Android 音頻錄製(三)-OpenSL ES

Android 系統爲我們提供了三種錄製音頻的方式

1. MediaRecord（ Java API）
2. AudioRecord（ Java API）
3. OpenSL ES（ Native API）

上兩節說了 MediaRecord 和 AudioRecord，他們都是應用層提供的 Java Api。這節呢就來說下 native 層爲我們提供的相關 api。

什麼是 OpenSL ES

先來看下 OpenSL ES 官方文檔 是怎麼介紹的。

This library allows you to use C or C++ to implement high-performance, low-latency audio, whether you are writing a synthesizer, digital audio workstation, karaoke, game, or other real-time app.
The OpenSL ES™ standard exposes audio features similar to those in the MediaPlayer and MediaRecorder APIs in the Android Java framework. OpenSL ES provides a C language interface as well as C++ bindings, allowing you to call the API from code written in either language.

這兩段話的大致意思就是。我這個庫在音頻處理方面老牛逼了，性能高不說，延時還短的不要不要的。而且使用方法和 MediaPlayer 與 MeidaRecord 基本上差別不大，唯一的要求就是得會 C 或者 C艹 呀。

OpenSL ES 在哪

要進行 NDK 開發就必須得先集成相應環境，正如需要 Android SDK 一樣。同樣需要 Android NDK，如果已經科學上網，則直接在 Android Studio 的 SDk Manager 中直接下載即可

下載完成後新建項目時就會看到有一個 Native C++ 的選項，創建出來的項目是一個 Native 版本的 Hello Word。

OpenSL ES 錄製

OpenSL ES 錄製音頻有如下相關流程

1. 創建接口對象

   void createEngine() {
       SLresult result;
   
       result = slCreateEngine(&engineObj, 0, nullptr, 0, nullptr, nullptr);
       assert(result == SL_RESULT_SUCCESS);
       (void) result;
   
       result = (*engineObj)->Realize(engineObj, SL_BOOLEAN_FALSE);
       assert(SL_RESULT_SUCCESS == result);
       (void) result;
   
       result = (*engineObj)->GetInterface(engineObj, SL_IID_ENGINE, &engine);
       assert(SL_RESULT_SUCCESS == result);
       (void) result;
   
       const SLInterfaceID ids[1] = {SL_IID_ENVIRONMENTALREVERB};
       const SLboolean req[1] = {SL_BOOLEAN_FALSE};
       // outputMixObj 用於輸出聲音數據
       (*engine)->CreateOutputMix(engine, &outputMixObj, 1, ids, req);
   
       result = (*outputMixObj)->Realize(outputMixObj, SL_BOOLEAN_FALSE);
       assert(SL_RESULT_SUCCESS == result);
       (void) result;
   }
   

2. 創建錄音器

   result = (*mAudioEngine->engine)->CreateAudioRecorder(mAudioEngine->engine, &mRecorderObj,
           &audioSrc, &audioSink, 1, id, req);
   if (SL_RESULT_SUCCESS != result) {
       return false;
   }
   
   result = (*mRecorderObj)->Realize(mRecorderObj, SL_BOOLEAN_FALSE);
   if (SL_RESULT_SUCCESS != result) {
       return false;
   }
   

3. 設置緩衝隊列和回調函數

   result = (*mBufferQueue)->RegisterCallback(mBufferQueue, recorderCallback, this);
   

4. 設置錄音狀態，開始錄製

   bool AudioRecorder::start() {
       if (!mIsInitialized) {
           if (!initRecorder()) {
               return false;
           }
           mIsInitialized = true;
       }
   
       SLresult result;
   
       result = (*mRecorder)->SetRecordState(mRecorder, SL_RECORDSTATE_STOPPED);
       if (SL_RESULT_SUCCESS != result) {
           return false;
       }
   
       result = (*mBufferQueue)->Clear(mBufferQueue);
       if (SL_RESULT_SUCCESS != result) {
           return false;
       }
   
       // enqueue an empty buffer to be filled by the recorder
       // (for streaming recording, we would enqueue at least 2 empty buffers to start things off)
       result = (*mBufferQueue)->Enqueue(mBufferQueue, mBuffers[mIndex], mBufSize * sizeof(short));
       if (SL_RESULT_SUCCESS != result) {
           return false;
       }
   
       // 開始錄製
       result = (*mRecorder)->SetRecordState(mRecorder, SL_RECORDSTATE_RECORDING);
       if (SL_RESULT_SUCCESS != result) {
           return false;
       }
   
       mIsRecording = true;
       LOGI("start recording...");
   
       return true;
   }
   

最後附上 完整源碼

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音頻基礎知識

Android 音頻錄製-MeidaRecord

Android 音頻錄製-AudioRecord

Android 音頻錄製-OpenSL ES