安卓MediaRecorder(3)音频采集编码写入详细源码分析

本文首发地址 https://blog.csdn.net/CSqingchen/article/details/134896808
最新更新地址 https://gitee.com/chenjim/chenjimblog

前言

通过安卓MediaRecorder(2)录制源码分析，我们知道 MediaRecorder 相关接口是在 StagefrightRecorder.cpp 中实现，本文进一步分析音频采集、编码、写入文件详细流程。

音频采集

音频初始化

通过前文，我们知道 setupAudioEncoder 在 setupMPEG4orWEBMRecording 中初始化，相关源码如下

// frameworks/av/media/libmediaplayerservice/StagefrightRecorder.cpp
status_t StagefrightRecorder::setupAudioEncoder() {sp<MediaCodecSource> audioEncoder = createAudioSource();return OK;
}
sp<MediaCodecSource> StagefrightRecorder::createAudioSource() {...// 通过 AVFactory 工厂创建 AudioSource,并初始化 sp<AudioSource> audioSource = AVFactory::get()->createAudioSource(&attr,mAttributionSource,sourceSampleRate,mAudioChannels,mSampleRate,mSelectedDeviceId,mSelectedMicDirection,mSelectedMicFieldDimension);}

那 AudioSource 是如何初始化的呢

// frameworks/av/media/libstagefright/AudioSource.cpp
void AudioSource::set(const audio_attributes_t *attr, const AttributionSourceState& attributionSource,uint32_t sampleRate, uint32_t channelCount, uint32_t outSampleRate,audio_port_handle_t selectedDeviceId,audio_microphone_direction_t selectedMicDirection,float selectedMicFieldDimension)
{...// 构造了 一个 AudioRecord cpp 对象  mRecord = new AudioRecord(AUDIO_SOURCE_DEFAULT, sampleRate, AUDIO_FORMAT_PCM_16_BIT,audio_channel_in_mask_from_count(channelCount),attributionSource,(size_t) (bufCount * frameCount),// 采集的音频数据回调 wp<AudioRecord::IAudioRecordCallback>{this},frameCount /*notificationFrames*/,AUDIO_SESSION_ALLOCATE,AudioRecord::TRANSFER_DEFAULT,AUDIO_INPUT_FLAG_NONE,attr,selectedDeviceId,selectedMicDirection,selectedMicFieldDimension);...
}

AudioRecord.java 底层的实现也是 AudioSource.cpp
AudioRecord 主要是负责从麦克风设备采集音频 PCM 帧

AudioRecord 分析

// frameworks/av/media/libaudioclient/AudioRecord.cpp
status_t AudioRecord::set(...) {...if (mCallback != nullptr) {// 启动录制的线程 mAudioRecordThread = new AudioRecordThread(*this);mAudioRecordThread->run("AudioRecord", ANDROID_PRIORITY_AUDIO);}...
}
bool AudioRecord::AudioRecordThread::threadLoop() {...nsecs_t ns =  mReceiver.processAudioBuffer();...
}nsecs_t AudioRecord::processAudioBuffer() {... // 回调 AudioRecord::IAudioRecordCallback if (newOverrun) {callback->onOverrun();}if (markerReached) {callback->onMarker(markerPosition.value());}while (newPosCount > 0) {callback->onNewPos(newPosition.value());newPosition += updatePeriod;newPosCount--;}if (mObservedSequence != sequence) {mObservedSequence = sequence;callback->onNewIAudioRecord();}while (mRemainingFrames > 0) {// 获取 audioBuffer status_t err = obtainBuffer(&audioBuffer, requested, NULL, &nonContig);// 回调 取到的 buffer 到 AudioSource 中 onMoreData  const size_t readSize = callback->onMoreData(*buffer);// 释放 buffer releaseBuffer(&audioBuffer);}
}

AudioSource 采集到音频

// frameworks/av/media/libstagefright/AudioSource.cpp
size_t AudioSource::onMoreData(const AudioRecord::Buffer& audioBuffer) { ...// 将AudioRecord::Buffer 放入 MediaBufferMediaBuffer *buffer = new MediaBuffer(audioBuffer.size());memcpy((uint8_t *) buffer->data(),audioBuffer.data(), audioBuffer.size());buffer->set_range(0, audioBuffer.size());// 将 buffer 放入缓存queueInputBuffer_l(buffer, timeUs);return audioBuffer.size();
}
void AudioSource::queueInputBuffer_l(MediaBuffer *buffer, int64_t timeUs) {...// 将 buffer 放入缓存 mBuffersReceived 中mBuffersReceived.push_back(buffer);mFrameAvailableCondition.signal();
}// 如下接口可以读取采集到的 buffer
status_t AudioSource::read(MediaBufferBase **out, const ReadOptions * /* options */) {...MediaBuffer *buffer = *mBuffersReceived.begin();mBuffersReceived.erase(mBuffersReceived.begin());buffer->setObserver(this);...*out = buffer;
}

音频编码

编码器创建如下

sp<MediaCodecSource> StagefrightRecorder::createAudioSource() {sp<MediaCodecSource> audioEncoder = MediaCodecSource::Create(mLooper, format, audioSource);
}
// MediaCodecSource 构造如下  
MediaCodecSource::MediaCodecSource(const sp<ALooper> &looper,const sp<AMessage> &outputFormat,const sp<MediaSource> &source,const sp<PersistentSurface> &persistentSurface,uint32_t flags){if (!(mFlags & FLAG_USE_SURFACE_INPUT)) {// 将 AudioSource 放入 Puller 中mPuller = new Puller(source);}
}

MediaCodecSource::start 发送 kWhatStart 消息

status_t MediaCodecSource::start(MetaData* params) {sp<AMessage> msg = new AMessage(kWhatStart, mReflector);msg->setObject("meta", params);// 发消息 kWhatStart 到 MediaCodecSource::onMessageReceived // 进而传递到 MediaCodecSource::onStart return postSynchronouslyAndReturnError(msg);
}
void MediaCodecSource::onMessageReceived(const sp<AMessage> &msg) {switch (msg->what()) {case kWhatStart: {sp<AMessage> response = new AMessage;// 调用 MediaCodecSource::onStart response->setInt32("err", onStart(params));response->postReply(replyID);}}
}
status_t MediaCodecSource::onStart(MetaData *params) {...// 创建 kWhatPullerNotify 消息,传入  MediaCodecSource::Puller::start sp<AMessage> notify = new AMessage(kWhatPullerNotify, mReflector);err = mPuller->start(meta.get(), notify);
}

MediaCodecSource::Puller::start 流程如下

status_t MediaCodecSource::Puller::start(const sp<MetaData> &meta, const sp<AMessage> &notify) {mNotify = notify;// 发送 kWhatStart 消息 到 MediaCodecSource::Puller::onMessageReceivedsp<AMessage> msg = new AMessage(kWhatStart, this);msg->setObject("meta", meta);return postSynchronouslyAndReturnError(msg);
}
void MediaCodecSource::Puller::onMessageReceived(const sp<AMessage> &msg) {switch (msg->what()) {case kWhatStart: {// start后，就开始 pull schedulePull();}case kWhatPull:{// 通过上文的 AudioSource::read 读取采集到的数据status_t err = mSource->read(&mbuf);// 将读取到的 mbuf 放入队列 queue->pushBuffer(mbuf);if (mbuf != NULL) {// 送到 MediaCodecSource::onMessageReceived， 通知编码器 pull 到数据mNotify->post();// 继续 pull msg->post();} else {// 结束 EndOfStream handleEOS();}}
}

MediaCodecSource::Puller 读取到数据后，mNotify 发消息 kWhatPullerNotify 通知编码

void MediaCodecSource::onMessageReceived(const sp<AMessage> &msg) {switch (msg->what()) {case kWhatPullerNotify:{...// 收到 通知，送去编码feedEncoderInputBuffers();}}
}
status_t MediaCodecSource::feedEncoderInputBuffers() {// 取数据编码while (!mAvailEncoderInputIndices.empty() && mPuller->readBuffer(&mbuf)) {...// inbuf 送到编码器status_t err = mEncoder->getInputBuffer(bufferIndex, &inbuf);...// 编码status_t err = mEncoder->queueInputBuffer(bufferIndex, 0, size, timeUs, flags);}
}

音频编码后数据处理

在创建编码器时，把 mEncoderActivityNotify 设置到编码器的 Callback，编码器的消息会通过 kWhatEncoderActivity 发送出来

status_t MediaCodecSource::initEncoder() {...mEncoderActivityNotify = new AMessage(kWhatEncoderActivity, mReflector);mEncoder->setCallback(mEncoderActivityNotify);...
}

当编码完成、状态变化，会收到 kWhatEncoderActivity 消息通知

void MediaCodecSource::onMessageReceived(const sp<AMessage> &msg) {switch (msg->what()) {case kWhatEncoderActivity:{if (cbID == MediaCodec::CB_INPUT_AVAILABLE) {// 输入不可用，继续给编码器送输入} else if (cbID == MediaCodec::CB_OUTPUT_FORMAT_CHANGED) {// 输出格式变化} else if (cbID == MediaCodec::CB_OUTPUT_AVAILABLE) {// 正常的输出数据// 获取编码器额输出status_t err = mEncoder->getOutputBuffer(index, &outbuf);// 将输出 buf 转  MediaBufferMediaBuffer *mbuf = new MediaBuffer(outbuf->size());// 提取 MetaDatasp<MetaData> meta = new MetaData(mbuf->meta_data());...// 将 编码数据 outbuf 填充到 mbufmemcpy(mbuf->data(), outbuf->data(), outbuf->size());// 将编码后的数据添加到队列output->mBufferQueue.push_back(mbuf);} else if (cbID == MediaCodec::CB_ERROR) {// ERROR 异常，退出signalEOS(err);}}}
}

当需要数据时,从输出队列取数据即可

status_t MediaCodecSource::read(MediaBufferBase** buffer, const ReadOptions* /* options */) {Mutexed<Output>::Locked output(mOutput);*buffer = NULL;while (output->mBufferQueue.size() == 0 && !output->mEncoderReachedEOS) {output.waitForCondition(output->mCond);}if (!output->mEncoderReachedEOS) {*buffer = *output->mBufferQueue.begin();output->mBufferQueue.erase(output->mBufferQueue.begin());return OK;}return output->mErrorCode;
}

MPEG4Writer写入音频编码后数据到文件

通过如下源码，我们知道了 MPEG4Writer 创建和写入线程启动

status_t StagefrightRecorder::setupMPEG4orWEBMRecording() {...writer = mp4writer = new MPEG4Writer(mOutputFd);
}
status_t StagefrightRecorder::start() {...status = mWriter->start(meta.get());
}
status_t MPEG4Writer::start(MetaData *param) {...err = startWriterThread();...// 这个 startTracks 主要为 MPEG4Writer::Track 做准备 err = startTracks(param);
}
status_t MPEG4Writer::startWriterThread() {mDone = false;mIsFirstChunk = true;mDriftTimeUs = 0;// 将 音、视频 Track 添加到 mChunkInfosfor (List<Track *>::iterator it = mTracks.begin();it != mTracks.end(); ++it) {ChunkInfo info;info.mTrack = *it;info.mPrevChunkTimestampUs = 0;info.mMaxInterChunkDurUs = 0;mChunkInfos.push_back(info);}...// 启动线程执行 ThreadWrapper pthread_attr_t attr;pthread_attr_init(&attr);pthread_attr_setdetachstate(&attr, PTHREAD_CREATE_JOINABLE);pthread_create(&mThread, &attr, ThreadWrapper, this);pthread_attr_destroy(&attr);return OK;
}
void *MPEG4Writer::ThreadWrapper(void *me) {MPEG4Writer *writer = static_cast<MPEG4Writer *>(me);// 最终执行的是 threadFunc()writer->threadFunc();return NULL;
}

写入线程开启后，一直循环，无数据时等待

void MPEG4Writer::threadFunc() {Mutex::Autolock autoLock(mLock);while (!mDone) {Chunk chunk;bool chunkFound = false;// findChunkToWrite 从 mChunkInfos 找到需要写入的 Chunkwhile (!mDone && !(chunkFound = findChunkToWrite(&chunk))) {mChunkReadyCondition.wait(mLock);}// 在实时记录模式下，写时不按顺序持有锁, 减少媒体跟踪线程的阻塞时间。// 否则，保持锁，直到现有的块被写入文件。if (chunkFound) {if (mIsRealTimeRecording) {mLock.unlock();}// 写入 Chunk writeChunkToFile(&chunk);if (mIsRealTimeRecording) {mLock.lock();}}}// 写入所有内存writeAllChunks();
}

写入到文件是在 writeChunkToFile 中完成

void MPEG4Writer::writeChunkToFile(Chunk* chunk) {while (!chunk->mSamples.empty()) {// 取一个 MediaBuffer List<MediaBuffer *>::iterator it = chunk->mSamples.begin();...// 写入 MediaBuffer off64_t offset = addSample_l(*it, usePrefix, tiffHdrOffset, &bytesWritten);...}// 写入后清空chunk->mSamples.clear();
}
off64_t MPEG4Writer::addSample_l(MediaBuffer *buffer, bool usePrefix,uint32_t tiffHdrOffset, size_t *bytesWritten) {...writeOrPostError(mFd, (const uint8_t*)buffer->data() + buffer->range_offset(),buffer->range_length());
}
void MPEG4Writer::writeOrPostError(int fd, const void* buf, size_t count) {...// 真正的写入 buf ssize_t bytesWritten = ::write(fd, buf, count);...// IO 异常时 抛出 ，通过消息传递到上层  sp<AMessage> msg = new AMessage(kWhatIOError, mReflector);msg->setInt32("err", ERROR_IO);
}

MPEG4Writer::Track 取编码后的音频编数据

MPEG4Writer::Track 启动源码如下

status_t MPEG4Writer::startTracks(MetaData *params) {...for (List<Track *>::iterator it = mTracks.begin();it != mTracks.end(); ++it) {// MPEG4Writer::Track  start status_t err = (*it)->start(params);...}return OK;
}
status_t MPEG4Writer::Track::start(MetaData *params) {...// 启动线程执行 ThreadWrapper pthread_create(&mThread, &attr, ThreadWrapper, this);
}
void *MPEG4Writer::Track::ThreadWrapper(void *me) {Track *track = static_cast<Track *>(me);status_t err = track->threadEntry();return (void *)(uintptr_t)err;
}

MPEG4Writer::Track::threadEntry 读取编码后的数据

status_t MPEG4Writer::Track::threadEntry() {// mSource->read 也就是 上文 MediaCodecSource::read ，一直不停的读取数据到 buffer  MediaBufferBase *buffer;while (!mDone && (err = mSource->read(&buffer)) == OK && buffer != NULL) {...// 将 buffer 转为 MediaBuffer MediaBuffer *copy = new MediaBuffer(buffer->range_length());if (sampleFileOffset != -1) {copy->meta_data().setInt64(kKeySampleFileOffset, sampleFileOffset);} else {memcpy(copy->data(), (uint8_t*)buffer->data() + buffer->range_offset(),buffer->range_length());}...// 将 copy 放入队列 mChunkSamplesmChunkSamples.push_back(copy);...// 将 mChunkSamples 转 为 ChunkbufferChunk(timestampUs);}
}
void MPEG4Writer::Track::bufferChunk(int64_t timestampUs) {Chunk chunk(this, timestampUs, mChunkSamples);// 也就是  MPEG4Writer::bufferChunk mOwner->bufferChunk(chunk);mChunkSamples.clear();
}
void MPEG4Writer::bufferChunk(const Chunk& chunk) {Mutex::Autolock autolock(mLock);for (List<ChunkInfo>::iterator it = mChunkInfos.begin();it != mChunkInfos.end(); ++it) {if (chunk.mTrack == it->mTrack) {// 将 Chunk 放入 ChunkInfo.mChunks 中，it->mChunks.push_back(chunk);// 数据准备好了，通知 mChunkReadyCondition.wait 继续执行// 进而 由 findChunkToWrite 读取写入文件  mChunkReadyCondition.signal();return;}}
}

结语

到这里，已经完成了 MediaRecorder 音频采集、编码、写入文件详细源码分析。
用一幅图概括如下

希望对你有所帮助。如果你在使用MediaRecorder的过程中遇到了其他问题，欢迎留言讨论。
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