
    SLDataSink audioSnk = {&loc_outmix, NULL};
 
    // 创建播放器
    const SLInterfaceID ids[3] = {SL_IID_SEEK, SL_IID_MUTESOLO, SL_IID_VOLUME};
    const SLboolean req[3] = {SL_BOOLEAN_TRUE, SL_BOOLEAN_TRUE, SL_BOOLEAN_TRUE};
    result = (*engineEngine)->CreateAudioPlayer(engineEngine, &fdPlayerObject, &audioSrc, &audioSnk, 3, ids, req);
    (void)result;
 
    // 实现播放器
    result = (*fdPlayerObject)->Realize(fdPlayerObject, SL_BOOLEAN_FALSE);
    (void)result;
 
    // 得到播放器接口
    result = (*fdPlayerObject)->GetInterface(fdPlayerObject, SL_IID_PLAY, &fdPlayerPlay);
    (void)result;
 
    // 得到声音控制接口
    result = (*fdPlayerObject)->GetInterface(fdPlayerObject, SL_IID_VOLUME, &fdPlayerVolume);
    (void)result;
 
    //第四步,设置播放状态
    if (NULL != fdPlayerPlay) {
 
        result = (*fdPlayerPlay)->SetPlayState(fdPlayerPlay, SL_PLAYSTATE_PLAYING);
        (void)result;
    }
 
    //设置播放音量 (100 * -50:静音 )
    (*fdPlayerVolume)->SetVolumeLevel(fdPlayerVolume, 20 * -50);
 
}

3.2 播放pcm文件

(集成到ffmpeg时,也是播放ffmpeg转换成的pcm格式的数据),这里为了模拟是直接读取的pcm格式的音频文件。

1. 创建播放器和混音器

//第一步,创建引擎
    createEngine();
 
    //第二步,创建混音器
    const SLInterfaceID mids[1] = {SL_IID_ENVIRONMENTALREVERB};
    const SLboolean mreq[1] = {SL_BOOLEAN_FALSE};
    result = (*engineEngine)->CreateOutputMix(engineEngine, &outputMixObject, 1, mids, mreq);
    (void)result;
    result = (*outputMixObject)->Realize(outputMixObject, SL_BOOLEAN_FALSE);
    (void)result;
    result = (*outputMixObject)->GetInterface(outputMixObject, SL_IID_ENVIRONMENTALREVERB, &outputMixEnvironmentalReverb);
    if (SL_RESULT_SUCCESS == result) {
        result = (*outputMixEnvironmentalReverb)->SetEnvironmentalReverbProperties(
                outputMixEnvironmentalReverb, &reverbSettings);
        (void)result;
    }
    SLDataLocator_OutputMix outputMix = {SL_DATALOCATOR_OUTPUTMIX, outputMixObject};
    SLDataSink audioSnk = {&outputMix, NULL};

2. 设置pcm格式的频率位数等信息并创建播放器

// 第三步,配置PCM格式信息
    SLDataLocator_AndroidSimpleBufferQueue android_queue={SL_DATALOCATOR_ANDROIDSIMPLEBUFFERQUEUE,2};
    SLDataFormat_PCM pcm={
            SL_DATAFORMAT_PCM,//播放pcm格式的数据
            2,//2个声道(立体声)
            SL_SAMPLINGRATE_44_1,//44100hz的频率
            SL_PCMSAMPLEFORMAT_FIXED_16,//位数 16位
            SL_PCMSAMPLEFORMAT_FIXED_16,//和位数一致就行
            SL_SPEAKER_FRONT_LEFT | SL_SPEAKER_FRONT_RIGHT,//立体声(前左前右)
            SL_BYTEORDER_LITTLEENDIAN//结束标志
    };
    SLDataSource slDataSource = {&android_queue, &pcm};
 
 
    const SLInterfaceID ids[3] = {SL_IID_BUFFERQUEUE, SL_IID_EFFECTSEND, SL_IID_VOLUME};
    const SLboolean req[3] = {SL_BOOLEAN_TRUE, SL_BOOLEAN_TRUE, SL_BOOLEAN_TRUE};
    
    result = (*engineEngine)->CreateAudioPlayer(engineEngine, &pcmPlayerObject, &slDataSource, &audioSnk, 3, ids, req);
    //初始化播放器
    (*pcmPlayerObject)->Realize(pcmPlayerObject, SL_BOOLEAN_FALSE);
 
//    得到接口后调用  获取Player接口
    (*pcmPlayerObject)->GetInterface(pcmPlayerObject, SL_IID_PLAY, &pcmPlayerPlay);

3. 设置缓冲队列和回调函数

//    注册回调缓冲区 获取缓冲队列接口
    (*pcmPlayerObject)->GetInterface(pcmPlayerObject, SL_IID_BUFFERQUEUE, &pcmBufferQueue);
    //缓冲接口回调
    (*pcmBufferQueue)->RegisterCallback(pcmBufferQueue, pcmBufferCallBack, NULL);

回调函数:

void * pcmBufferCallBack(SLAndroidBufferQueueItf bf, void * context)
{
    //assert(NULL == context);
    getPcmData(&buffer);
    // for streaming playback, replace this test by logic to find and fill the next buffer
    if (NULL != buffer) {
        SLresult result;
        // enqueue another buffer
        result = (*pcmBufferQueue)->Enqueue(pcmBufferQueue, buffer, 44100 * 2 * 2);
        // the most likely other result is SL_RESULT_BUFFER_INSUFFICIENT,
        // which for this code example would indicate a programming error
    }
}

读取pcm格式的文件:

void getPcmData(void **pcm)
{
    while(!feof(pcmFile))
    {
        fread(out_buffer, 44100 * 2 * 2, 1, pcmFile);
        if(out_buffer == NULL)
        {
            LOGI("%s", "read end");
            break;
        } else{
            LOGI("%s", "reading");
        }
        *pcm = out_buffer;
        break;
    }
}

4. 设置播放状态并手动开始调用回调函数

//    获取播放状态接口
    (*pcmPlayerPlay)->SetPlayState(pcmPlayerPlay, SL_PLAYSTATE_PLAYING);
 
//    主动调用回调函数开始工作
    pcmBufferCallBack(pcmBufferQueue, NULL);

注意:在回调函数中result = (*pcmBufferQueue)->Enqueue(pcmBufferQueue, buffer, 44100 * 2 * 2);,最后的“44100*2*2”是buffer的大小,因为我这里是指定了没读取一次就从pcm文件中读取了“44100*2*2”个字节,所以可以正常播放,如果是利用ffmpeg来获取pcm数据源,那么实际大小要根据每个AVframe的具体大小来定,这样才能正常播放出声音!!!(44100 * 2 * 2 表示:44100是频率HZ,2是立体声双通道,2是采用的16位采样即2个字节,所以总的字节数就是:44100 * 2 * 2)

Android OpenSL ES 开发:Android OpenSL 介绍和开发流程说明

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