OpenSL ES 是 Khronos Group 为嵌入式系统开发的调优的免版权费、跨平台、硬件加速的音频 API 规范。Android 提供了这一 API 规范的 Android 专有实现，并允许应用程序通过 NDK 调用这些 API。Android 应用可以借助这一 API，使用 C 或 C++ 实现高性能、低延迟的音频。OpenSL ES™ 标准与 Android Java 框架中的 MediaPlayer 和 MediaRecorder API 提供类似的音频功能。Android OpenSL ES 提供面向对象的 C 语言接口和 C++ 绑定，使我们可以从使用任意一种语言编写的代码中调用 API。
OpenSL ES 的使用
与在 Android 应用中使用其它的第三方 C/C++ 库类似，我们需要把 OpenSL ES 添加进我们的应用中，并编写调用 OpenSL ES API 的代码。把 OpenSL ES 添加进我们的应用，具体而言指添加相关的头文件，并配置我们的动态链接库的编译选项以正确的链接 OpenSL ES 库。
标准的核心 OpenSL ES 头文件有 <SLES/OpenSLES.h> 和 <SLES/OpenSLES_Platform.h>，而 <SLES/OpenSLES_Android.h> 和 <SLES/OpenSLES_AndroidConfiguration.h> 中则提供了其他 Android 专用功能。
添加核心 OpenSL ES 功能集的话，包含 <SLES/OpenSLES.h> 头文件即可，这个头文件包含了 <SLES/OpenSLES_Platform.h>。添加 OpenSL ES Android 扩展，则可以添加 <SLES/OpenSLES_Android.h> 头文件，这个头文件包含如下两个头文件：
#include <SLES/OpenSLES_AndroidConfiguration.h>
#include <SLES/OpenSLES_AndroidMetadata.h>
配置编译选项引入 OpenSL ES 库使得链接器能够正确链接这个库的方法，因我们使用的编译配置系统的不同而不同。如果我们使用 Android.mk 配置编译，需要为 Android.mk
文件添加如下行：
LOCAL_LDLIBS += -lOpenSLES
如果使用 CMake 的话，target_link_libraries 需要加上 OpenSLES，如：
target_link_libraries(echo
PRIVATE
OpenSLES )
Google 的 android ndk-samples 中有许多 NDK 相关的示例代码，其中包含有 OpenSL ES 相关的示例代码 audio-echo 和 native-audio。
这里通过一段基于 audio-echo 的实现了 Android 平台的耳返功能的代码看一下 OpenSL ES API 的基本使用方法。在这段示例代码中，启动声音录制设备录制声音，录得的声音数据被放进一个中间缓冲区，中间缓冲区中的数据随后又被送进播放设备播放出来。
OpenSL ES 是一套 C 的面向对象 API，它的基本使用方法如下：
- 创建 Engine 对象
result = slCreateEngine(&engine.slEngineObj_, 0, NULL, 0, NULL, NULL);
SLASSERT(result);
result =
(*engine.slEngineObj_)->Realize(engine.slEngineObj_, SL_BOOLEAN_FALSE);
SLASSERT(result);
result = (*engine.slEngineObj_)
->GetInterface(engine.slEngineObj_, SL_IID_ENGINE,
&engine.slEngineItf_);
SLASSERT(result);
先由 OpenSL ES API 的入口函数 slCreateEngine() 创建类型为 SLObjectItf 的 slEngineObj_，然后借助于 slEngineObj_ 获得类型为 SLEngineItf 的 Engine 对象 slEngineItf_。
- 创建 Recorder
AudioRecorder::AudioRecorder(SampleFormat *sampleFormat, SLEngineItf slEngine)
: freeQueue_(nullptr),
recQueue_(nullptr),
devShadowQueue_(nullptr),
callback_(nullptr) {
SLresult result;
sampleInfo_ = *sampleFormat;
SLAndroidDataFormat_PCM_EX format_pcm;
ConvertToSLSampleFormat(&format_pcm, &sampleInfo_);
// configure src.audio source
SLDataLocator_IODevice loc_dev = {SL_DATALOCATOR_IODEVICE,
SL_IODEVICE_AUDIOINPUT,
SL_DEFAULTDEVICEID_AUDIOINPUT, NULL};
SLDataSource audioSrc = {&loc_dev, NULL};
// configure src.audio sink
SLDataLocator_AndroidSimpleBufferQueue loc_bq = {
SL_DATALOCATOR_ANDROIDSIMPLEBUFFERQUEUE, DEVICE_SHADOW_BUFFER_QUEUE_LEN};
SLDataSink audioSnk = {&loc_bq, &format_pcm};
// create src.audio recorder
// (requires the RECORD_AUDIO permission)
const SLInterfaceID id[2] = {SL_IID_ANDROIDSIMPLEBUFFERQUEUE,
SL_IID_ANDROIDCONFIGURATION};
const SLboolean req[2] = {SL_BOOLEAN_TRUE, SL_BOOLEAN_TRUE};
result = (*slEngine)->CreateAudioRecorder(
slEngine, &recObjectItf_, &audioSrc, &audioSnk,
sizeof(id) / sizeof(id[0]), id, req);
SLASSERT(result);
// Configure the voice recognition preset which has no
// signal processing for lower latency.
SLAndroidConfigurationItf inputConfig;
result = (*recObjectItf_)
->GetInterface(recObjectItf_, SL_IID_ANDROIDCONFIGURATION,
&inputConfig);
if (SL_RESULT_SUCCESS == result) {
SLuint32 presetValue = SL_ANDROID_RECORDING_PRESET_VOICE_RECOGNITION;
(*inputConfig)
->SetConfiguration(inputConfig, SL_ANDROID_KEY_RECORDING_PRESET,
&presetValue, sizeof(SLuint32));
}
result = (*recObjectItf_)->Realize(recObjectItf_, SL_BOOLEAN_FALSE);
SLASSERT(result);
result =
(*recObjectItf_)->GetInterface(recObjectItf_, SL_IID_RECORD, &recItf_);
SLASSERT(result);
result = (*recObjectItf_)
->GetInterface(recObjectItf_, SL_IID_ANDROIDSIMPLEBUFFERQUEUE,
&recBufQueueItf_);
SLASSERT(result);
result = (*recBufQueueItf_)
->RegisterCallback(recBufQueueItf_, bqRecorderCallback, this);
SLASSERT(result);
devShadowQueue_ = new AudioQueue(DEVICE_SHADOW_BUFFER_QUEUE_LEN);
assert(devShadowQueue_);
#ifdef ENABLE_LOG
std::string name = "rec";
recLog_ = new AndroidLog(name);
#endif
}
创建 Recorder 的过程如下：
- 为 Recorder 定义流水线，即音频数据流动的方式 Audio Sink 和 Audio Source。
- 通过
SLEngineItf的CreateAudioRecorder()接口创建类型为SLObjectItf的 Recorder 对象。
- 通过
- 配置 Recorder。
- 获得类型为
SLRecordItf的recItf_。
- 获得类型为
上面这段代码创建的 recording pipeline 大体如下：
Recording Pipeline
配置 OpenSL ES recorder 从录音设备拿到音频数据，放入缓冲区，这些数据再由缓冲区对象通过回调传给外界。Android OpenSL ES 中用 SLAndroidSimpleBufferQueueItf 对象描述缓冲区，这个缓冲区本身不包含用于存放音频数据的内存，而是需要外部分配内存，并将分配好的内存给到缓冲区对象。
- 启动和停止 Recorder
上个步骤创建的类型为 SLRecordItf 的 recItf_ 对象用于控制 recorder 的状态，包括 recorder 的启动和停止：
SLboolean AudioRecorder::Start(void) {
if (!freeQueue_ || !recQueue_ || !devShadowQueue_) {
LOGE("====NULL poiter to Start(%p, %p, %p)", freeQueue_, recQueue_,
devShadowQueue_);
return SL_BOOLEAN_FALSE;
}
audioBufCount = 0;
SLresult result;
// in case already recording, stop recording and clear buffer queue
result = (*recItf_)->SetRecordState(recItf_, SL_RECORDSTATE_STOPPED);
SLASSERT(result);
result = (*recBufQueueItf_)->Clear(recBufQueueItf_);
SLASSERT(result);
for (int i = 0; i < RECORD_DEVICE_KICKSTART_BUF_COUNT; i++) {
sample_buf *buf = NULL;
if (!freeQueue_->front(&buf)) {
LOGE("=====OutOfFreeBuffers @ startingRecording @ (%d)", i);
break;
}
freeQueue_->pop();
assert(buf->buf_ && buf->cap_ && !buf->size_);
result = (*recBufQueueItf_)->Enqueue(recBufQueueItf_, buf->buf_, buf->cap_);
SLASSERT(result);
devShadowQueue_->push(buf);
}
result = (*recItf_)->SetRecordState(recItf_, SL_RECORDSTATE_RECORDING);
SLASSERT(result);
return (result == SL_RESULT_SUCCESS ? SL_BOOLEAN_TRUE : SL_BOOLEAN_FALSE);
}
SLboolean AudioRecorder::Stop(void) {
// in case already recording, stop recording and clear buffer queue
SLuint32 curState;
SLresult result = (*recItf_)->GetRecordState(recItf_, &curState);
SLASSERT(result);
if (curState == SL_RECORDSTATE_STOPPED) {
return SL_BOOLEAN_TRUE;
}
result = (*recItf_)->SetRecordState(recItf_, SL_RECORDSTATE_STOPPED);
SLASSERT(result);
result = (*recBufQueueItf_)->Clear(recBufQueueItf_);
SLASSERT(result);
#ifdef ENABLE_LOG
recLog_->flush();
#endif
return SL_BOOLEAN_TRUE;
}
OpenSL ES 没有为启动和停止 recorder 创建专门的接口，而是通过修改 recorder 的状态的接口实现的。
- 处理录制数据
可以给 SLAndroidSimpleBufferQueueItf 对象配置多块分配好的内存缓冲区，当一块缓冲区中填满了录制数据时，注册的回调被调用。当回调被调用时，可以这样来处理数据：
void bqRecorderCallback(SLAndroidSimpleBufferQueueItf bq, void *rec) {
(static_cast<AudioRecorder *>(rec))->ProcessSLCallback(bq);
}
void AudioRecorder::ProcessSLCallback(SLAndroidSimpleBufferQueueItf bq) {
#ifdef ENABLE_LOG
recLog_->logTime();
#endif
assert(bq == recBufQueueItf_);
sample_buf *dataBuf = NULL;
devShadowQueue_->front(&dataBuf);
devShadowQueue_->pop();
dataBuf->size_ = dataBuf->cap_; // device only calls us when it is really
// full
callback_(ctx_, ENGINE_SERVICE_MSG_RECORDED_AUDIO_AVAILABLE, dataBuf);
recQueue_->push(dataBuf);
sample_buf *freeBuf;
while (freeQueue_->front(&freeBuf) && devShadowQueue_->push(freeBuf)) {
freeQueue_->pop();
SLresult result = (*bq)->Enqueue(bq, freeBuf->buf_, freeBuf->cap_);
SLASSERT(result);
}
++audioBufCount;
// should leave the device to sleep to save power if no buffers
if (devShadowQueue_->size() == 0) {
LOGI("devShadowQueue size is 0, stop recording");
(*recItf_)->SetRecordState(recItf_, SL_RECORDSTATE_STOPPED);
}
}
由于 OpenSL ES 总是在缓冲区满时调用回调，则配置的缓冲区大小对录制的延迟及回调调用的间隔有着重大影响。
- 销毁 Recorder
录制结束时，销毁 recorder：
AudioRecorder::~AudioRecorder() {
// destroy src.audio recorder object, and invalidate all associated interfaces
if (recObjectItf_ != NULL) {
(*recObjectItf_)->Destroy(recObjectItf_);
}
if (devShadowQueue_) {
sample_buf *buf = NULL;
while (devShadowQueue_->front(&buf)) {
devShadowQueue_->pop();
freeQueue_->push(buf);
}
delete (devShadowQueue_);
}
#ifdef ENABLE_LOG
if (recLog_) {
delete recLog_;
}
#endif
}
- 创建 Player
AudioPlayer::AudioPlayer(SampleFormat *sampleFormat, SLEngineItf slEngine)
: freeQueue_(nullptr),
playQueue_(nullptr),
devShadowQueue_(nullptr),
callback_(nullptr) {
SLresult result;
assert(sampleFormat);
sampleInfo_ = *sampleFormat;
result = (*slEngine)
->CreateOutputMix(slEngine, &outputMixObjectItf_, 0, NULL, NULL);
SLASSERT(result);
// realize the output mix
result =
(*outputMixObjectItf_)->Realize(outputMixObjectItf_, SL_BOOLEAN_FALSE);
SLASSERT(result);
// configure src.audio source
SLDataLocator_AndroidSimpleBufferQueue loc_bufq = {
SL_DATALOCATOR_ANDROIDSIMPLEBUFFERQUEUE, DEVICE_SHADOW_BUFFER_QUEUE_LEN};
SLAndroidDataFormat_PCM_EX format_pcm;
ConvertToSLSampleFormat(&format_pcm, &sampleInfo_);
SLDataSource audioSrc = {&loc_bufq, &format_pcm};
// configure src.audio sink
SLDataLocator_OutputMix loc_outmix = {SL_DATALOCATOR_OUTPUTMIX,
outputMixObjectItf_};
SLDataSink audioSnk = {&loc_outmix, NULL};
/*
* create fast path src.audio player: SL_IID_BUFFERQUEUE and SL_IID_VOLUME
* and other non-signal processing interfaces are ok.
*/
SLInterfaceID ids[2] = {SL_IID_BUFFERQUEUE, SL_IID_VOLUME};
SLboolean req[2] = {SL_BOOLEAN_TRUE, SL_BOOLEAN_TRUE};
result = (*slEngine)->CreateAudioPlayer(
slEngine, &playerObjectItf_, &audioSrc, &audioSnk,
sizeof(ids) / sizeof(ids[0]), ids, req);
SLASSERT(result);
// realize the player
result = (*playerObjectItf_)->Realize(playerObjectItf_, SL_BOOLEAN_FALSE);
SLASSERT(result);
// get the play interface
result = (*playerObjectItf_)
->GetInterface(playerObjectItf_, SL_IID_PLAY, &playItf_);
SLASSERT(result);
// get the buffer queue interface
result = (*playerObjectItf_)
->GetInterface(playerObjectItf_, SL_IID_BUFFERQUEUE,
&playBufferQueueItf_);
SLASSERT(result);
// register callback on the buffer queue
result = (*playBufferQueueItf_)
->RegisterCallback(playBufferQueueItf_, bqPlayerCallback, this);
SLASSERT(result);
result = (*playItf_)->SetPlayState(playItf_, SL_PLAYSTATE_STOPPED);
SLASSERT(result);
// create an empty queue to track deviceQueue
devShadowQueue_ = new AudioQueue(DEVICE_SHADOW_BUFFER_QUEUE_LEN);
assert(devShadowQueue_);
silentBuf_.cap_ = (format_pcm.containerSize >> 3) * format_pcm.numChannels *
sampleInfo_.framesPerBuf_;
silentBuf_.buf_ = new uint8_t[silentBuf_.cap_];
memset(silentBuf_.buf_, 0, silentBuf_.cap_);
silentBuf_.size_ = silentBuf_.cap_;
#ifdef ENABLE_LOG
std::string name = "play";
logFile_ = new AndroidLog(name);
#endif
}
创建 Player 的过程与创建 Recorder 的过程类似，同样要定义流水线，只是流水线的样子有差别。Player 的流水线如下面这样：
OpenSL ES Playout Pipeline
这个过程最终获得类型为 SLPlayItf 的 playItf_ 来控制播放。
- 播放的启动和停止
SLresult AudioPlayer::Start(void) {
SLuint32 state;
SLresult result = (*playItf_)->GetPlayState(playItf_, &state);
if (result != SL_RESULT_SUCCESS) {
return SL_BOOLEAN_FALSE;
}
if (state == SL_PLAYSTATE_PLAYING) {
return SL_BOOLEAN_TRUE;
}
result = (*playItf_)->SetPlayState(playItf_, SL_PLAYSTATE_STOPPED);
SLASSERT(result);
result =
(*playBufferQueueItf_)
->Enqueue(playBufferQueueItf_, silentBuf_.buf_, silentBuf_.size_);
SLASSERT(result);
devShadowQueue_->push(&silentBuf_);
result = (*playItf_)->SetPlayState(playItf_, SL_PLAYSTATE_PLAYING);
SLASSERT(result);
return SL_BOOLEAN_TRUE;
}
void AudioPlayer::Stop(void) {
SLuint32 state;
SLresult result = (*playItf_)->GetPlayState(playItf_, &state);
SLASSERT(result);
if (state == SL_PLAYSTATE_STOPPED) return;
std::lock_guard<std::mutex> lock(stopMutex_);
result = (*playItf_)->SetPlayState(playItf_, SL_PLAYSTATE_STOPPED);
SLASSERT(result);
(*playBufferQueueItf_)->Clear(playBufferQueueItf_);
#ifdef ENABLE_LOG
if (logFile_) {
delete logFile_;
logFile_ = nullptr;
}
#endif
}
这个过程同样与 recorder 的对应过程类似，通过改变状态来控制播放的启动与停止。
- 播放回调的处理
void bqPlayerCallback(SLAndroidSimpleBufferQueueItf bq, void *ctx) {
(static_cast<AudioPlayer *>(ctx))->ProcessSLCallback(bq);
}
void AudioPlayer::ProcessSLCallback(SLAndroidSimpleBufferQueueItf bq) {
LOGI("AudioPlayer::ProcessSLCallback");
#ifdef ENABLE_LOG
logFile_->logTime();
#endif
std::lock_guard<std::mutex> lock(stopMutex_);
// retrieve the finished device buf and put onto the free queue
// so recorder could re-use it
sample_buf *buf;
if (!devShadowQueue_->front(&buf)) {
/*
* This should not happen: we got a callback,
* but we have no buffer in deviceShadowedQueue
* we lost buffers this way...(ERROR)
*/
if (callback_) {
uint32_t count;
callback_(ctx_, ENGINE_SERVICE_MSG_RETRIEVE_DUMP_BUFS, &count);
}
return;
}
devShadowQueue_->pop();
if (buf != &silentBuf_) {
buf->size_ = 0;
freeQueue_->push(buf);
if (!playQueue_->front(&buf)) {
#ifdef ENABLE_LOG
logFile_->log("%s", "====Warning: running out of the Audio buffers");
#endif
return;
}
devShadowQueue_->push(buf);
(*bq)->Enqueue(bq, buf->buf_, buf->size_);
playQueue_->pop();
return;
}
if (playQueue_->size() < PLAY_KICKSTART_BUFFER_COUNT) {
(*bq)->Enqueue(bq, buf->buf_, buf->size_);
devShadowQueue_->push(&silentBuf_);
return;
}
assert(PLAY_KICKSTART_BUFFER_COUNT <=
(DEVICE_SHADOW_BUFFER_QUEUE_LEN - devShadowQueue_->size()));
for (int32_t idx = 0; idx < PLAY_KICKSTART_BUFFER_COUNT; idx++) {
playQueue_->front(&buf);
playQueue_->pop();
devShadowQueue_->push(buf);
(*bq)->Enqueue(bq, buf->buf_, buf->size_);
}
}
在录制回调中，将录制的数据传给外界，在播放的回调中，则将外部数据拷贝给 OpenSL ES 使用。
- 销毁 Player
AudioPlayer::~AudioPlayer() {
std::lock_guard<std::mutex> lock(stopMutex_);
// destroy buffer queue src.audio player object, and invalidate all associated
// interfaces
if (playerObjectItf_ != NULL) {
(*playerObjectItf_)->Destroy(playerObjectItf_);
}
// Consume all non-completed src.audio buffers
sample_buf *buf = NULL;
while (devShadowQueue_->front(&buf)) {
buf->size_ = 0;
devShadowQueue_->pop();
if(buf != &silentBuf_) {
freeQueue_->push(buf);
}
}
delete devShadowQueue_;
while (playQueue_->front(&buf)) {
buf->size_ = 0;
playQueue_->pop();
freeQueue_->push(buf);
}
// destroy output mix object, and invalidate all associated interfaces
if (outputMixObjectItf_) {
(*outputMixObjectItf_)->Destroy(outputMixObjectItf_);
}
delete[] silentBuf_.buf_;
}
播放结束之后，需要销毁相关的对象。
- 销毁 engine
JNIEXPORT void JNICALL MainActivity_deleteSLEngine(
JNIEnv *env, jclass type) {
LOGI("MainActivity_deleteSLEngine");
delete engine.recBufQueue_;
delete engine.freeBufQueue_;
releaseSampleBufs(engine.bufs_, engine.bufCount_);
if (engine.slEngineObj_ != NULL) {
(*engine.slEngineObj_)->Destroy(engine.slEngineObj_);
engine.slEngineObj_ = NULL;
engine.slEngineItf_ = NULL;
}
if (engine.delayEffect_) {
delete engine.delayEffect_;
engine.delayEffect_ = nullptr;
}
}
OpenSL ES 的主要对象结构
OpenSL ES 的对象可以大体总结如下：
opensles_object.png
上面的示例代码只是展示了 OpenSL ES 支持搭建的一种流水线，对于 OpenSL ES API 的其它使用方式更详细的信息可以参考官方的文档。
上面的代码可以在 GitHub 的 Repo audio-echo 种找到。
参考文档：
OpenSL ES
OpenSL ES 使用入门
OpenSL ES for Android
OpenSL ES Android 扩展
OpenSL ES 编程说明
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